The Cosmic Perspective: Exam 4 Flashcards

1) What is the diameter of the disk of the Milky Way?
A) 100 light-years
B) 1,000 light-years
C) 10,000 light-years
D) 100,000 light-years
E) 1,000,000 light-years

2) What is the thickness of the disk of the Milky Way?
A) 100 light-years
B) 1,000 light-years
C) 10,000 light-years
D) 100,000 light-years
E) 1,000,000 light-years

3) What kinds of objects lie in the halo of our galaxy?
A) open clusters
B) O and B stars
C) globular clusters
D) gas and dust
E) all of the above

4) What kinds of objects lie in the disk of our galaxy?
A) open clusters
B) O and B stars
C) old K and M stars
D) gas and dust
E) all of the above

5) Which of the following comprise the oldest members of the Milky Way?
A) the Sun and other solar mass stars
B) O stars
C) red giant stars in spiral arms
D) Cepheid variables
E) globular clusters

6) What makes up the interstellar medium?
A) open clusters
B) O and B stars
C) K and M stars
D) gas and dust
E) all of the above

7) If you were to take a voyage across the Milky Way, what kind of material would you spend most of your time in?
A) empty space–a pure vacuum
B) dusty molecular clouds
C) star clusters
D) warm, rarefied clouds of atomic hydrogen
E) cool, dense clouds of atomic hydrogen

8) How does the interstellar medium obscure our view of most of the galaxy?
A) It produces so much visible light that it is opaque and blocks our view of anything beyond it.
B) It reflects most light from far distances of the galaxy away from our line of sight.
C) It absorbs all wavelengths of light.
D) It absorbs visible, ultraviolet, and some infrared light.
E) all of the above

9) How can we see through the interstellar medium?
A) by observing in high-energy wavelengths such as X rays and long wavelengths of light such as radio waves
B) by observing only the brightest visible sources
C) by using only the biggest telescopes
D) by using telescopes above the Earth's atmosphere
E) We cannot see through the interstellar medium.

10) Harlow Shapley concluded that the Sun was not in the center of the Milky Way Galaxy by
A) looking at the shape of the "milky band" across the sky.
B) mapping the distribution of stars in the galaxy.
C) mapping the distribution of globular clusters in the galaxy.
D) mapping the distribution of gas clouds in the spiral arms.
E) looking at other nearby spiral galaxies.

11) Approximately how far is the Sun from the center of the galaxy?
A) 27 light-years
B) 270 light-years
C) 2,700 light-years
D) 27,000 light-years
E) 27 million light-years

12) What do astronomers consider heavy elements?
A) elements that are heavier than iron
B) elements that are heavier than carbon
C) elements that are heavier than hydrogen
D) elements that are heavier than uranium
E) all elements besides hydrogen and helium

13) Where are most heavy elements made?
A) in the interstellar medium
B) in stars and supernovae
C) in the Big Bang, when the universe first began
D) none of the above
E) all of the above

14) Why are we unlikely to find Earth-like planets around halo stars in the Galaxy?
A) Planets around stars are known to be extremely rare.
B) Halo stars formed in an environment where there were few heavy elements to create rocky planets.
C) Any such planets would have been ejected long ago by galactic mergers.
D) Halo stars do not have enough mass to hold onto planets.
E) Halo stars formed in a different way from disk stars.

15) How are interstellar bubbles made?
A) by the collapse of a gas cloud to form stars
B) by planetary nebulae from low-mass stars
C) by the winds of massive stars and supernovae
D) by collisions between galaxies
E) by the rapidly rotating magnetic fields of pulsars

16) What is a superbubble?
A) a very low-density region of interstellar space, formed by the merger of several bubbles
B) a very high-density region of interstellar space, filled with gas ejected from nearby star systems
C) a bubble so large that it fills much of the galactic halo
D) the region of space cleared by a powerful supernova
E) a cloud of gas that can form a million or more stars

17) Sound waves in space
A) do not exist.
B) travel so slowly that they are unnoticeable.
C) travel much faster than sound on Earth but have such low density that they are inaudible.
D) travel much faster than sound on Earth and are therefore very loud.
E) can travel through the halo but not the disk of the galaxy.

18) What is a shock front?
A) a wave of pressure that moves faster than the speed of sound
B) a wave of pressure that moves slightly slower than the speed of sound
C) a wave of pressure that moves faster than the speed of light
D) a wave of electromagnetic energy that can create electrical shocks
E) the wave created when protons slam into electrons

20) What can cause a galactic fountain?
A) winds and jets from newly-formed protostars
B) a supernova occurring in the halo
C) multiple supernovae occurring together
D) the combined effect of spiral density waves
E) molecular clouds falling towards the galactic center

21) What is the galactic fountain model?
A) the idea that there is a lot of interstellar water vapor
B) the theory that the Milky Way is a spiral galaxy and looks like a whirlpool from above
C) the theory that hot, ionized gas blows out of the galactic center like a jet or fountain
D) the theory that hot, ionized gas blown out of the galactic disk and into the halo by superbubbles cools down and falls back into the disk
E) none of the above

22) What evidence supports the galactic fountain model?
A) We see a jet of ionized gas shooting out of the bulge of our galaxy.
B) We have mapped several spiral arms of the Milky Way Galaxy.
C) We see hot gas above the disk of the galaxy and cool gas that appears to be raining down from the halo.
D) We have observed a lot of water molecules in the interstellar medium.
E) We have no evidence yet for the galactic fountain model.

23) What is the most common form of gas in the interstellar medium?
A) molecular hydrogen
B) molecular helium
C) atomic hydrogen
D) atomic helium
E) ionized hydrogen

24) What produces the 21-cm line that we use to map out the Milky Way Galaxy?
A) atomic hydrogen
B) ionized hydrogen
C) molecular hydrogen
D) carbon monoxide
E) helium

25) Where do most dust grains form?
A) in supernovae
B) in the winds of red giant stars
C) in planetary nebulae
D) in molecular clouds
E) all of the above

26) Suppose you read somewhere that 10 percent of the matter in the Milky Way is in the form of dust grains. Should you be surprised? If so, why?
A) There is nothing surprising about 10 percent of the matter being dust grains because dust grains are the material from which stars are born.
B) Given how easily dust grains form, 10 percent is a surprisingly low fraction of material to be in that form.
C) Ten percent is surprisingly high because dust grains can form only at low temperatures.
D) The 10 percent figure cannot be correct, because dust grains are solid but only about 2 percent of the matter in our galaxy is made of anything besides hydrogen and helium.

27) The image of our galaxy in radio emission from CO, mapping the distribution of molecular clouds, is closest to the image of our galaxy in
A) 21-cm-line radio emission from atomic hydrogen.
B) visible light, showing the edges of supernova bubbles.
C) visible light, which is closest to how the night sky appears from Earth.
D) X rays from hot gas bubbles in the disk.
E) infrared emission from interstellar dust grains.

28) Compared with our Sun, most stars in the halo are
A) young, red, and dim and have fewer heavy elements.
B) young, blue, and bright and have much more heavy element material.
C) old, red, and dim and have fewer heavy elements.
D) old, red, and dim and have much more heavy element material.
E) old, red, and bright and have fewer heavy elements.

29) Compared with stars in the disk, orbits of stars in the halo
A) are relatively uniform to each other.
B) are elliptical, with random orientation.
C) are elliptical but orbiting in the same direction.
D) do not have to be around the galactic center.
E) do not have to pass through the plane of the galaxy.

30) Approximately how long does it take the Sun to orbit the Milky Way Galaxy?
A) 23,000 years
B) 230,000 years
C) 2.3 million years
D) 230 million years
E) 23 billion years

31) Where does most star formation occur in the Milky Way today?
A) in the halo
B) in the bulge
C) in the spiral arms
D) in the Galactic center
E) uniformly throughout the Galaxy

32) How do we know that spheroidal stars are older, on average, than disk stars?
A) Spheroidal stars orbit in random directions but disk stars have more ordered orbits.
B) There are no blue spheroidal stars.
C) There are no red disk stars.
D) Theories of galaxy formation tell us that the spheroid formed earlier than the disk.
E) We see evidence for new stars forming in the disk today.

33) Which of the following statements about globular clusters is false?
A) Globular clusters contain many thousands of stars.
B) Globular cluster stars are more than 12 billion years old.
C) Globular cluster ages increase with distance from the Milky Way.
D) Globular clusters are distributed spherically around the Milky Way.
E) Globular cluster stars are very metal-poor relative to the Sun.

34) Which of the following statements about the disk of the Milky Way is false?
A) The average age of disk stars is less than that of halo stars.
B) Disk stars are all younger than 5 billion years.
C) Disk stars have a higher proportion of heavy elements, on average, than halo stars.
D) Disk stars orbit in the same direction around the Galactic center.
E) The length of the disk is about 100 times its thickness.

35) Which of the following statements about halo stars is false?
A) Halo stars have random orbits about the Milky Way center.
B) Halo stars are no longer being formed at the current epoch.
C) All halo stars are less massive than our Sun.
D) Halo stars are made entirely of hydrogen and helium with no heavy elements.
E) Halo stars are some of the oldest known objects in the universe.

36) What evidence suggests that the protogalactic cloud that formed the Milky Way resulted from several collisions among smaller clouds?
A) The stars in the halo of the Milky Way are organized into several dense clusters arranged throughout the halo.
B) The Milky Way resembles an elliptical galaxy more than other spirals do.
C) Halo stars differ in age and heavy-element content, but these variations do not seem to depend on the stars' distance from the galactic center.
D) The bulge of the Milky Way is surrounded by many globular clusters, just as elliptical galaxies are.
E) The Milky Way is the central galaxy of a cluster of galaxies.

37) Which constellation lies in the direction toward the galactic center?
A) Orion
B) the Big Dipper
C) Leo
D) Sagittarius
E) Taurus

38) How do we learn about what is going on in the center of our own galaxy (the Milky Way)?
A) We have learned it only recently, thanks to the great photographs obtained by the Hubble Space Telescope.
B) We cannot see the galactic center with visible or ultraviolet light, but radio and X rays from the center can be detected.
C) The gas and dust in the Milky Way prevent any type of direct observation of the galactic center, but theoretical models allow us to predict what is happening there.
D) We must look at the centers of other galaxies and hope that ours is just like others.
E) We can study it with visible telescopes as with any other star in the Galaxy.

39) Which of the following does not accurately describe what we observe toward the Galactic center?
A) at radio wavelengths, we see giant gas clouds threaded by powerful magnetic fields
B) at infrared wavelengths, we see a massive stellar cluster
C) at optical wavelengths, we see a cluster of old, red stars
D) at X rays, we see faint emission from an accretion disk around a black hole

40) What evidence supports the theory that there is a black hole at the center of our galaxy?
A) We observe an extremely bright X-ray source at the center of our galaxy.
B) We can see gas falling into an accretion disk and central mass at the center of our galaxy.
C) The motions of the gas and stars at the center indicate that it contains a million solar masses within a region only about 1 parsec across.
D) We observe a large, dark object that absorbs all light at the center of our galaxy.
E) all of the above

41) What is SgrA*?
A) a source of bright X-ray emission coming from the entire constellation of Sagittarius
B) a source of bright radio emission in the center of our galaxy
C) a source that is bright in the visible wavelengths in the center of our galaxy
D) the brightest star in the constellation Sagittarius
E) the bulge at the center of our galaxy

42) What evidence do we have that the spheroidal population of stars are older than other stars in the galaxy?
A) They are farther away
B) They have higher masses than other stars in the galaxy
C) They have fewer planets
D) They have a smaller proportion of heavy elements
E) They move slower than other stars in the galaxy

1) Open clusters and young stars are generally found only in the disk of the galaxy and not in the halo.

2) We can see most of the galaxy with visible light.

3) Observing the galaxy at radio wavelengths allows us to see beyond the dust in the disk of the galaxy that obscures our view.

4) The Milky Way looks the same in X rays as it does at infrared wavelengths.

5) The Sun is located at the edge of the galaxy, approximately 50,000 light-years from the galactic center.

6) Shapley used the distribution of globular clusters in the galaxy to determine that the Sun was not at the center of the Milky Way.

7) All heavy elements are made during supernova events.

8) The star-gas-star cycle will continue forever because stars are continually recycling gas.

9) Almost all elements heavier than hydrogen and helium were made inside stars.

10) Most of the current star formation in the Milky Way occurs in spiral arms.

1) How does the diameter of the disk of Milky Way Galaxy compare to its thickness?
A) The diameter and thickness are roughly equal.
B) The diameter is about 100 times as great as the thickness.
C) The diameter is about 10 times as great as the thickness.
D) The diameter is about 100,000 times as great as the thickness.

2) What do we call the bright, sphere-shaped region of stars that occupies the central few thousand light-years of the Milky Way Galaxy?
A) the galaxy's disk
B) the galaxy's bulge
C) a globular cluster
D) the galaxy's halo

3) The Sun's location in the Milky Way Galaxy is
A) very near the galactic center.
B) in the halo of the galaxy, about 28,000 light-years above the galactic disk.
C) at the very outer edge of the galactic disk.
D) in the galactic disk, roughly halfway between the center and the outer edge of the disk.

4) What do we mean by the interstellar medium?
A) the dust that fills the halo of the Milky Way Galaxy
B) the middle section of the Milky Way Galaxy
C) the gas and dust that lies in between the stars in the Milky Way Galaxy
D) the name of an oracle who can channel messages from beings that live near the star called Vega

5) What are the Magellanic Clouds?
A) two small galaxies that probably orbit the Milky Way Galaxy
B) two nebulae located in the disk of the Milky Way Galaxy and visible only from the Southern Hemisphere
C) star-forming clouds found in the constellation Orion
D) the clouds of dust and gas found interspersed in many places throughout the Milky Way Galaxy

6) How do disk stars orbit the center of the galaxy?
A) They all orbit in roughly the same plane and in the same direction.
B) They have orbits randomly inclined and in different directions relative to the galactic center.
C) They follow spiral paths along the spiral arms.
D) They follow orbits that move up and down through the disk, typically taking them about 50,000 light-years above and below the disk on each orbit.

7) How do we know the total mass of the Milky Way Galaxy that is contained within the Sun's orbital path?
A) by counting the number of stars visible in this region of the galaxy
B) by estimating the amount of gas and dust in between the stars
C) by using the law of conservation of angular momentum to calculate the orbital speeds of nearby stars
D) by applying Newton's version of Kepler's third law to the orbits of the Sun or other nearby stars around the center of the Galaxy

8) Elements heavier than hydrogen and helium constitute about ________ of the mass of the interstellar medium.
A) 0.002%
B) 2%
C) 70%
D) 98%

9) What do we mean by the star-gas-star cycle?
A) It is the idea that stars in close binary systems can exchange gas with one another.
B) It is the set of nuclear reactions by which heavy elements are produced in the cores of massive stars.
C) It describes the orbits of the stars and interstellar medium around the center of the galaxy.
D) It is the continuous recycling of gas in the galactic disk between stars and the interstellar medium.

10) What are cosmic rays?
A) another name for gamma rays and X rays
B) fast moving dust particles in the interstellar medium
C) subatomic particles that travel close to the speed of light
D) lasers used as weapons by extraterrestrials

11) The primary way that we observe the atomic hydrogen that makes up most of the interstellar gas in the Milky Way is with
A) ground-based visible-light telescopes.
B) space-based ultraviolet telescopes.
C) X-ray telescopes.
D) radio telescopes observing at a wavelength of 21 centimeters.

12) Which of the following analogies best describes how the structure of the galaxy's spiral arms is maintained?
A) Like military jets flying in formation above a football stadium, the stars in the spiral arms keep a spiral-shaped formation as they orbit the galaxy.
B) Like cars slowing in traffic to look at an accident, stars slow as they pass through the spiral arms.
C) Like a coiling rope, the spiral arms wind up tighter with every galactic rotation.
D) Like the fins of a giant pinwheel toy, the spiral arms carry a set of bright stars around as they sweep through the galaxy.

13) What do we mean by a protogalactic cloud?
A) a cloud of hydrogen and helium that contracts to become a galaxy
B) a term once used historically to refer to any galaxy
C) the cloud-like halo that surrounds the disks of spiral galaxies
D) a cloud of gas that was once a galaxy

14) Most stars in the Milky Way's halo are
A) very old.
B) found inside molecular clouds.
C) very young.
D) blue or white in color.

15) What is an ionization nebula?
A) a region of very hot, low-density gas surrounding a recent supernova
B) a clump of gas that will soon give birth to a new star
C) a colorful cloud of gas that glows because it is heated by light from nearby hot stars
D) a name sometimes used to describe spiral galaxies besides the Milky Way

16) What do halo stars do differently from disk stars?
A) They remain stationary, quite unlike disk stars that orbit the galactic center.
B) They orbit the galactic center with many different inclinations, while disk stars all orbit in nearly the same plane.
C) Halo stars explode as supernovae much more frequently than disk stars.
D) They orbit the center of the galaxy at much lower speeds than disk star.

17) Where does most star formation occur in the Milky Way Galaxy?
A) everywhere throughout the galactic disk
B) in the central bulge
C) within the halo
D) in the spiral arms

18) Based on observations, which of the following statements about stars in the Milky Way is generally true?
A) The older the star, the bluer its color.
B) The older the star, the faster its orbital speed.
C) The older the star, the lower its abundance of heavy elements.
D) The younger the star, the higher its mass.

19) What kind of object do we think lies in the center of the Milky Way Galaxy?
A) a 3- to 4-million-solar-mass black hole
B) a gigantic X-ray binary system
C) a dense cluster of young, hot stars
D) an enormous collection of dark matter, which explains why we detect no light at all from the galactic center

1) If we could see our own galaxy from 2 million light-years away, it would appear
A) as a flattened disk with a central bulge and spiral arms.
B) as a faintly glowing band of light stretching all the way around the sky.
C) to fill the sky with widely spaced stars.
D) like a single, dim star.

2) How does the interstellar medium affect our view of most of the galaxy?
A) It prevents us from seeing most of the galactic disk with visible and ultraviolet light.
B) It absorbs all wavelengths of light.
C) It produces so much visible light that it blocks our view of anything beyond it.
D) It has no effect on visible-light observations, but prevents us from studying the galactic center with radio waves or X rays.

3) Applying the Newton's version of Kepler's third law (or the orbital velocity law) to the a star orbiting 40,000 light-years from the center of the Milky Way Galaxy allows us to determine
A) the total mass of the entire Milky Way Galaxy.
B) the mass of the black hole thought to reside in the center of the galaxy.
C) the percentage of the galaxy's mass that is made of dark matter.
D) the mass of the Milky Way Galaxy that lies within 40,000 light-years of the galactic center.

4) How would you expect a star that formed recently in the disk of the galaxy to differ from one that formed early in the history of the disk?
A) It should be higher in mass.
B) It should have a higher fraction of elements heavier than hydrogen and helium.
C) It should be much brighter.
D) It should orbit the galactic center at a much higher rate of speed.
E) All of the above would be expected.

5) Suppose a scientist holds a press conference at which he claims that 10% of the matter in the Milky Way is in the form of dust grains. Does his claim seem reasonable? Why or why not?
A) It is reasonable, because we already know that interstellar dust obscures our view through the disk of the galaxy.
B) The 10% figure is too low, because most of the mass of the galaxy is in the form of interstellar dust.
C) The 10% figure is too high because there are not enough heavy elements to make that much dust.
D) It seems reasonable as long as we assume that red giant stars—which produce dust grains in their stellar winds—are more common than we thought.

6) The most common form of gas in the disk of the Milky Way Galaxy is
A) molecular hydrogen.
B) gas in hot bubbles.
C) atomic hydrogen gas.
D) gas in stellar winds.

7) How should we expect the Milky Way's interstellar medium to be different in 50 billion years than it is today?
A) The total amount of gas will be about the same, but it will contain a much higher percentage of elements heavier than hydrogen and helium.
B) The total amount of gas will be much less than it is today.
C) The total amount of gas will be much greater, since many stars will undergo supernovae between now and then.
D) Thanks to the recycling of the star-gas-star cycle, the interstellar medium should look about the same in 50 billion years as it does today.

8) Over time, the star-gas-star cycle leads the gas in the Milky Way to
A) have a greater abundance of heavy elements.
B) have a lower abundance of heavy elements.
C) become denser and have a greater abundance of heavy elements.
D) become denser and hotter.

9) Suppose you want to observe and study the radiation from gas inside an interstellar bubble created by a supernova. Which of the following observatories will be most useful?
A) the Chandra X-ray Observatory
B) the Keck I telescope on the summit of Mauna Kea
C) the SOFIA airborne infrared observatory
D) the Hubble Space Telescope

10) If you could watch a time-lapse movie of the interstellar medium over hundreds of millions of years, what would you see?
A) Gas that changes only in very slow and steady ways, so that the movie would in fact be quite boring.
B) The entire disk of the Milky Way would pulsate in and out as it contracts to form stars and then blows out in supernovae and then contracts to form stars again and so on.
C) The movie would alternate back and forth between being very bright when there is a lot of gas and very dark when there is very little gas.
D) Gas that is often moving at high speed, particularly after one or more supernovae, and constantly changing form between molecular clouds, atomic hydrogen, and hot, ionized bubbles and superbubbles.

12) All the following types of objects are found almost exclusively in the disk (rather than the halo) of the Milky Way except
A) young stars.
B) globular clusters.
C) X-ray binaries.
D) high-mass, red supergiant stars.

13) Red and orange stars are found evenly spread throughout the galactic disk, but blue stars are typically found
A) in the halo.
B) only in or near star-forming clouds.
C) only in the central bulge.
D) evenly spread throughout the galactic disk.

14) Which of the following statements comparing halo stars to our Sun is not true?
A) Most stars in the halo have cooler surface temperatures than the Sun.
B) Most stars in the halo are less luminous than the Sun.
C) Most stars in the halo contain a much lower percentage of heavy elements than the Sun.
D) Most stars in the halo have either died or are in their final stages of life, while the Sun is only in about the middle of its lifetime.

15) Most nearby stars move relative to the Sun at speeds below about 30 km/s. Suppose you observe a nearby star that is moving much faster than this (say, 300 km/s). Which of the following is a likely explanation for its high speed?
A) It is probably a halo star that is currently passing through the disk.
B) It is a very young star, recently formed.
C) It has been pushed to high speed by the shock wave from a nearby supernova.
D) It is a very high mass star.

16) Why do we believe that most of the mass of the Milky Way is in the form of dark matter?
A) Although dark matter emits no visible light, we have detected its radio emissions.
B) The orbital speeds of stars far from the galactic center are surprisingly high.
C) Theoretical models of galaxy formation suggest that a galaxy cannot form unless it has at least 10 times as much matter as we see in the Milky Way disk.
D) Our view of distant galaxies is often obscured by dark blotches, which are presumably made of dark matter.

17) Spiral arms appear bright because
A) they contain more hot young stars than other parts of the disk.
B) they contain far more stars than other parts of the galactic disk.
C) they contain more molecular clouds than other parts of the disk.
D) they are the only places where we find stars within the disk of the galaxy.

18) How did star formation likely proceed in the protogalactic cloud that formed the Milky Way?
A) The stars that formed first eventually settled into a galactic disk, circling the center of the galaxy.
B) The protogalactic cloud gradually formed stars, starting from the center of the galaxy working outwards.
C) The stars that formed first could orbit the center of the galaxy in any direction at any inclination.
D) The protogalactic cloud gradually formed stars, starting from the outer edges of the spiral arms and working inward.

19) If we could watch spiral arms from a telescope situated above the Milky Way over 500 million years, what would we see happen?
A) The spiral arms will seem to "wind up," to wrap more and more tightly around the center of the Galaxy.
B) The spiral arms will eventually dissipate and fade away, since they are a temporary phenomenon that should only last for a million years or so.
C) Stars will move through the spiral arms, bunching up closer as they pass through. Young hot stars will form and die within the arms before having a chance to move out.
D) The spiral arms will eventually unwind, as centripetal forces send the stars flying outwards into intergalactic space.

20) What is the best evidence for an extremely massive black hole in the center of the Milky Way?
A) Huge amounts of X-rays are pouring out of the center of the galaxy.
B) The center of our galaxy hosts a pulsar that is spinning so fast that it could only be a black hole.
C) We observe stars vanishing in the center of the Galaxy as they are sucked into the black hole.
D) The orbits of stars in the center of the galaxy indicate that the presence of 3- to 4-million-solar-mass object in a region no larger than our Solar System.

21) Which of the following statements is not true of the object known as Sgr A* in the center of our Galaxy?
A) It is by far the brightest source of visible light lying in the direction of the galactic center.
B) It is thought to harbor a black hole of more than 3 million solar masses.
C) It is a source of X-ray emission that we have observed with telescopes in space.
D) It is a source of bright radio emission.

2) Suppose that we look at a photograph of many galaxies. Assuming that all galaxies formed at about the same time, which galaxy in the picture is the youngest?
A) the one that is reddest in color
B) the one that is bluest in color
C) the one that is farthest away
D) the one that is closest to us
E) the one that appears smallest in size

3) Which of the following types of galaxies are most spherical in shape?
A) spirals
B) ellipticals
C) lenticulars
D) irregulars

4) Which of the following types of galaxies are reddest in color?
A) spirals
B) ellipticals
C) lenticulars
D) irregulars

5) Which of the following statements about galaxies is true?
A) Small galaxies outnumber large galaxies and produce most of the light in the universe.
B) Small galaxies outnumber large galaxies but large galaxies produce most of the light in the universe.
C) There is an approximately equal number of small and large galaxies in the universe and together they each contribute an equal amount of light.
D) Most galaxies in the universe are about the same size as the Milky Way.
E) Galaxies come in a wide variety of shapes and sizes but are all very blue in color.

6) Which types of galaxies have a clearly defined spheroidal component?
A) spirals only
B) ellipticals only
C) lenticulars only
D) irregulars only
E) all but irregulars

7) Which types of galaxies have a clearly defined disk component?
A) spirals only
B) ellipticals only
C) lenticulars only
D) irregulars only
E) spirals and lenticulars

8) Compared to spiral galaxies, elliptical galaxies are
A) redder and rounder.
B) redder and flattened.
C) bluer and rounder.
D) bluer and flattened.
E) always much smaller.

9) The disk component of a spiral galaxy includes which of the following parts?
A) halo
B) bulge
C) spiral arms
D) globular clusters
E) all of the above

10) How does a lenticular galaxy differ from a normal spiral galaxy?
A) It has no bulge.
B) It has an elongated bulge resembling a bar more than a sphere.
C) It is flatter in shape.
D) It has no gas or dust.
E) It has no spiral arms.

11) What is the major difference between an elliptical galaxy and a spiral galaxy?
A) A spiral galaxy contains mostly younger stars.
B) A spiral galaxy has a spherical halo.
C) An elliptical galaxy lacks a disk component.
D) Elliptical galaxies are not as big as spiral galaxies.
E) There are no dwarf spiral galaxies, but there are dwarf ellipticals.

12) Most large galaxies in the universe are
A) elliptical.
B) spiral or lenticular.
C) irregular.
D) abnormal.

13) Which of the following types of galaxies are most commonly found in large clusters?
A) spirals
B) ellipticals
C) lenticulars
D) irregulars

14) Approximately how many stars does a dwarf elliptical galaxy have?
A) 1 trillion
B) 100 billion
C) 10 billion
D) less than a billion
E) less than a million

15) Which of the following is true about irregular galaxies?
A) They are composed solely of old stars.
B) They generally have significant bulge populations.
C) They were more common when the universe was younger.
D) They have reddish colors.
E) They have well defined spiral arms.

16) Why are Cepheid variables important?
A) Cepheid variables are stars that vary in brightness because they harbor a black hole.
B) Cepheids are pulsating variable stars, and their pulsation periods are directly related to their true luminosities. Hence, we can use Cepheids as "standard candles" for distance measurements.
C) Cepheids are a type of young galaxy that helps us understand how galaxies form.
D) Cepheids are supermassive stars that are on the verge of becoming supernovae and therefore allow us to choose candidates to watch if we hope to observe a supernova in the near future.

17) What is a standard candle?
A) an object for which we are likely to know the true luminosity
B) an object for which we can easily measure the apparent brightness
C) a class of objects in astronomy that all have exactly the same luminosity
D) any star for which we know the exact apparent brightness
E) a long, tapered candle that lights easily

18) Why is the Hyades Cluster important for building up a catalog of the true luminosities of main-sequence stars?
A) It is an extremely bright cluster.
B) It is close enough to us that the distance to the cluster stars can be found by stellar parallax.
C) It is an old globular cluster that has been around our galaxy for several billion years.
D) We have brightness measurements for the stars in the cluster over many decades, so we know how the stars vary in brightness.
E) It contains many Cepheid variables.

20) How was Edwin Hubble able to use his discovery of a Cepheid in Andromeda to prove that the "spiral nebulae" were actually entire galaxies?
A) There are no Cepheids in the Milky Way, so his discovery proved that it had to be in another galaxy.
B) He measured the stellar parallax of the Cepheid in Andromeda, was able to determine the distance to it, and showed that it was far outside the Milky Way Galaxy.
C) He used main-sequence fitting to determine the distance to Andromeda and show that it was far outside the Milky Way Galaxy.
D) From the period-luminosity relation for Cepheids, he was able to determine the distance to Andromeda and show that it was far outside the Milky Way Galaxy.
E) Since a Cepheid is a type of luminous galaxy, when he found it in Andromeda he was able to prove that Andromeda was a separate galaxy from the Milky Way.

21) What two quantities did Edwin Hubble plot against each other to discover the expansion of the Universe?
A) velocity and distance
B) luminosity and distance
C) velocity and temperature
D) luminosity and temperature
E) age and distance

22) What is Hubble's law?
A) The longer the time period between peaks in brightness, the greater the luminosity of the Cepheid variable star.
B) The recession velocity of a galaxy is directly proportional to its distance from us.
C) The recession velocity of a galaxy is inversely proportional to its distance from us.
D) The faster a spiral galaxy's rotation speed, the more luminous it is.
E) The faster a spiral galaxy's rotation speed, the less luminous it is.

24) What is the primary practical difficulty that limits the use of Hubble's law for measuring distances?
A) Redshifts of galaxies are difficult to measure.
B) The recession velocities of distant galaxies are so great that they are hard to measure.
C) We do not know Hubble's constant very accurately yet.
D) Hubble's law is only useful theoretically; it is difficult to use in practice.
E) The motion of Earth relative to the Milky Way is difficult to account for.

25) White-dwarf supernovae are good standard candles for distance measurements for all the following reasons except which?
A) All white-dwarf supernovae involve the explosion of stars of nearly the same mass.
B) White-dwarf supernovae are so bright that they can be detected even in very distant galaxies.
C) White-dwarf supernovae are common enough that we detect several every year.
D) White-dwarf supernovae occur only among young and extremely bright stars.
E) All white-dwarf supernovae have similar light curves, which makes them easy to distinguish from massive-star supernovae.

26) What makes white-dwarf supernovae good standard candles?
A) They are very bright, so they can be used to determine the distances to galaxies billions of light-years away.
B) They should all have approximately the same luminosity.
C) They occur so frequently that we can use them to measure the distances to virtually all galaxies.
D) We have had several occur close to us in the Milky Way, so we have been able to determine their luminosities very accurately.
E) both A and B

27) What is the most accurate way to determine the distance to a nearby star?
A) radar ranging
B) stellar parallax
C) main-sequence fitting
D) using Cepheid variables
E) Hubble's law

28) What is the most accurate way to determine the distance to a nearby galaxy?
A) radar ranging
B) stellar parallax
C) using Cepheid variables
D) main sequence fitting
E) Hubble's law

29) What is the most accurate way to determine the distance to a very distant irregular galaxy?
A) main-sequence fitting
B) using Cepheid variables
C) using a white-dwarf supernova as a standard candle
D) main sequence fitting
E) Hubble's law

30) Which of the following sequences lists the methods for determining distance in the correct order from nearest to farthest?
A) main-sequence fitting, parallax, Cepheid variables, Hubble's law
B) parallax, main-sequence fitting, Cepheid variables, Hubble's law
C) parallax, main-sequence fitting, Hubble's law, Cepheid variables
D) parallax, main-sequence fitting, white-dwarf supernovae, Hubble's law
E) main-sequence fitting, parallax, Hubble's law, white-dwarf supernovae

31) Dr. X believes that the Hubble constant is H0 = 55 km/s/Mpc. Dr. Y believes it is H0 = 80 km/s/Mpc. Which statement below automatically follows?
A) Dr. X believes that the universe is expanding, but Dr. Y does not.
B) Dr. X believes that the Andromeda Galaxy (a member of our Local Group) is moving away from us at a slower speed than Dr. Y believes.
C) Dr. X believes that the universe is older than Dr. Y believes.
D) Dr. X believes that the universe will someday stop expanding, while Dr. Y believes it will expand forever.
E) Dr. X believes that the universe has a much higher density than Dr. Y believes.

32) Dr. Smith believes that the Hubble constant is H0 = 70 km/s/Mpc. Dr. Jones believes it is H0 = 50 km/s/Mpc. Which statement below automatically follows?
A) Dr. Smith believes that the universe is expanding, but Dr. Jones does not.
B) Dr. Smith believes that the Andromeda Galaxy (a member of our Local Group) is moving away from us at a faster speed than Dr. Jones believes.
C) Dr. Smith believes that the universe is older than Dr. Jones believes.
D) Dr. Smith believes that the universe is younger than Dr. Jones believes.
E) Dr. Smith believes that the universe will someday stop expanding, while Dr. Jones believes it will expand forever.

33) Recall that Hubble's law is written v = H0d, where v is the recession velocity of a galaxy located a distance d away from us, and H0 is Hubble's constant. Suppose H0 = 65 km/s/Mpc. How fast would a galaxy located 500 megaparsecs distant be receding from us?
A) 65 km/s
B) 65 Mpc/s
C) 32,500 km/s
D) 9 km/s
E) 0.65 times the speed of light

34) Hubble's "constant" is constant in
A) time.
B) space.
C) space and time.
D) our Galaxy but different in others.

35) Based on current estimates of the value of Hubble's constant, how old is the universe?
A) between 4 and 6 billion years old
B) between 8 and 12 billion years old
C) between 12 and 16 billion years old
D) between 16 and 20 billion years old
E) between 20 and 40 billion years old

37) What does the equivalent of an H-R diagram for galaxies, plotting luminosity versus color, show?
A) galaxies fill the diagram showing that there is no correlation between luminosity and color
B) two clumps, one blue with relatively low luminosity, one red with relatively high luminosity, and a valley in between with few galaxies
C) a continuum from faint, blue galaxies to bright, red galaxies
D) a continuum from faint, red galaxies to bright, blue galaxies
E) A main sequence, just as for stars

1) Although it is difficult to tell from our vantage point inside the galaxy, astronomers suspect that the Milky Way is a barred spiral.

2) Spiral galaxies have more gas, dust, and younger stars than elliptical galaxies do.

3) Stars are continually forming in the halo of our Galaxy today.

4) A lenticular galaxy is another name for an elongated elliptical galaxy.

5) There are more large spiral galaxies than there are large elliptical galaxies.

6) Elliptical galaxies are more likely to be found in clusters than are spiral galaxies.

7) Massive-star supernovae and white-dwarf supernovae work equally well as standard candles for measuring cosmic distances.

8) The larger the value of Hubble's constant, the more rapid the expansion of the universe and hence the younger the universe.

1) Based on the number of galaxies visible in the Hubble Deep Field (Figure 20.1 in your textbook), the estimated number of galaxies in our observable universe is
A) about 50,000.
B) about 100 million.
C) about 100 billion.
D) an infinite number.

2) Which of the following is not one of the three major categories of galaxies?
A) elliptical galaxies
B) globular galaxies
C) spiral galaxies
D) irregular galaxies

3) Galaxies with disks but no evident spiral arms are called
A) irregular galaxies.
B) lenticular galaxies.
C) barred spiral galaxies.
D) spheroidal components.

4) Which of the following best describes the status of the Milky Way in our Local Group of galaxies?
A) It is one of the two largest galaxies in the group.
B) It is one of about a dozen large spiral galaxies in the group.
C) It is by far the largest galaxy in the group.
D) It is quite average among the galaxies in the group.

5) A standard candle is
A) a 7-cm-long wax candle.
B) another name for a main-sequence star.
C) another name for a barred-spiral galaxy.
D) a light source of known luminosity.

6) What is main-sequence fitting?
A) It is a method for determining the distance to a star cluster by assuming that its main sequence should line up with the main sequence on a standard H-R diagram.
B) It is a method for determining the age of a star cluster.
C) It is a way of forcing stars to fit into a standard main sequence, even when they have some unusual characteristics.
D) It is the way we construct an H-R diagram by plotting the surface temperatures and luminosities of stars.

7) What is a Cepheid variable?
A) It is a bright source of variable X-ray emission, thought to harbor a supermassive black hole.
B) It is a type of very luminous star that makes an excellent standard candle.
C) It is a main-sequence star of spectral type B5.
D) It is a type of galaxy that varies in its light output.

8) What two observable properties of a Cepheid variable are directly related to one another?
A) the period between its peaks of brightness and its distance
B) its luminosity and its mass
C) the period between its peaks of brightness and its luminosity
D) its mass and its distance

9) What does Hubble's law tell us?
A) The faster a spiral galaxy's rotation speed, the more luminous it is.
B) The longer the period of a Cepheid variable, the greater its luminosity.
C) For every force, there is an equal and opposite reaction force.
D) The more distant a galaxy, the faster it is moving away from us.

10) Given that white dwarf supernovae are such good standard candles, why don't we use them to measure the distance to all galaxies?
A) They are rare events, so we have observed them in only a tiny fraction of all galaxies.
B) We cannot see them beyond a distance of about 100 million light-years.
C) They can occur only in spiral galaxies, not elliptical galaxies.
D) We would, but we don't have enough telescopes.

12) When we use an analogy that represents the expanding universe with the surface of an expanding balloon, what does the inside of the balloon represent?
A) It represents the center of the universe.
B) It represents the entire universe.
C) It represents regions of the universe beyond the Milky Way Galaxy.
D) The inside of the balloon does not represent any part of our universe.

13) If we say that a galaxy has a lookback time of 1 billion years, we mean that
A) its light traveled through space for 1 billion years to reach us.
B) is now 1 billion light-years away.
C) it was 1 billion light-years away when the light left the galaxy.
D) it is 400 million years old.

14) Cosmological redshift is the result of
A) the high speeds at which galaxies move within clusters.
B) the expansion of the universe.
C) very old, red stars in distant galaxies.
D) supermassive black holes.

17) You observe the peak brightnesses of two white dwarf supernovae. Supernova A is only 1/4 as bright as Supernova B. What can you say about their relative distances?
A) Supernova A is twice as far away as Supernova B.
B) Supernova A is 4 times as far away as Supernova B.
C) Supernova B is 4 times as far away as Supernova A.
D) Supernova B is twice as far away as Supernova A.
E) We can't say anything about their relative distances because we do not have enough information.

18) The fact that the universe is expanding means that space itself is growing within
A) the Milky Way.
B) clusters of galaxies.
C) the observable universe.
D) the Local Group.

19) Spectral lines from Galaxy B are redshifted from their rest wavelengths twice as much as the spectral lines from Galaxy A/B. According to Hubble's law, what can you say about their approximate relative distances?
A) Galaxy A is twice as far as Galaxy B.
B) Galaxy B is four times as far as Galaxy A.
C) Galaxy A is four times as far as Galaxy B.
D) Galaxy B is twice as far as Galaxy A.
E) Not enough information to say—you need to know Hubble's constant to answer this question.

1) In a photo like the Hubble Deep Field (Figure 20.1 in your textbook), we see galaxies in many different stages of their lives. In general, which galaxies are seen in the earliest (youngest) stages of their lives?
A) the galaxies that are farthest away
B) the galaxies that have the most hot, young O and B stars
C) the galaxies that are the reddest in color
D) the galaxies that are nearest to us

2) Which of the following statements about types of galaxies is not true?
A) Spiral galaxies have younger stars than elliptical galaxies.
B) Among the large galaxies in the universe outside of clusters, most are spiral.
C) Large elliptical galaxies are more common in clusters of galaxies than they are outside of clusters.
D) Elliptical galaxies are bluer and contain more dust than spiral galaxies.

3) The most basic difference between elliptical galaxies and spiral galaxies is that
A) elliptical galaxies lack anything resembling the halo of a spiral galaxy.
B) elliptical galaxies have a spheroidal component (of stars distributed spherically about the galactic center), and spiral galaxies do not.
C) elliptical galaxies lack anything resembling the disk of a spiral galaxy.
D) elliptical galaxies are very old and spiral galaxies are very young.

4) Hubble's galaxy classification diagram (the "tuning fork")
A) explains active galactic nuclei.
B) shows how galaxies evolve from one form to another.
C) suggests the existence of black holes.
D) relates galaxies according to their shapes, but not according to any evolutionary status.

5) Using the technique of main-sequence fitting to determine the distance to a star cluster requires that
A) we have telescopes powerful enough to allow us to identify the spectral types of main-sequence stars of many masses in the cluster.
B) the cluster be near enough for us to measure the parallax of its stars.
C) we use ultraviolet and X-ray telescopes.
D) we have a well-calibrated period-luminosity relation for Cepheid variable stars.

6) Suppose that we suddenly discovered that all these years we'd been wrong about the distance from Earth to the Sun, and it is actually 10% greater than we'd thought. How would that affect our estimate of the distance to the Andromeda Galaxy?
A) It would not have any effect on our estimate of the distance to the Andromeda Galaxy.
B) It would mean the distance to the Andromeda Galaxy is also 10% greater than we thought.
C) It would mean the distance to the Andromeda Galaxy is 10% less than we thought.
D) It would mean that all the objects we've assumed are standard candles really are not good standard candles, and therefore that we have no idea of the true distance to the Andromeda Galaxy.

7) Suppose we observe a Cepheid variable in a distant galaxy. The Cepheid brightens and dims with a regular period of about 10 days. What can we learn from this observation?
A) It will allow us to calculate the rotation rate of the galaxy.
B) It will allow us to determine the mass of the galaxy.
C) We can learn the distance to the galaxy.
D) Under the rules of the International Astronomical Union, we will be entitled to naming rights for the galaxy.

9) Suppose that Hubble's constant were 20 kilometers per second per million light-years. How fast would we expect a galaxy 100 million light-years away to be moving? (Assume the motion is due only to Hubble's law.)
A) away from us at 200 km/s
B) toward us at 2,000 km/s
C) away from us at 2,000 km/s
D) away from us at 20,000 km/s

10) Does Hubble's law work well for galaxies in the Local Group? Why or why not?
A) No, because Hubble did not know the Local Group existed when he discovered his law.
B) No, because galaxies in the Local Group are gravitationally bound together.
C) No, because we do not know the precise value of Hubble's constant.
D) Yes, it works so well that we have never detected any measurable deviations from its predictions.

11) Why are white dwarf supernovae more useful than massive star supernovae for measuring cosmic distances?
A) We can see only white dwarf supernovae in distant galaxies, not massive star supernovae.
B) White dwarf supernovae are much more common than massive star supernovae.
C) White dwarf supernovae follow a period-luminosity relation, while massive supernovae do not.
D) White dwarf supernovae all have roughly the same true peak luminosity, while massive supernovae come in a wide range of peak luminosities.

12) Suppose an elliptical galaxy is so far away that we cannot see even its brightest stars individually. Which of the following techniques might allow us to measure its distance?
A) We could use main-sequence fitting.
B) We could use Cepheid variables as standard candles.
C) We could use a white dwarf supernova as a standard candle.
D) We could use radar ranging.

13) What is the best way to determine a galaxy's redshift?
A) Find the galaxy's apparent distance, and look up the redshift based on Hubble's law.
B) Find the color of the galaxy, and estimate its distance based on how red the galaxy is.
C) Take a spectrum of the galaxy, and measure the difference in wavelength of spectral lines from the wavelengths of those same lines as measured in the laboratory.
D) Measure the magnitude of the galaxy, estimate its distance, and calculate its redshift using Hubble's law.

14) Which statement below correctly describes the relationship between expansion rate and age for the universe?
A) The faster the rate of expansion, the younger the age of the universe.
B) The faster the rate of expansion, the older the age of the universe.
C) Age is independent of the expansion rate.

15) What does cosmological redshift do to light?
A) makes it brighter
B) stretches its wavelength
C) makes it slow down
D) makes all light infrared

16) The lookback time of the cosmological horizon is
A) the Big Bang.
B) the age of the universe.
C) 1 billion years.
D) 10 billion years.

17) Why can't we see past the cosmological horizon?
A) Beyond the cosmological horizon, we would be looking back to a time before the universe was born.
B) We do not have big enough telescopes.
C) The cosmological horizon is infinitely far away, and we can't see to infinity.
D) Every galaxy in the entire universe (not just the observable universe) exists within the cosmological horizon, so there's nothing to see beyond it.

18) Hubble's constant is about 22 km/s/million light-years, implying an age of about 14 billion years for the universe. If Hubble's constant were 11 km/s/million light-years, the age of the universe would be about
A) 7 billion years.
B) 14 billion years.
C) 28 billion years.
D) Impossible to say, because Hubble's constant has no relationship to the age of the universe.

19) Given that the universe is about 14 billion years old, which of the following statements is logically valid?
A) All galaxies nearby us are about 14 billion years old.
B) All galaxies that we can see are about 14 billion years old.
C) All galaxies that we see have an age that is approximately equal to the age of the universe today, minus the lookback time (or light travel time) corresponding to the redshift of that galaxy.
D) The oldest galaxies we see at great distances are younger than the oldest galaxies we see nearby.

1) To date, physicists have investigated the behavior of matter and energy at temperatures as high as those that existed in the universe as far back as ________ after the Big Bang.
A) 1 million years
B) 300,000 years
C) 300 years
D) 3 minutes
E) 10-10 second

2) How long after the Big Bang was the Planck time, before which our current theories are completely unable to describe conditions in the universe?
A) 10-10 second
B) 10-35 second
C) 10-43 second
D) 3 minutes
E) 300,000 years

3) The Planck era refers to the time period
A) before the Big Bang.
B) before the Planck time.
C) after the Planck time.
D) after inflation.
E) after the GUT era.

5) A GUT (grand unified theory) refers to theories that
A) unify all four forces.
B) unify gravity and the electromagnetic and weak forces.
C) unify gravity and the strong and weak forces.
D) unify the strong force and the electromagnetic and weak forces.
E) unify the electromagnetic and weak forces.

6) When we say that the electromagnetic and weak forces "freeze out" from the electroweak force at 10-10 seconds after the Big Bang, we mean that
A) these forces are important only at temperatures below the freezing point of water–a temperature that the universe reached at an age of about 10-10 second.
B) "freezing out" was a term coined by particle physicists who think that the Big Bang theory is really cool.
C) prior to this time the electromagnetic and weak forces maintained a single identity, but they possessed separate identities following this time.
D) following this time neither the electromagnetic nor the weak force was ever important in the universe again.
E) quantum fluctuations by high-speed, relativistic particles in a state of false vacuum cause disturbances in the spacetime continuum, leading to the process described in the question this answer refers to.

7) How many forces operated in the universe during the GUT era?
A) one, what we call the "super force"
B) two, gravity and the GUT force
C) two, gravity and the electroweak force
D) three, gravity, the strong force, and the electroweak force
E) all of the above forces

8) Which forces have physicists shown to be the same force under conditions of very high temperature or energy, as confirmed by experiments in particle accelerators?
A) gravity and the weak force
B) gravity and the strong force
C) the strong and weak forces
D) the strong and electromagnetic forces
E) the electromagnetic and weak forces

10) (From a science quiz that appeared in the weekly magazine The Economist.) Economic history is easier to write than the history of the universe. Nevertheless, most cosmologists now think that when the universe was formed,
A) first there was a Big Bang, then inflation (of space) caused recession (of all matter, away from the Big Bang).
B) first there was inflation, which caused the Big Bang, then recession.
C) first there was a Big Bang. There has not been any inflation yet, but if it comes it will cause recession.

11) Why might inflation have occurred at the end of the GUT era?
A) Gravity was an extremely weak force at this period in time.
B) Large amounts of matter and antimatter annihilated at this time.
C) There wasn't enough matter present to slow down the expansion at that time.
D) The universe was too small and needed to grow quickly.
E) An enormous amount of energy was released when the strong force froze out from the GUT force.

12) What direct evidence do we have that the weak and electromagnetic forces were once unified as a single electroweak force?
A) The most advanced telescopes are able to see back to this era in the universe.
B) Detectors on Earth have received photons and high-energy particles from this era.
C) Temperatures in the center of the Sun can reproduce the conditions during this era.
D) Particle accelerators on Earth can reach energies equivalent to the high temperatures of this era and have produced particles predicted by the electroweak theory.
E) We have no direct evidence of the electroweak force.

13) What happened to the quarks that existed freely during the particle era?
A) They combined in groups to make protons, neutrons, and their antiparticles.
B) They froze out of the soup of particles at the end of the era.
C) They evaporated.
D) They combined in groups to make electrons and neutrinos.
E) They combined in groups to make W and Z bosons.

14) Approximately how long did the era of nucleosynthesis last?
A) 10-10 second
B) 0.001 second
C) 5 seconds
D) 5 minutes
E) 5 years

15) What kinds of atomic nuclei formed during the era of nucleosynthesis?
A) only hydrogen
B) only helium
C) hydrogen and helium and trace amounts of deuterium and lithium
D) roughly equal amounts of each of the following: hydrogen, helium, deuterium and lithium
E) nuclei of all the chemical elements

16) Why is the era of nucleosynthesis so important in determining the chemical composition of the universe?
A) All the elements except hydrogen were produced after the era of nucleosynthesis.
B) We can observe spectra from this era to determine what the primordial mix of the elements was at the beginning of the universe.
C) Except for the small amount of matter produced later by stars, the chemical composition of the universe is the same now as at the end of the era of nucleosynthesis.
D) We can study the processes that occurred during the era of nucleosynthesis to determine how most of the elements in the universe were created.
E) By knowing how much matter was created during the era of nucleosynthesis, we can determine whether the universe is open or closed.

17) Why did the era of nuclei end when the universe was about 300,000 years old?
A) All the free particles had combined to form the nuclei of atoms.
B) The universe had expanded and cooled to a temperature of about 3,000 K, cool enough for stable, neutral atoms to form.
C) Neutrinos and electrons were finally able to escape the plasma of the early universe and no longer heated the other particles.
D) Photons were finally able to escape the plasma of the early universe and no longer heated the hydrogen and helium ions.
E) No theory can explain this.

18) Evidence that the cosmic background radiation really is the remnant of a Big Bang comes from predicting characteristics of remnant radiation from the Big Bang and comparing these predictions with observations. Four of the five statements below are real. Which one is fictitious?
A) The cosmic background radiation is expected to have a temperature just a few degrees above absolute zero, and its actual temperature turns out to be about 3 K (actually 2.7 K).
B) The cosmic background radiation is expected to have a perfect thermal spectrum, and observations from the COBE spacecraft verify this prediction.
C) The cosmic background radiation is expected to contain spectral lines of hydrogen and helium, and it does.
D) The cosmic background radiation is expected to look essentially the same in all directions, and it does.
E) The cosmic background radiation is expected to have tiny temperature fluctuations at the level of about 1 part in 100,000. Such fluctuations were found in the COBE data.

19) Which of the following statements about the cosmic background radiation is not true?
A) It has a temperature of about 3 degrees K above absolute zero.
B) It is the result of a mixture of radiation from many independent sources, such as stars and galaxies.
C) It had a much higher temperature in the past.
D) It was discovered by Penzias and Wilson in the early 1960s.
E) It appears essentially the same in all directions (it is isotropic).

20) Where do the photons in the cosmic background radiation originate?
A) the moment of the Big Bang
B) the end of the Planck era
C) during the era of nucleosynthesis
D) the end of the era of nuclei
E) during the era of galaxy formation

21) Why does the Big Bang theory predict that the cosmic background radiation should have a perfect thermal radiation spectrum?
A) The background radiation came from the heat of the universe, with a peak corresponding to the temperature of the universe.
B) The spectrum of pure hydrogen is a perfect thermal radiation spectrum.
C) The spectrum of 75 percent hydrogen and 25 percent helium is a perfect thermal radiation spectrum.
D) The light from all the stars and gas in the sky averaged over the entire universe is a perfect thermal radiation spectrum.
E) It doesn't predict that the cosmic background radiation should have a perfect thermal radiation spectrum.

22) Why do we expect the cosmic background radiation to be almost, but not quite, the same in all directions?
A) The overall structure of the universe is very uniform, but the universe must have contained some regions of higher density in order for galaxies to form.
B) The temperature of the universe can be found by taking an average over the entire sky, but individual stars will create peaks in the spectrum over small angles.
C) Dark matter consisting of WIMPs greatly smooths out the spectrum, but the small patches of "light" matter create peaks in the spectrum.
D) The overall structure of the universe is very uniform, but the synthesis of different elements produces varying signatures within the background spectrum.
E) The overall structure of the universe is very uniform, but intervening gas between us and the era of nuclei absorbs wavelengths depending on the composition and redshift of the gas.

23) Helium originates from
A) stellar nucleosynthesis only.
B) the Big Bang only.
C) stellar nucleosynthesis with a small contribution from the Big Bang.
D) the Big Bang with a small contribution from stellar nucleosynthesis.
E) radioactive decay of heavier elements only.

24) What are the two key observational facts that led to widespread acceptance of the Big Bang model?
A) the cosmic background radiation and the high helium content of the universe
B) the cosmic background radiation and the expansion of the universe
C) the cosmic background radiation and the near-critical density of the universe
D) the predominance of matter over antimatter and the near-critical density of the universe
E) the predominance of matter over antimatter and the large scale structure of galaxies

25) Why do we think tiny quantum ripples should have been present in the very early universe?
A) The shock wave of the Big Bang caused ripples that expanded outward with time.
B) The energy released when the strong force froze out of the GUT force caused shock waves that produced ripples in the universe.
C) Matter and antimatter particles that spontaneously formed from high-energy photons caused perturbations in the radiation field.
D) The annihilation of matter and antimatter particles caused tiny explosions that perturbed the radiation field.
E) Quantum mechanics requires that the energy fields at any point in space be continually fluctuating as a result of the uncertainty principle.

26) What is postulated to have caused a sudden inflation of the early universe?
A) the annihilation of matter and antimatter
B) the separation of the electromagnetic and weak forces
C) the "freezing out" of the strong force from the GUT force
D) the energy released in the fusion of protons and neutrons to produce helium
E) giant quantum fluctuations

Lost in Spacetime. Just when you thought it was safe to take final exams . . . a vindictive multi-dimensional being reaches down (up? over? through?) to Earth and pulls you out of the universe. You are thrown back into the universe at a place of this being's choosing, and she permits you to leave only after you have identified your surroundings. You are subject to several tests.
Through a scientifically unexplainable miracle, you are able to survive in every one of the places you are tested. (Lest you become too comfortable, however, you certainly are able to feel any associated pain due to high temperature, pressure, gravity, etc.) In each case described below, identify your surroundings. In some cases, the surroundings described may exist only during eras of the universe (past or future) other than our own time; in those cases, you should identify both the place and the time where you are located.

28) You find yourself in a place that looks (except for your own presence) perfectly symmetrical. There is no way to distinguish one place from another, and all forces are one. With this perfect symmetry, there is no obvious way to define the flow of time. Where are you?
A) You are in the center of a young star.
B) You are in the early universe before the Planck time.
C) You are floating somewhere in the universe near its end, 10100 years from now.
D) You are inside the nucleus of an atom.
E) You are in the universe shortly after inflation.

29) You are in a place that is extremely hot and dense, making you feel quite sweaty and claustrophobic. You can't see far because your surroundings are opaque to light. Around you, nuclear fusion is converting carbon into oxygen and other elements. Where are you?
A) You are in the center of a star very much like our Sun.
B) You are in the early universe during the era of nucleosynthesis.
C) You are inside a nuclear power plant on Earth.
D) You are in the center of a massive star near the end of its life.
E) You are in the center of a star much smaller than the Sun.

30) You are on the surface of an object, and you have a fairly clear view out into space. It might be very nice, except for one major drawback: You are very squashed. Also, light you observe from distant objects is apparently slightly blueshifted (compared to what it normally looks like). The surface of the object is composed primarily of carbon and oxygen, and the horizon distance is about the same as that on Earth. By observing the stellar background for a few weeks, you realize that there are several planets orbiting your object. Where are you?
A) You are on the surface of Earth.
B) You are on the surface of a planet that is somewhat more massive than Earth.
C) You are on the surface of a white dwarf.
D) You are "on" an accretion disk around a black hole.
E) You are on the surface of a neutron star.

31) It sure is bright everywhere; you've been able to travel around a bit, and it's clear that you are not in a star. Yet it is as bright as looking directly at the Sun. In your extensive travels through your current surroundings, you cannot find a single neutral atom anywhere, nor can you find a nucleus besides hydrogen or helium. And, while it is hot (a few thousand degrees Kelvin), it is nowhere near the temperature needed for nuclear fusion. Where are you?
A) You are in the universe during its first 300,000 years.
B) You are in the universe more than 10100 years in the future.
C) You are in an accretion disk around a supermassive black hole.
D) You are in the central regions of a quasar.
E) You are where the Sun should be located, but about 5 billion years from now.

32) You are feeling like spaghetti. Although normally only about 2 meters tall, you are now about 25 meters long. (How fortunate, if painful, that the being has arranged for your body to become elastic enough so that it is not ripped apart under these conditions.) As you look up over your head, you see things moving pretty quickly in the universe–but that lasts only for a brief instant, and then all contact with the universe is lost. Where are you?
A) You are plunging into the Sun.
B) You are crossing the event horizon of a black hole.
C) You are being consumed by a "crack" in the universe caused by inflation.
D) You are near the center of a star that has just developed an iron core, leading to a supernova.
E) You are in a medieval torture chamber somewhere in western Europe.

33) You are once again in a hot, dense place. You are surrounded by protons and neutrons, some rapidly fusing into helium. You notice that your surroundings are cooling (good, because it's really hot!) and rapidly dropping in density. Within about 3 minutes, the fusion reactions stop. Where are you?
A) You are in the center of a star very much like our Sun.
B) You are in the early universe during the era of nucleosynthesis.
C) You are inside a nuclear power plant on Earth.
D) You are in the center of a star much smaller than the Sun.
E) You are in the center of a massive star near the end of its life.

34) At last you are in a place where the heat and high density are no longer bothering you. However, although the density is very low, the gas around you is extremely high in temperature. In fact, the temperature is so high that it is emitting lots of X rays, which are creating cancer-causing mutations in your body at a rapid rate. Well, at least the view is great! There are no stars anywhere within about 10,000 light-years of you, but at slightly greater distances your sky is brightened by many beautiful, star-filled structures, some with majestic spiral shapes. Where are you?
A) You are in the universe when it was about 200 million years old, just before galaxies began forming.
B) You are in the center of the Milky Way Galaxy, looking outward into the Local Group.
C) You are somewhere between the Andromeda and Milky Way galaxies in the Local Group.
D) You are in intergalactic space within a rich cluster of thousands of galaxies.
E) You are in the outskirts of a galaxy whose nucleus is a powerful quasar.

35) At last, someplace fairly comfortable. Very weak gravity is holding you to the surface of the small object on which you sit. Your object is apparently moving away from a star, perhaps one that it orbits with a period of thousands of years. Around you, geysers are spouting gas into space. Looking back along the object's orbit, you see particles of dust that the geysers apparently blew off the object when it was nearer to the star that it is now leaving behind. You conclude that the geysers were recently much more active but are now settling down into a quiescent state that may last for millennia. You also soon realize that you are closer to home than you have been in all your previous journeys. Perhaps if you can somehow find a small rocket, a heat shield, and a good parachute, you can escape and head home for your final exam. Where are you?
A) You are on an asteroid near the center of a galaxy, heading in toward a massive black hole.
B) You have been shrunk in size and are riding a grain of interstellar dust that is carrying you on an orbit about our very own Sun.
C) You are riding a jet of gas from a quasar that is headed in the direction of an ordinary star.
D) You are on comet Hale-Bopp, circa May 1997.
E) You are at Disneyland on the Moon, riding the new "wild and wet" roller coaster.

36) Which of the following observations is not a piece of evidence supporting the Big Bang theory?
A) Darkness of the night sky
B) Recession speeds of far away galaxies relative to close ones
C) Observed helium abundance in the universe
D) Relative motions of galaxies in the Local Group

1) The Planck era is another name for the present period of time in the universe.

2) The observed composition of ordinary matter in the universe–roughly 75 percent hydrogen and 25 percent helium–closely matches theoretical predictions based on the Big Bang model.

3) GUT theories predict that protons will eventually decay, causing all solid objects in the universe to fall apart if the universe keeps expanding forever.

4) The theory that inflation occurred in the early universe is incompatible with the theory of relativity.

5) If inflation really occurred, then our observable universe is only a tiny portion of the entire universe born in the Big Bang.

6) Observations of the cosmic background radiation from the COBE satellite revealed tiny variations in its temperature from one place to another (corresponding to a few millionths of a degree Kelvin).

7) The Big Bang predicts that one in four atoms in the universe is helium.

8) Current measurements of the density of the universe support the prediction of the theory of inflation that the universe should be flat.

9) The fact that the sky is dark at night shows that the observable universe cannot extend forever.

10) Process of Science: Inflation can explain some general features of the Universe but it is not directly testable and cannot be considered a theory.

1) Based on our current understanding of physics, we can understand the conditions that prevailed in the early universe as far back in time as about
A) 380,000 years after the Big Bang.
B) one ten-billionth of a second after the Big Bang.
C) 10-45 seconds after the Big Bang.
D) 10 billion years ago.

2) What happens when a particle of matter meets its corresponding antiparticle of antimatter?
A) They can form a complete atom.
B) The combined mass of the two particles is completely transformed into energy (photons).
C) They fuse to make a heavier particle.
D) The question makes no sense, since antimatter does not really exist.

3) What is the significance of the Planck time?
A) It is the time at which inflation is thought to have occurred.
B) Before it, conditions were so extreme that our current understanding of physics is insufficient to predict what might have occurred.
C) It is the time when the cosmic microwave background was released.
D) It is the amount of time required for two protons to fuse to make deuterium.

4) The four fundamental forces that operate in the universe today are
A) strong force, weak force, electromagnetic force, gravity.
B) strong force, weak force, electric force, magnetic force.
C) nuclear force, electromagnetic force, gravity, tidal force.
D) nuclear force, gravity, electric force, magnetic force.

5) A "GUT" (grand unified theory) refers to theories that
A) unify gravity with the strong and weak forces.
B) unify the electromagnetic and weak forces.
C) unify all four forces together.
D) unify the strong force with the electromagnetic and weak forces.

6) What do we mean by inflation?
A) the expansion of the universe that we still observe today
B) the sudden release of photons when a particle and antiparticle annihilate one another
C) a sudden and extremely rapid expansion of the universe that occurred in a tiny fraction of a second during the universe's first second of existence
D) quantum fluctuations by high speed, relativistic particles in a state of false vacuum

7) Which of the following statements correctly summarizes the events in the early universe according to the Big Bang theory?
A) The universe began with the forces unified. During the first fraction of a second, the forces separated and there was a brief but important episode of inflation. Subatomic particles of both matter and antimatter then began to appear from the energy present in the universe. Most of the particles annihilated to make photons, but some became protons, neutrons, electrons, and neutrinos. The protons and neutrons underwent some fusion during the first three minutes, thereby determining the basic chemical composition of the universe.
B) An episode of what we call inflation initiated the event of the Big Bang. Once the Big Bang got underway, particles and forces began to appear one by one. The forces produced protons, which fused to make hydrogen and helium until the universe was about 380,000 years old. Then gravity began to act, turning the hydrogen and helium into galaxies.
C) Forces and various subatomic particles began to appear during the first second after the Big Bang. For reasons not understood, the particles were all made of ordinary matter and none were made of antimatter, thus explaining why we live in a universe made of matter. The particles underwent some fusion for the first 380,000 years after the Big Bang, at which time the first stars were born.
D) The Big Bang began with the initiation of what we call inflation, which gradually slowed to the current expansion rate of the universe. Forces came to exist for a different reason, having to do with quantum fluctuations in the space-time continuum. Particles came to exist as a result of cracks made when forces froze. Once there were particles, gravity brought them together to make stars, and the stars then turned the particles into hydrogen, helium, and other elements.

9) The Big Bang theory is supported by two major lines of evidence that alternative models have not successfully explained. What are they?
A) (1) the theory correctly predicts that the universe should be expanding; (2) the theory predicts the existence of and the specific characteristics of the observed cosmic microwave background
B) (1) the theory predicts the episode of inflation that we think occurred in the early universe; (2) the theory predicts the existence of large quantities of dark matter.
C) (1) the theory correctly predicts that the universe should be expanding; (2) the theory correctly predicts the observed ratio of spiral to elliptical galaxies in the universe.
D) (1) the theory predicts the existence of and the specific characteristics of the observed cosmic microwave background; (2) the theory correctly predicts the observed overall chemical composition of the universe.

11) The idea of dark matter arose to explain gravitational effects observed in galaxies and clusters of galaxies. However, studies of the early universe (especially of the cosmic microwave background and of chemical abundances) also tell us something about dark matter. What do they tell us?
A) They add further support to the idea that dark matter really exists and is made of non-ordinary (nonbaryonic) matter, such as WIMPs.
B) They do not support the conclusion that dark matter is the dominant form of matter in the universe.
C) They tell us that dark matter probably exists, but that it must be made of ordinary (baryonic) matter in the form of MACHOs.
D) They tell us that dark matter was produced during the era of nuclei.

12) Which of the following observations cannot be explained by the Big Bang theory unless we assume that an episode of inflation occurred?
A) the fact that the temperature of the cosmic microwave background is almost the same everywhere
B) the fact that about 25% of the ordinary matter in the universe consists of helium
C) the existence of the cosmic microwave background
D) the fact that the universe is expanding

13) The idea of inflation makes one clear prediction that, until the discovery of an accelerating expansion, seemed to contradict the available observations. What is this prediction?
A) Inflation predicts that the early universe should have regions of enhanced density that could have acted as "seeds" for the formation of galaxies and large structures.
B) The universe should be geometrically "flat" (in the four dimensions of spacetime).
C) Inflation predicts that the temperature of the cosmic microwave background should be almost (but not exactly) the same everywhere.
D) Inflation predicts that the entire universe must be far larger than the observable universe.

14) Olbers's paradox is an apparently simple question, but its resolution suggests that the universe is finite in age. What is the question?
A) What would it be like to ride on a beam of light?
B) How many stars are in the universe?
C) Can we measure the position and momentum of an electron at the same time?
D) Why is the sky dark at night?

15) What is the temperature of the universe (as a whole) today?
A) 3K
B) 300K
C) 3000K
D) The universe cannot be said to have a single temperature.

16) Which of the following statements cannot be tested by science today?
A) Our universe is flat.
B) Prior to the Planck time, our universe sprouted from another universe.
C) The universe is 14 billion years old.
D) The expansion of the universe is now accelerating.

1) How do we determine the conditions that existed in the very early universe?
A) We look all the way to the cosmological horizon, where we can see the actual conditions that prevailed all the way back to the first instant of the Big Bang.
B) The conditions in the very early universe must have been much like those found in stars today, so we learn about them by studying stars.
C) We work backward from current conditions to calculate what temperatures and densities must have been when the observable universe was much smaller in size.
D) We can only guess at the conditions, since we have no way to calculate or observe what they were.

2) Why can't current theories describe what happened during the Planck era?
A) We do not know how hot or dense the universe was during that time.
B) We do not understand the properties of antimatter.
C) We do not yet have a theory that links quantum mechanics and general relativity.
D) The Planck era was the time before the Big Bang, and we cannot describe what happened before that instant.

3) Which of the following statements best explains what we mean when we say that the electroweak and strong forces "froze out" at 10-38 second after the Big Bang?
A) These two forces first became distinct at this time.
B) These forces are important only at temperatures below the freezing point of water—a temperature that the universe reached at an age of about at 10-38 second.
C) Freezing out was a term coined by particle physicists who think that the Big Bang theory is really cool.
D) Following this time, neither the strong nor electroweak forces were ever important in the universe again.

4) According to the Big Bang theory, how many forces—and which ones—operated in the universe during the GUT era?
A) 1 force that represented the unification of all four forces that operate today
B) 3 forces: gravity, the strong force, and the electroweak force
C) 2 forces: the strong force and the electroweak force
D) 2 forces: gravity and a single force that later became the strong, weak, and electromagnetic forces

5) Laboratory experiments conducted with particle accelerators confirm predictions made by the theory that unifies
A) the electromagnetic and weak forces into the electroweak force.
B) the strong, weak, and electromagnetic forces into the GUT force.
C) the unification of all four forces into a single "superforce."
D) the strong and weak forces into the combined nuclear force.

6) What was the significance of the end of the era of nucleosynthesis, when the universe was about 5 minutes old?
A) The proportions of dark matter and luminous matter had been determined.
B) The basic chemical composition of the universe had been determined.
C) It marks the time at which the first stars formed.
D) It marks the time at which the expansion of the universe had settled down to its current rate.

7) According to the Big Bang theory, why do we live in a universe that is made of almost entirely of matter rather than antimatter?
A) During the first 0.001 second after the Big Bang, particles and antiparticles were made in almost but not perfectly equal numbers. Everything annihilated except the very slight excess of matter particles.
B) GUT theories predict that under the conditions that prevailed in the early universe, the normal laws of physics would have been suspended so that only matter particles were created, and no particles of antimatter.
C) The fact that we live in a universe made of matter is not surprising, because antimatter has never been shown to exist for real.
D) Einstein's famous equation E = mc2 tells us that energy can turn into matter, but does not tell us that it can turn into antimatter.

8) Which of the following is not an observed characteristic of the cosmic microwave background?
A) It has a perfect thermal radiation spectrum.
B) Its temperature is the same everywhere, except for small variations at the level of 1 part in 100,000.
C) Its temperature is a little less than 3 Kelvin (3 degrees above absolute zero).
D) It contains prominent spectral lines of hydrogen, the primary chemical ingredient of the universe.

9) In principle, if we could see all the way to the cosmological horizon we could see the Big Bang taking place. However, our view is blocked for times prior to about 380,000 years after the Big Bang. Why?
A) Before that time, the universe was too crowded with stars.
B) Before that time, the gas in the universe was dense and ionized and therefore did not allow light to travel freely.
C) Before that time, the universe was dark so there was no light to illuminate anything.
D) 380,000 years after the Big Bang marks the time when stars were first born, and thus began to shine the light by which we can see the universe.

10) If observations had shown that the cosmic microwave background was perfectly smooth (rather than having very slight variations in temperature), then we would have no way to account for
A) the relationship between the strong and the weak force.
B) the fact that our universe is expanding.
C) how galaxies came to exist.
D) the existence of helium in the universe.

11) In stars, helium can sometimes be fused into carbon and heavier elements (in their final stages of life). Why didn't the same fusion processes produce carbon and heavier elements in the early universe?
A) By the time stable helium nuclei had formed, the temperature and density had already dropped too low for helium fusion to occur.
B) Helium fusion occurred, but the carbon nuclei that were made were later destroyed by the intense radiation in the early universe.
C) Temperatures in the early universe were never above the roughly 100 million Kelvin required for helium fusion.
D) No one knows—this is one of the major mysteries in astronomy.

12) How does the idea of inflation account for the existence of the "seeds" of density from which galaxies and other large structures formed?
A) Inflation predicts that gravity would have been very strong and thereby would have concentrated mass into seeds.
B) Inflation tells us that the universe should have a "flat" overall geometry, and this led to the flat disks of galaxies.
C) Inflation predicts that temperatures and densities should have become nearly equal throughout the universe.
D) Inflation would have caused random, microscopic quantum fluctuations to grow so large in size that they became the seeds of structure.

13) Which of the following is not consistent with recent observations of the cosmic microwave background by the WMAP satellite?
A) The universe is geometrically "flat" (in the four dimensions of spacetime).
B) The matter density (both luminous and dark matter combined) in the universe is only about one-fourth of the critical density.
C) Dark energy, whatever it is, represents the majority of the energy content of the universe.
D) The universe is at least 20 billion years old.

14) Based on the results from the WMAP satellite, the overall composition of the universe is
A) 100% ordinary (baryonic) matter.
B) 15% ordinary (baryonic) matter, 85% nonbaryonic dark matter.
C) 1% ordinary (baryonic) matter, 99% nonbaryonic dark matter.
D) 4% ordinary (baryonic) matter, 23% nonbaryonic dark matter, 73% dark energy.

15) Which adjective does not necessarily describe a known feature of the early universe? (Be sure to consider the universe as a whole, not just the observable universe.)
A) dense
B) small
C) hot
D) filled with intense radiation

16) The Big Bang theory seems to explain how elements were formed during the first few minutes after the Big Bang. Which hypothetical observation below (these are not real observations) would call our current theory into question?
A) the discovery of a star-like object made entirely of carbon and oxygen
B) the discovery of a planet that with no helium in its atmosphere
C) the discovery of a galaxy with a helium abundance of only 10% by mass
D) the discovery of a galaxy with 27% helium rather than the 25% that theory tells us was produced in the Big Bang

1) Why do we call dark matter "dark"?
A) It emits no visible light.
B) We cannot detect the type of radiation that it emits.
C) It emits no or very little radiation of any wavelength.
D) It blocks out the light of stars in a galaxy.

2) What is meant by "dark energy"?
A) the energy associated with dark matter through E=mc2
B) any unknown force that opposes gravity
C) the agent causing the universal expansion to accelerate
D) highly energetic particles that are believed to constitute dark matter
E) the total energy in the Universe after the Big Bang but before the first stars

3) Why do we believe 90 percent of the mass of the Milky Way is in the form of dark matter?
A) The orbital speeds of stars far from the galactic center are surprisingly high, suggesting that these stars are feeling gravitational effects from unseen matter in the halo.
B) Although dark matter emits no visible light, it can be seen with radio wavelengths, and such observations confirm that the halo is full of this material.
C) Theoretical models of galaxy formation suggest that a galaxy cannot form unless it has at least 10 times as much matter as we see in the Milky Way disk, suggesting that the halo is full of dark matter.
D) Our view of distant galaxies is sometimes obscured by dark blotches in the sky, and we believe these blotches are dark matter located in the halo.

4) How do we know that there is much more mass in the halo of our galaxy than in the disk?
A) There are so many globular clusters in the halo that their total mass is greater than the mass of stars in the disk.
B) Stars in the outskirts of the Milky Way orbit the galaxy at much higher speeds than we would expect if all the mass were concentrated in the disk.
C) Although the question of mass in the halo was long mysterious, we now know it exists because we see so many brown dwarfs in the halo.
D) The recent discovery of photinos, combined with theoretical predictions, tells us that there must be a huge mass of photinos in the halo.
E) We don't know that there is more mass in the halo; it is only a guess based on theory.

5) What evidence suggests that the Milky Way contains dark matter?
A) We observe clouds of atomic hydrogen far from the galactic center orbiting the galaxy at unexpectedly high speeds, higher speeds than they would have if they felt only the gravitational attraction from objects that we can see.
B) We see many lanes of dark material blocking out the light of stars behind them along the band of the Milky Way.
C) We see many dark voids between the stars in the halo of the Milky Way.
D) When we observe in different wavelengths, such as infrared or radio, we see objects that don't appear in visible-light observations.
E) When we look at the galactic center, we are able to observe a large black hole that is composed of dark matter.

6) If there is no dark matter in the Milky Way Galaxy, what is the best alternative explanation for the observations?
A) We are not measuring the orbital velocities of atomic clouds and stars properly.
B) We are not measuring the distances to atomic clouds and stars properly.
C) We are not attributing enough mass to the visible or "bright" matter.
D) We are not observing all the visible or "bright" matter in the galaxy.
E) Our understanding of gravity is not correct for galaxy-size scales.

7) How are rotation curves of spiral galaxies determined beyond radii where starlight can be detected?
A) by extrapolation
B) through observations of the 21 cm line of atomic hydrogen
C) through observations of spectral lines of dark matter
D) by watching the galaxies rotate over a period of years
E) by measuring the broadening of absorption lines

8) The distribution of the dark matter in a spiral galaxy is
A) approximately spherical and about the same size as the galaxy halo.
B) approximately spherical and about ten times the size of the galaxy halo.
C) flattened in a disk and about the same size as the stellar disk.
D) flattened in a disk but about ten times larger than the stellar disk.
E) predominantly concentrated in the spiral arms.

9) How do we determine the amount of dark matter in elliptical galaxies?
A) We measure the orbital velocities of star-forming gas clouds around the outer portions of the galaxy.
B) We measure the speeds of stars at different radii from the galactic center and determine how much mass is interior to the orbit.
C) We count the number of stars in the galaxy and determine its volume, so that we can calculate the galaxy's density.
D) We search for dark lanes of dust and black holes within the galaxy.
E) We measure how fast the galaxy rotates as a whole.

10) When we see that a spectral line of a galaxy is broadened, that is, spanning a range of wavelengths, we conclude that
A) we do not have very good resolution of a star's orbital velocity.
B) there are many stars traveling at extremely high orbital velocities.
C) there are different Doppler shifts among the individual stars in the galaxy.
D) we are actually measuring the orbital velocity of a cloud of atomic gas.
E) we are actually measuring the orbital velocity of dark matter.

11) A large mass-to-light ratio for a galaxy indicates that
A) the galaxy is very massive.
B) the galaxy is not very massive.
C) on average, each solar mass of matter in the galaxy emits less light than our Sun.
D) on average, each solar mass of matter in the galaxy emits more light than our Sun.
E) most stars in the galaxy are more massive than our Sun.

12) What is the mass-to-light ratio for the inner region of the Milky Way Galaxy, in units of solar masses per solar luminosity?
A) 1,000
B) 600
C) 100
D) 6
E) 0.1

13) Compared to the central regions of spiral galaxies, we expect elliptical galaxies to have
A) higher mass-to-light ratios because stars in elliptical galaxies are dimmer than those in spirals.
B) lower mass-to-light ratios because stars in elliptical galaxies are dimmer than those in spirals.
C) higher mass-to-light ratios because stars in elliptical galaxies do not have high orbital velocities.
D) lower mass-to-light ratios because elliptical galaxies have less gas and dust than spirals.
E) the same mass-to-light ratio because they are made of the same material, stars and dark matter.

14) If a galaxy's overall mass-to-light ratio is 100 solar masses per solar luminosity, and its stars account for only 5 solar masses per solar luminosity, how much of the galaxy's mass must be dark matter?
A) 100 percent
B) 95 percent
C) 80 percent
D) 50 percent
E) 5 percent

15) Which of the following methods used to determine the mass of a cluster does not depend on Newton's laws of gravity?
A) measuring the orbital velocities of galaxies in a cluster
B) measuring the temperature of X-ray gas in the intracluster medium
C) measuring the amount of distortion caused by a gravitational lens
D) none of the above

16) Why wasn't the intracluster medium in galaxy clusters discovered until the 1960s?
A) We did not know how much dark matter existed before then.
B) We didn't have the resolution to observe galaxy clusters until then.
C) The Milky Way was blocking our view of distant galaxy clusters.
D) The medium emits X rays, which are blocked by the Earth's atmosphere and require X-ray satellites in space in order to be observed.
E) Radiation emitted by the medium was so dim that we couldn't detect it until we built much larger telescopes.

17) Which of the following statements about rich clusters of galaxies (those with thousands of galaxies) is not true?
A) They are sources of X-ray emission due to the presence of hot, intergalactic gas.
B) There likely have been numerous collisions among the member galaxies at some time in the past.
C) Galaxies in the central regions are predominantly spirals, while elliptical galaxies roam the outskirts.
D) They often have a very large, central dominant galaxy near their center, perhaps formed by the merger of several individual galaxies.
E) The speeds of the galaxies in the cluster indicate that most of the cluster mass is dark matter.

18) Gravitational lensing occurs when
A) massive objects bend light beams that are passing nearby.
B) massive objects cause more distant objects to appear much larger than they should and we can observe the distant objects with better resolution.
C) dark matter builds up in a particular region of space, leading to a very dense region and an extremely high mass-to-light ratio.
D) telescope lenses are distorted by gravity.

19) Which of the following is not evidence for dark matter?
A) the flat rotation curves of spiral galaxies
B) the broadening of absorption lines in an elliptical galaxy's spectrum
C) X-ray observations of hot gas in galaxy clusters
D) gravitational lensing around galaxy clusters
E) the expansion of the universe

20) Which of the following particles are baryons?
A) electrons
B) neutrinos
C) protons
D) quarks
E) photons

21) Which of the following is an example of baryonic matter?
A) you
B) the particles produced by physicists in particle accelerators
C) electrons and positrons produced by pair production
D) WIMPs
E) neutrinos

22) Measuring the amount of deuterium in the universe allows us to set a limit on
A) the temperature of the universe at the end of the era of nuclei.
B) the total amount of mass in the universe.
C) the density of ordinary (baryonic) matter in the universe.
D) the expansion rate of the universe.
E) the current age of the universe.

23) Based on current evidence concerning the amount of deuterium in the universe, we can conclude that
A) ordinary (baryonic) matter makes up most of the mass of the universe.
B) neutrons greatly outnumber protons in the universe.
C) most of the deuterium that was created during the era of nucleosynthesis has since been destroyed.
D) the density of ordinary (baryonic) matter is between 1 percent and 10 percent of the critical density.
E) we live in a critical-density universe.

24) What do we mean when we say that a particle is a weakly interacting particle?
A) It interacts only through the weak force.
B) It interacts only through the weak force and the force of gravity.
C) It is so small that it doesn't affect objects in the universe.
D) It doesn't interact with any type of baryonic matter.
E) It is the only type of particle that interacts through the weak force.

25) Why can't the dark matter in galaxies be made of neutrinos?
A) There are not enough neutrinos to make up all the dark matter.
B) Neutrinos do not have any mass; they interact only through the weak force.
C) We know that dark massive objects such as planets and neutron stars are not made of neutrinos.
D) Neutrinos travel at extremely high speeds and can escape a galaxy's gravitational pull.
E) Big Bang nucleosynthesis constrains how many neutrinos there are in the Universe.

26) Which of the following are candidates for dark matter?
A) brown dwarfs
B) Jupiter-size objects
C) WIMPs
D) faint red stars
E) all of the above

27) Why do we expect WIMPs to be distributed throughout galactic halos, rather than settled into a disk?
A) They are light enough that they have expanded out into the halo.
B) WIMPs were produced at the early stages of galaxy evolution, and objects in the halo, such as globular clusters, were formed at the beginning of the galaxy.
C) Since they do not interact with the electromagnetic force, they do not feel friction or drag and hence do not contract with the rest of the protogalactic cloud.
D) Shock waves from supernovae have blown the WIMPs out into the halo.
E) Jets from the early active stage of a galaxy's life shot out most of the WIMPs from the disk.

28) Why isn't space expanding within systems such as our solar system or the Milky Way?
A) Hubble's law of expansion applies only to the space between galaxies.
B) We are so close to these systems that we don't observe their expansion.
C) The universe is not old enough yet for these objects to begin their expansion.
D) Their gravity is strong enough to hold them together against the expansion of the universe.

29) What are peculiar velocities?
A) velocities perpendicular to our line of sight
B) velocities directly along our line of sight
C) velocities that we cannot explain by only the force of gravity
D) velocities caused by the expansion of the universe
E) velocities of distant objects that are not caused by the expansion of the universe

30) What do peculiar velocities reveal?
A) the amount of dark matter in a galaxy
B) the distribution of dark matter in large-scale structures
C) the composition of dark matter
D) the error in our observations of Hubble's law
E) the critical density of the universe

31) How do astronomers create three-dimensional maps of the universe?
A) through comparison of computer models of the structure formation with observations
B) by using the position on the sky and the redshift to determine a distance along the line of sight
C) by using the position on the sky and the galaxy brightness as a measure of distance along the line of sight
D) by interpreting the peculiar velocities of each galaxy
E) by carefully measuring the parallax of each galaxy

32) What does the universe look like on very large scales?
A) Galaxies are uniformly distributed.
B) Galaxies are randomly distributed.
C) Galaxies are distributed in a hierarchy of clusters, superclusters, and hyperclusters.
D) Galaxies appear to be distributed in chains and sheets that surround great voids.
E) Galaxies are distributed in a great shell expanding outward from the center of the universe.

33) What fraction of the mass needed to halt expansion is known to exist in the form of visible mass in the universe?
A) 1 percent
B) 4 percent
C) 22 percent
D) 74 percent
E) 100 percent

34) Based on inventoried matter in the universe, including dark matter known to exist in galaxies and clusters, the actual density of the universe is what fraction of the critical density?
A) 1 percent
B) 10 percent
C) 26 percent
D) 74 percent
E) 100 percent

35) If all the "dark matter" in the Universe were to be, somehow, instantaneously removed, which of the following would not happen?
A) The Solar System would fly apart.
B) The Milky Way would fly apart.
C) Clusters of galaxies would fly apart.
D) The Universe would expand forever.
E) all of the above

36) Which model of the universe gives the youngest age for its present size?
A) a recollapsing universe
B) a coasting universe
C) a critical universe
D) an accelerating universe
E) all models give the same age

37) What is the ultimate fate of an open universe?
A) the Big Crunch
B) Stars will expand away from each other and galaxies effectively "evaporate."
C) All matter decays to a low-density sea of photons and subatomic particles.
D) All matter eventually ends up in massive black holes.
E) Individual stars die but their gas is recycled through the interstellar medium and new stars form in a never-ending process.

38) Recent measurements of the expansion rate of the universe reveal that the expansion rate of the universe is doing something astronomers did not expect. What is that?
A) The measurements show that the universe may not be expanding at all.
B) The measurements show that the universe may be shrinking rather than expanding.
C) The measurements show that the expansion is accelerating, rather than slowing under the influence of gravity.
D) The measurements indicate that the universe is at least 30 billion years old, meaning that more than 10 billion years passed between the Big Bang and the formation of the first stars and galaxies.
E) The data show that the expansion rate varies widely in different parts of the universe.

39) What is the evidence for an accelerating universe?
A) White-dwarf supernovae are the same brightness regardless of redshift.
B) White-dwarf supernovae are slightly brighter than expected for a coasting universe.
C) White-dwarf supernovae are slightly dimmer than expected for a coasting universe.
D) The Andromeda Galaxy is moving away from the Milky Way at an ever-increasing speed.
E) There is far more dark matter than visible matter in the universe.

40) What might be causing the universe to accelerate?
A) WIMPs
B) neutrinos
C) white-dwarf supernovae
D) dark gravity
E) We don't know!–but we call it "dark energy."

41) What is Einstein's cosmological constant?
A) the value of the expansion rate of the universe
B) the value of the acceleration of the universe
C) the value that measures the strength of gravity across the universe
D) the size of the cosmological horizon
E) a repulsive force that counteracts gravity and was introduced to allow for a static universe

42) What is not a main source of evidence for the existence of dark matter?
A) massive blue stars
B) rotation curves of disk galaxies
C) stellar motions in elliptical galaxies
D) velocities and positions of galaxies in clusters of galaxies
E) gravitational lensing by clusters of galaxies

1) Approximately 90 percent of the mass of the Milky Way is located in the halo of the galaxy in the form of dark matter.

2) Dark matter is purely hypothetical, because we have no way of detecting its presence.

3) If the universe is accelerating, it will expand forever.

4) If we learn that the universe is a recollapsing universe, it will mean that the universe is presently contracting, rather than expanding as generally believed.

5) By definition, our Sun has a mass-to-light ratio of 1 solar mass per solar luminosity.

6) One possible ingredient of dark matter is known as WIMPs, or weakly interacting massive particles. WIMPs probably are made of protons and neutrons.

7) Although we don't know exactly when clusters, galaxies, or stars began forming, we do know that clusters came first, with galaxies and stars forming later.

8) Individual galaxies generally have higher mass-to-light ratios than clusters of galaxies.

9) Some galaxy clusters are still growing today.

10) The visible parts of galaxies contribute about one-tenth of the critical density of the universe.

11) The only possible geometry of an accelerating universe is open.

1) Which of the following best summarizes what we mean by dark matter?
A) matter that we have identified from its gravitational effects but that we cannot see in any wavelength of light
B) matter that may inhabit dark areas of the cosmos where we see nothing at all
C) matter consisting of black holes
D) matter for which we have theoretical reason to think it exists, but no observational evidence for its existence

3) The text states that luminous matter in the Milky Way seems to be much like the tip of an iceberg. This refers to the idea that
A) luminous matter emits white light, much like the light reflected from icebergs.
B) black holes are much more interesting than ordinary stars that give off light.
C) dark matter represents much more mass and extends much further from the galactic center than the visible stars of the Milky Way.
D) the luminous matter of the Milky Way is essentially floating on the surface of a great sea of dark matter.

4) What is a rotation curve?
A) a precise description of the shape of a star's orbit around the center of the Milky Way Galaxy
B) a graph showing how orbital velocity depends on distance from the center for a spiral galaxy
C) a curve used to decide whether a star's orbit places it in the disk or the halo of a spiral galaxy
D) a graph that shows a galaxy's mass on the vertical axis and size on the horizontal axis

5) What is the primary way in which we determine the mass distribution of a spiral galaxy?
A) We calculate its mass-to-light ratio.
B) We apply Newton's version of Kepler's third law to the orbits of globular clusters in the galaxy's halo.
C) We count the number of stars we can see at different distances from the galaxy's center.
D) We construct its rotation curve by measuring Doppler shifts from gas clouds at different distances from the galaxy's center.

6) What do we mean when we say that the rotation curve for a spiral galaxy is "flat"?
A) The amount of light emitted by stars at different distances is about the same throughout the galaxy.
B) Gas clouds orbiting far from the galactic center have approximately the same orbital speed as gas clouds located further inward.
C) The disk of a spiral galaxy is quite flat rather than spherical like the halo.
D) All the galaxy's mass is concentrated in its flat, gaseous disk.

7) Although we know less about dark matter in elliptical galaxies than in spiral galaxies, what does current evidence suggest?
A) Elliptical galaxies probably contain about the same proportion of their mass in the form of dark matter as do spiral galaxies.
B) Elliptical galaxies probably contain far less dark matter than spiral galaxies.
C) Elliptical galaxies probably contain far more dark matter than spiral galaxies.
D) Unlike the broad distribution of dark matter in spiral galaxies, elliptical galaxies probably contain dark matter only near their centers.

8) In general, when we compare the mass of a galaxy or cluster of galaxies to the amount of light it emits (that is, when we look at it mass-to-light ratio), we expect that
A) the higher amount of mass relative to light (higher mass-to-light ratio), the lower the proportion of dark matter.
B) the higher the amount of mass relative to light (higher mass-to-light ratio), the greater the proportion of dark matter.
C) the amount of light should be at least one solar luminosity for each solar mass of matter (mass-to-light ratio less than or equal to 1).
D) the higher the amount of mass relative to light (higher mass-to-light ratio), the older the galaxy or cluster.

9) Which of the following is not one of the three main strategies used to measure the mass of a galaxy clusters?
A) measuring the speeds of galaxies orbiting the cluster's center
B) studying X-ray emission from hot gas inside the cluster
C) observing how the cluster bends light from galaxies located behind it
D) measuring the temperatures of stars in the halos of the galaxies

10) When we say that a cluster of galaxies is acting as a gravitational lens, what do we mean?
A) It magnifies the effects of gravity that we see in the cluster.
B) It is an unusually large cluster that has a lot of gravity.
C) It bends or distorts the light coming from galaxies located behind it.
D) The overall shape of the cluster is that of a lens.

11) Which of the following statements best summarizes current evidence concerning dark matter in individual galaxies and in clusters of galaxies?
A) Dark matter is the dominant form of mass in both clusters and in individual galaxies.
B) Dark matter is present between galaxies in clusters, but not within individual galaxies.
C) Dark matter is present in individual galaxies, but there is no evidence that it can exist between the galaxies in a cluster.
D) Within individual galaxies, dark matter is always concentrated near the galactic center, and within clusters it is always concentrated near the cluster center.

12) What is the distinguishing characteristic of what we call ordinary or baryonic matter?
A) It emits a great deal of light.
B) It can attract other matter through the force of gravity.
C) It is made of subatomic particles that scientists call WIMPs.
D) It consists of atoms or ions with nuclei made from protons and neutrons.

13) What do we mean when we say that particles such as neutrinos or WIMPs are weakly interacting?
A) The light that they emit is so weak that it is undetectable to our telescopes.
B) They are only weakly bound by gravity, which means they can fly off and escape from galaxies quite easily.
C) They respond to the weak force but not to the electromagnetic force, which means they cannot emit light.
D) They interact with other matter only through the weak force and not through gravity or any other force.

14) Which of the following best sums up current scientific thinking about the nature of dark matter?
A) Most dark matter probably consists of weakly interacting particles of a type that we have not yet identified.
B) Dark matter consists 90% of neutrinos and 10% of WIMPs.
C) There is no longer any doubt that dark matter is made mostly of WIMPs.
D) Dark matter probably does not really exist, and rather indicates a fundamental problem in our understanding of gravity.

15) When we speak of the large-scale structure of the universe, we mean
A) the structure of any large galaxy.
B) the structure of any individual cluster of galaxies.
C) the overall shape of the observable universe.
D) the overall arrangement of galaxies, clusters of galaxies, and superclusters in the universe.

16) The critical density of the universe is the
A) average density the universe would need for gravity to someday halt the current expansion if dark energy did not exist.
B) actual average density of the universe.
C) density of dark matter in the universe.
D) density of water.

17) What is the primary form of evidence that has led astronomers to conclude that the expansion of the universe is accelerating?
A) observations of the speeds of individual galaxies in clusters
B) measurements of the rotation curve for the universe
C) measurements of how galaxy speeds away from the Milky Way have increased during the past century
D) observations of white dwarf supernovae

3) Spiral galaxy rotation curves are generally fairly flat out to large distances. Suppose that spiral galaxies did not contain dark matter. How would their rotation curves be different?
A) The orbital speeds would fall off sharply with increasing distance from the galactic center.
B) The orbital speeds would rise upward with increasing distance from the galactic center, rather than remaining approximately constant.
C) The rotation curve would be a straight, upward sloping diagonal line, like the rotation curve of a merry-go-round.
D) The rotation curve would look the same with or without the presence of dark matter.

4) The flat rotation curves of spiral galaxies tell us that they contain a lot of dark matter. Do they tell us anything about where the dark matter is located within the galaxy?
A) Yes, they tell us that dark matter is concentrated near the center of the galaxy.
B) Yes, they tell us that dark matter is spread uniformly throughout the galactic disk.
C) Yes, they tell us that dark matter is spread throughout the galaxy, with most located at large distances from the galactic center.
D) No, we cannot determine anything about the location of dark matter from the rotation curve.

5) It is more difficult to determine the total amount of dark matter in an elliptical galaxy than in a spiral galaxy. Why?
A) Elliptical galaxies lack the atomic hydrogen gas that we use to determine orbital speeds at great distances from the centers of spiral galaxies.
B) Elliptical galaxies contain much less dark matter than spiral galaxies, so it's much more difficult to measure.
C) Stars in elliptical galaxies are dimmer, making them harder to study.
D) We cannot observe spectral lines for elliptical galaxies.

6) How do we know that galaxy clusters contain a lot of mass in the form of hot gas that fills spaces between individual galaxies?
A) We infer its existence by observing its gravitational effects on the galaxy motions.
B) The hot gas shows up as bright pink in visible-light photos of galaxy clusters.
C) We can observe the frictional effects of the hot gas in slowing the speeds of galaxies in the clusters.
D) We detect this gas with X-ray telescopes.

7) Why does the temperature of the gas between galaxies in galaxy clusters tell us about the mass of the cluster?
A) The temperature is always directly related to mass, which is why massive objects are always hotter than less massive objects.
B) The temperature tells us the average speeds of the gas particles, which are held in the cluster by gravity, so we can use these speeds to determine the cluster mass.
C) The temperature of the gas tells us the gas density, so we can use the density to determine the cluster's mass.
D) The question is nonsense—gas temperature cannot possibly tell us anything about mass.

8) How does gravitational lensing tell us about the mass of a galaxy cluster?
A) The lensing allows us to determine the orbital speeds of galaxies in the cluster, so that we can determine the mass of the cluster from the orbital velocity law.
B) The lensing broadens spectral lines, and we can use the broadening to "weigh" the cluster.
C) Using Einstein's general theory of relativity, we can calculate the cluster's mass from the precise way in which it distorts the light of galaxies behind it.
D) Newton's universal law of gravitation predicts how mass can distort light, so we can apply Newton's law to determine the mass of the cluster.

9) If WIMPs really exist and make up most of the dark matter in galaxies, which of the following is not one of their characteristics?
A) They travel at speeds close to the speed of light.
B) They are subatomic particles.
C) They can neither emit nor absorb light.
D) They tend to orbit at large distances from the galactic center.

11) Which of the following statements about large-scale structure is probably not true?
A) Galaxies and clusters have grown around tiny density enhancements that were present in the early universe.
B) Voids between superclusters began their existence as regions in the universe with a slightly lower density than the rest of the universe.
C) Many cluster and superclusters are still in the process of formation as their gravity gradually pulls in new members.
D) Clusters and superclusters appear to be randomly scattered about the universe, like dots sprinkled randomly on a wall.

12) Based on current evidence, a supercluster is most likely to have formed in regions of space where
A) the density of dark matter was slightly higher than average when the universe was very young.
B) there was an excess concentration of hydrogen gas when the universe was very young.
C) supermassive black holes were present in the very early universe.
D) the acceleration of the expansion was proceeding faster than elsewhere.

13) Based on current evidence, how does the actual average density of matter in the universe compare to the critical density?
A) If we include dark matter, the actual density equals the critical density.
B) The actual density, even with dark matter included, is less than about a third of the critical density.
C) The actual density of dark matter and luminous matter combined is no more than about 1% of the critical density.
D) The actual density of matter is many times higher than the critical density.

14) Which of the following statements best describes the current state of understanding regarding the apparent acceleration of the expansion of the universe?
A) The cause of the acceleration is well-understood, and attributed to the particles that make up dark energy.
B) We have moderately strong evidence that the acceleration is real, but essentially no idea what is causing it.
C) The acceleration is very important in the cosmos today, but the evidence indicates that it will eventually slow down, allowing the universe to recollapse.
D) The acceleration probably is not real, and what we attribute to acceleration is probably just a misinterpretation of the data.

15) Some people wish that we lived in a recollapsing universe that would eventually stop expanding and start contracting. For this to be the case, which of the following would have to be true (based on current understanding)?
A) Dark energy is the dominant form of energy in the cosmos.
B) Dark energy does not exist and there is much more dark matter than we are aware of to date.
C) Neither dark energy nor dark matter really exist.
D) Dark energy exists but dark matter does not.

16) Hubble's constant is related to the age of the universe, but the precise relationship depends on the way in which the expansion rate changes with time. For a given value of Hubble's constant today (such as 24 km/s/Mly), the age of the universe is oldest if what is true?
A) The expansion rate has remained nearly constant with time (a coasting universe).
B) The expansion rate has slowed by the amount expected for a universe with the critical density (a critical universe).
C) The expansion rate has been increasing with time (an accelerating universe).
D) The expansion rate is slowing dramatically with time (a recollapsing universe).

17) Imagine that it turns out that dark matter (not dark energy) is made up of an unstable form of matter that decays into photons or other forms of energy about 50 billion years from now. Based on current understanding, how would that affect the universe at that time?
A) Stars would cease to exist when the dark matter is gone.
B) Planetary systems would expand and disperse.
C) The galaxies in clusters would begin to fly apart.
D) The universe would cease its expansion.