Chapter 19: Cosmology

Section: 19-1

1. What do cosmologists study?

A) formation, structure, and evolution of stars

B) origin, structure, and evolution of the solar system

C) formation, structure, and evolution of galaxies

D) large-scale structure and evolution of the universe

2. Newton reached the conclusion that the universe must consist of an infinite expanse of stars because

A) he and his colleagues had observed the uniform distribution of stars all over the sky.

B) he was unable to detect the movement of stars around a common center, which his theory required for stability against collapse in a finite universe.

C) of his religious conviction that the creator would create nothing less than an infinite universe.

D) he reasoned that if stars were not uniformly distributed everywhere, then denser portions of the universe would clump together under their own gravity.

3. Which of these BEST describes the universe as envisioned by Isaac Newton?

A) static and clumpy

B) expanding slowly and clumpy

C) expanding slowly and uniform

D) static and uniform

4. Which scientist discovered that the equations he had derived predicted an expanding universe, then modified his equations to eliminate this expansion?

A) Albert Einstein

B) Isaac Newton

C) Stephen Hawking

D) Edwin Hubble

5. Einstein’s general theory of relativity, as originally formulated without the cosmological constant, predicts a universe that

A) is static.

B) must expand.

C) must contract.

D) must either expand or contract.

6. Why did Einstein introduce his cosmological constant into the original equations for general relativity?

A) The constant was necessary to make the solutions describe an expanding universe.

B) Experimental evidence at this time (before Hubble) suggested a static universe, and this constant was required to produce static solutions.

C) The constant represented the acceleration of gravity near Earth’s surface, and it was necessary to make the nonrelativistic limit of the solutions compatible with Newton’s results.

D) The constant represented the speed of light, and its inclusion was necessary to produce solutions that were compatible with the postulates of special relativity.

7. When did Einstein originally introduce his theory of general relativity, including the cosmological constant?

A) in 1896, before the modern stellar spectral classification scheme was devised

B) in 1915, before the question of the nature of the Milky Way Galaxy had been resolved

C) in 1930, after Hubble had discovered evidence for the expanding universe

D) in 1968, after the discovery of the cosmic microwave background gave evidence of the Big Bang

8. Which of these statements BEST describes the history of the cosmological constant?

A) Einstein introduced the constant to make the universe static, but many cosmologists now believe it makes the expansion of the universe accelerate.

B) Einstein introduced the constant to make the universe expand, and it is still necessary to cosmological theories today.

C) Einstein introduced the constant to make the universe static, but Hubble’s discovery of the expanding universe proved that it the constant actually vanishes.

D) Einstein introduced the constant to make the universe expand, and it is now known that it has a much higher value today because the expansion is accelerating.

Section: 19-2

9. Why is the universe expanding?

A) The energy from all the stars is heating the universe, making it expand like a gas that is heated.

B) Spacetime itself is expanding, carrying the galaxies (or superclusters of galaxies) with it.

C) The universe is not expanding—it is the Galaxy that is getting smaller, making the universe seem bigger and bigger.

D) An infinitely dense clump of matter exploded, sending the galaxies (or superclusters of galaxies) hurtling out through space.

10. Which of these statements is a correct description of the expansion of the universe?

A) Spacetime is something real, with galaxies inside it; as spacetime expands, the galaxies (or superclusters of galaxies) are carried along by the expansion.

B) Space is a vacuum, but the vacuum has real properties; as galaxies (or superclusters of galaxies) hurtle outward, the expansion is gradually slowing down by the resistance of space to the passage of the galaxies.

C) Spacetime is static, but dark matter, which is concentrated in the outer part of the universe, pulls outward on the galaxies.

D) Space is a vacuum, which is really nothing at all; the galaxies (or superclusters of galaxies) are hurtling outward through this nothingness.

11. When astronomers view a distant quasar with highly redshifted emission lines in its spectrum, this wavelength shift is a

A) Doppler shift.

B) gravitational redshift.

C) cosmological redshift.

D) Doppler shift or a gravitational redshift or a cosmological redshift—they are one and the same thing.

12. Suppose an object is at rest and that it is emitting radiation. An observer at rest with respect to the object observes the radiation to be blueshifted from its original frequency. This blueshift can be caused by

A) the Doppler shift only.

B) the gravitational wavelength shift only.

C) the cosmological wavelength shift only.

D) either the gravitational or the cosmological wavelength shift

13. What is the “cosmological redshift”?

A) stretching of the wavelengths of photons as they travel through expanding space

B) stretching of the wavelengths of photons by the Doppler shift because they are emitted by galaxies that are moving away from Earth

C) loss of energy from photons interacting with virtual particles in the vacuum, resulting in the wavelength of the photons gradually increasing as they travel toward Earth through space

D) stretching of the wavelengths of photons as they pass through absorbing matter in galaxies between Earth and the emitting galaxy

14. The farther away a galaxy is, the more its light is redshifted, as observed from Earth. This relationship between redshift and distance is caused by

A) the Doppler shift of light leaving a moving object. More distant galaxies are moving faster through space, so their light is more strongly Doppler-shifted (redshifted).

B) energy losses. The universe does not really expand; photons simply lose energy (wavelength lengthens) as they travel. Photons from more distant galaxies have traveled farther and so are more redshifted.

C) the expansion of space itself, which stretches the wavelength of the photon. The longer the time the photon has traveled, the more space has expanded and therefore the more the photon has been redshifted.

D) the gravitational redshift. Photons leaving a more distant galaxy have traveled farther through the galaxy’s gravitational field, so they have lost more energy and are more redshifted.

15. The cosmological redshift in the light from distant galaxies is explained by which of these effects?

A) A photon’s wavelength is a distance and is therefore lengthened by the general expansion of the universe, making the light appear reddened.

B) The light observed on Earth was Doppler-shifted to longer wavelengths by the motion of the objects (e.g., galaxies) away from Earth.

C) The light spreads out over larger areas as distance increases according to 1/(distance)², which causes the wavelength to increase in proportion to distance.

D) The light from more distant galaxies has traveled through the gravitational fields of more galaxies in getting to Earth and is therefore more gravitationally redshifted.

16. The cosmological redshift of the light from very distant galaxies is caused by the

A) rotation of the universe around its center (faster at greater distances from Earth).

B) expansion of space, stretching the photon’s wavelength while the photon is traveling toward Earth.

C) absorption of blue light by interstellar dust between Earth and the galaxy so that only the red wavelengths reach Earth.

D) Doppler shift, in which the photon’s wavelength is stretched by the galaxy’s motion through space, away from Earth, while the photon is being emitted.

17. What causes the cosmological redshift of photons that reach Earth from distant galaxies?

A) The photons have traveled through space that has been expanding and their wavelengths have expanded with it, becoming redder.

B) The photons were emitted from the galaxies much earlier in time when the overall temperature of matter was much lower. Hence, the observed photons are redder the farther away from Earth they were produced.

C) The photons were emitted by objects that were moving rapidly away from Earth and thereby have been reddened by the Doppler effect.

D) The photons have moved from high-gravitational-field regions toward lower fields, thereby becoming reddened.

18. When Hubble did his early work on the expanding universe, he knew nothing about the cosmological redshift. He used the Doppler shift equations instead, but these gave him the correct answer. Why?

A) These two sets of equations (Doppler shift and cosmological redshift) are the same and always give the same result for all cases.

B) Hubble was using only ordinary galaxies. If he had used galaxies with AGNs, the equations would not have worked.

C) Hubble was fortunate enough to use only galaxies for which the gravitational redshift and the cosmological redshift nearly cancel, so the Doppler shift provided essentially the whole effect.

D) Hubble was using only relatively nearby galaxies for which the nonrelativistic cosmological redshift and the Doppler shift provide the same result.

19. The expansion of the universe takes place

A) only between objects separated by vacuum; as a result, human bodies do not expand, but the Earth–Moon system does.

B) between all objects, even between the atoms in human bodies, although the expansion of a person is too small to be measured reliably.

C) only over distances the size of a galaxy or larger; consequently, the Milky Way Galaxy expands, but the solar system does not.

D) primarily in the huge spaces between clusters of galaxies; “small” objects like galaxies or Earth do not expand.

20. Consider four different regimes of space in which distances between objects might be changing as a result of the general expansion of the universe: (1) distances between different parts of Earth; (2) distance between planets in the solar system; (3) distances between stars in the Milky Way; (4) distances between clusters of galaxies. In which of these regimes are the distances changing because of the universe’s expansion?

A) 4, 3, 2, 1

B) 4

C) 4, 3, 2

D) 4, 3

21. What is it that keeps localized regions of space, such as things on Earth, planetary systems, star clusters, and whole galaxies, from participating in the general expansion of the universe?

A) their locations in places where irregularities in the chaotic Big Bang explosion permitted matter to condense

B) mutual gravitational attraction between objects in these systems

C) centrifugal force produced by the motion of the regions around a massive central object (e.g., the Sun, a supermassive black hole, etc.)

D) powerful and all-pervading gravitational pull from the central supermassive black holes of galaxies that holds everything in the galaxies in place

22. When Hubble first calculated the recessional speeds of galaxies in the 1920s, he used

A) the Doppler shift relationship. This was an error, and the calculations had to be repeated using the cosmological redshift relationship.

B) the Doppler shift relationship. This was an error, and the calculations had to be repeated using the gravitational redshift relationship.

C) the Doppler shift relationship. This was correct because the nonrelativistic cosmological redshift relationship gives the same result.

D) the cosmological redshift relationship. This was an error, and the calculations had to be repeated using the Doppler shift relationship.

Section: 19-3

23. According to Hubble’s law, how old is the universe? (H₀ = Hubble constant)

A) v/H₀ (where v = recession velocity in km/s)

B) r/H₀ (where r = distance in Mpc)

C) H₀

D) 1/H₀

24. In cosmology, the constant that is intimately related to the present “age” of the universe is

A) the constant in Wien’s law of radiation.

B) 1/G, the inverse of the universal gravitational constant.

C) the Planck time, 10^–43 s, in which space and time came into existence.

D) 1/H₀, the inverse of the Hubble constant of expansion.

25. For any object moving uniformly, velocity = distance/time. So in the Hubble relationship for the expansion of the universe, v = H₀r, what is the significance of the constant 1/H₀?

A) This quantity represents the average spacing between objects in the universe at the present time.

B) This quantity represents the time since the expansion began, or the age of the universe.

C) This quantity is merely a constant of proportionality to allow for the different units in v and r.

D) This quantity is the inverse of the velocity that the object would have at a standard distance of 10 parsecs.

26. Suppose new experimental evidence were to point to a Hubble constant value of H₀ = 50 km/s/Mpc. A calculation of the age of the universe using this value would give

A) the same value now calculated, namely 13.4 billion years.

B) an age younger than 13.4 billion years.

C) an age older than 13.4 billion years.

D) a different value of the age, but more information is needed to estimate what it would be.

27. Suppose the expansion of the universe has been slowing since the Big Bang, and the value H₀ is the Hubble constant that is appropriate to the universe today. What would this say about the age of the universe?

A) The age should still be 1/H₀ = 13.4 billion years.

B) The age would be less than 13.4 billion years.

C) The age would be greater than 13.4 billion years.

D) It is not possible to answer the question based only on this information.

28. Suppose that the expansion rate were slower in the past. Taking this fact into account when estimating the age of the universe from the presently accepted value of the Hubble constant, H₀ = 73.5 km/s/Mpc, yields

A) 13.4 billion years, as before.

B) a value less than 13.4 billion years.

C) a value greater than 13.4 billion years.

D) a value different from 13.4 billion years, but additional information is needed to determine whether it is larger or smaller.

29. In order to estimate the age of the universe as 1/H₀ , astronomers must make a number of assumptions. Which of these is NOT one of the necessary assumptions?

A) The universe’s expansion rate is the same in all directions.

B) The Milky Way Galaxy is the center of the universal expansion.

C) The value of H₀ is indeed constant.

D) The speed of the expansion has been constant through time.

30. An elliptical galaxy in Boötes (shown at the bottom of Figure 17-32 in the text) is at a distance of 1.6 billion ly and has an apparent recession speed of 39,000 km/s. What upper limit does this imply to the age of the universe? (Be careful with units.)

A) 0.47 billion years

B) 12 million years

C) 47 billion years

D) 12 billion years

31. If the Hubble constant is 75 km/s/Mpc, then the age of the universe is 13 billion years. Suppose it were discovered that the Hubble constant is actually larger than 75 km/s/Mpc. What effect would this fact have on the estimated age of the universe?

A) The estimated age could be increased or decreased, depending on the recession velocity of the galaxy being investigated.

B) The estimated age would be the same.

C) The estimated age would be decreased.

D) The estimated age would be increased.

32. Astronomer A claims that the Hubble constant is 84 km/s/Mpc, while astronomer B claims that it is 63 km/s/Mpc. The age of the universe calculated by astronomer A would be

A) 1.33 times the age calculated by astronomer B.

B) 2/3 of the age calculated by astronomer B.

C) 3/4 of the age calculated by astronomer B.

D) 1.25 times the age calculated by astronomer B.

33. What would the age of the universe be if the Hubble constant, H₀, is 90 km/s/Mpc?

A) 17 billion years

B) 11 billion years

C) 15 billion years

D) 9 billion years

34. What would the age of the universe be if the Hubble constant, H₀, is 60 km/s/Mpc?

A) 15.0 billion years

B) 16.6 billion years

C) 60 billion years

D) 11.1 billion years

35. Suppose H₀ = 71 km/s/Mpc. What is H₀ in m/s/ly?

A) 2.2  10^–3

B) 2.2  10^–2

C) 22

D) 2.2  10³

36. Where is Earth?

A) at the exact center of an expanding universe, as shown by the universal expansion away from Earth in all directions

B) near the edge of an expanding universe, as shown by the microwave radiation coming to Earth from the edge

C) near but probably not right at the center of the universe, as shown by the fact that the edge is so far away from Earth

D) somewhere in an expanding universe but not in any special part of it

37. Because of the general expansion of space, all distant galaxies appear to be moving away from Earth, with speeds that increase with distance from the Milky Way Galaxy. What would an observer in one of these distant galaxies see?

A) same thing as seen from Earth: all galaxies moving away, with more distant galaxies moving faster

B) all galaxies moving nearer, with more distant galaxies moving faster

C) all galaxies moving away, with closer galaxies moving faster

D) all galaxies on one side of the sky moving nearer and all galaxies on the other side moving away, with more distant galaxies moving faster

Section: 19-4

38. Who first proposed that the observed motion of galaxies away from Earth originated as an expansion outward from an extremely dense state of matter at the beginning of the universe?

A) Edwin Hubble

B) George Gamow

C) Fred Hoyle

D) Georges Lemaître

39. Who first proposed that the observed motion of galaxies away from Earth implied the existence of a radio background pervading the universe?

A) Robert Dicke

B) George Gamow

C) Fred Hoyle

D) Edwin Hubble

40. Who developed the steady-state theory of the universe?

A) Dicke and Peebles

B) Alpher and Gamow

C) Hoyle, Bondi, and Gold

D) Penzias and Wilson

41. How does the universe behave, according to the steady-state theory?

A) As the universe expands, new matter is created from which new galaxies form, thus maintaining a “steady state.”

B) New matter is being continuously created, which adds to the absorption of light in the universe and makes distant galaxies seem farther and farther away.

C) The universe is static, not expanding or contracting, but new matter is being created so that as old galaxies die, new galaxies form to take their place.

D) The universe is static, neither expanding nor contracting, thus maintaining a “steady state” in which no change takes place.

42. Which single observation is perhaps the strongest argument against the steady-state model of the universe and for the Big Bang model?

A) The number of supporters of the steady-state model is less than the number of supporters of the Big Bang model.

B) Astronomers have not observed matter being created from nothing in the space around Earth.

C) The universe is expanding.

D) The universe is bathed in a sea of microwaves coming from the edge of the visible universe.

43. The Big Bang theory is the most widely accepted model of the early universe today. Which of these is NOT one of the reasons for this?

A) The observed expansion of the universe is consistent with the Big Bang theory.

B) No alternate theories have been proposed.

C) The observed cosmic microwave background is consistent with the Big Bang theory.

D) The observed cosmic neutrino background is consistent with the Big Bang theory.

44. What is the cosmic microwave background radiation?

A) uniform background of radiation from electrons spiraling in weak intergalactic magnetic fields

B) radiation left over from the Big Bang after the universe expanded and cooled

C) almost uniform background of radiation from distant, unresolved, overlapping galaxies

D) radiation from a very tenuous, ionized gas that fills the universe equally in all directions

45. The cosmic background radiation is

A) the flux of visible radiation in empty space, contributed by all visible stars in the universe.

B) the radio noise generated by Earthbound transmitters, spreading out into space since about 1920.

C) the beam of atomic nuclei known as cosmic rays that continuously rain down on Earth from all directions in space.

D) low-intensity radio noise, with a 3 K blackbody temperature, almost uniform in intensity in all directions.

46. The cosmic background radiation is the

A) result of the radioactive decay of heavier, unstable elements produced in supernova explosions.

B) faint glow along the elliptic, caused by sunlight scattering from dust particles.

C) electromagnetic remnants of the incredibly hot, dense early universe.

D) radio noise from hot gas in rich clusters of galaxies.

47. Good evidence for an original Big Bang that “created” this universe comes from

A) the rapid motions of some nearby stars, such as Barnard’s Star.

B) a background “glow” of microwaves, with a blackbody temperature of about 3 K.

C) the measurement of the rotation of the Milky Way Galaxy.

D) the amount of gas and dust in the solar neighborhood.

48. The cosmic microwave background was discovered by

A) rocket-borne telescopes that also discovered X-ray sources in space.

B) the Voyager 2 spacecraft during one of its “coasting” periods between planetary encounters.

C) scientists testing a new antenna and receiver for satellite communications.

D) the Infrared Astronomical Satellite (IRAS), which produced an all-sky infrared survey.

49. Who discovered the cosmic microwave background radiation, the radiation left over from the Big Bang?

A) Ralph Alpher and George Gamow

B) Arno Penzias and Robert Wilson

C) Anthony Hewish and Jocelyn Bell

D) Robert Dicke and P. J. E. Peebles

50. Who first predicted the existence of the cosmic microwave background radiation, the radiation left over from the Big Bang?

A) Ralph Alpher and George Gamow

B) Arno Penzias and Robert Wilson

C) Anthony Hewish and Jocelyn Bell

D) Robert Dicke and P. J. E. Peebles

51. How was the cosmic microwave background radiation discovered?

A) using a microwave detector on the Hubble Space Telescope

B) using the Cosmic Background Explorer (COBE) spacecraft

C) using a communications antenna on Earth’s surface

D) using the Ulysses spacecraft observing from above the Sun’s north pole

52. What is the temperature of the blackbody radiation that Earth receives from the Big Bang (the cosmic microwave background radiation)?

A) 30 K

B) 30 billion K

C) 3 K

D) 3000 K

53. The cosmic background radiation that was left over after the Big Bang of the universe and that pervades all observable space has an effective blackbody temperature of approximately

A) 273 K.

B) 0 K.

C) 10 K.

D) 3 K.

54. The cosmic microwave background radiation forms a blackbody curve corresponding to a temperature of 2.73 K. What does Wien’s law predict for the peak wavelength in this blackbody curve?

A) 0.654 mm

B) 1.07 mm

C) 21.1 cm

D) 2.73 m

55. If the cosmic microwave background radiation is the radiation left over from the Big Bang, why is its temperature only 3 K?

A) The universe has expanded and cooled to the temperature now observed for the background radiation.

B) The cosmic microwave background is only the cool tail of the background radiation, but the higher energies are blocked by galaxies and intergalactic clouds.

C) The Big Bang was a cold explosion, not a hot explosion—its temperature was the same as the current temperature observed from the cosmic background radiation.

D) The cosmic background radiation observed now is not from the Big Bang at all—it is from cold, intergalactic hydrogen clouds that cover the sky.

56. What was the COBE satellite designed to measure?

A) redshifts of objects at cosmological distances to obtain an accurate measurement of the Hubble constant

B) 21-cm radio radiation from intergalactic hydrogen

C) X-rays from quasars and other objects at cosmological distances

D) cosmic microwave background radiation

57. When the intensity of the cosmic microwave background radiation is plotted against wavelength, what is the shape of the resulting curve?

A) emission lines, strongest and most densely concentrated in the microwave region

B) composite of many overlapping blackbody curves from gas clouds of different temperatures peaking in the microwaves

C) blackbody curve modified by many deep, overlapping absorption lines and several emission lines

D) essentially perfect blackbody spectrum peaking in the microwave region

58. Astronomers studying the cosmic microwave background have recently inferred the existence of another relic of the Big Bang that pervades the universe. What makes up this other background?

A) gamma rays

B) cosmic rays

C) magnetic fields

D) neutrinos

59. The greatest support for the Big Bang theory of cosmology came from the discovery of

A) the cosmic microwave background.

B) the cosmic neutrino background.

C) isotropy in the universe.

D) homogeneity in the universe.

Section: 19-5

60. Astronomers detect the slight anisotropy in the cosmic microwave background by making redshift measurements: wavelengths coming from the direction of Aquarius are slightly longer and those from the direction of Leo are slightly shorter. This redshift is a

A) Doppler shift.

B) gravitational redshift.

C) cosmological redshift.

D) Doppler shift, gravitational redshift, or cosmological redshift since all three are one and the same thing.

61. The cosmic microwave background radiation is not uniform over the sky—it is slightly hotter toward the constellation Leo and slightly cooler in the opposite direction, toward Aquarius. Why?

A) The difference is probably a statistical fluctuation and therefore not real.

B) The background radiation really is uniform; the observed difference is due to Earth’s motion through the universe.

C) Earth is slightly off-center in the universe, so one side of the universe is a bit closer and the other is a bit farther away.

D) That is the way the universe began—hotter in one direction and cooler in the other.

62. In relation to the universe, what does “isotropy” mean?

A) The universe has the same expansion speed at all distances.

B) The universe is the same at all distances.

C) The universe at any given distance is the same at all times.

D) The universe looks the same in all directions.

63. What do astronomers mean when they say that the universe is homogeneous?

A) The number of galaxies, averaged over large scales, is the same at any distance from Earth.

B) There are no mass concentrations anywhere in the universe.

C) The universe is static and unchanging.

D) At any given time, the universe looks the same in all directions.

64. Why does the cosmic microwave background appear to be slightly warmer in one direction in the sky and slightly cooler in the opposite direction?

A) The warmer direction is the direction in which the Big Bang occurred; hence, astronomers are seeing the remnant of the explosion in this direction.

B) The universe is younger in one direction and therefore warmer.

C) The radiation in one direction is Doppler-shifted to shorter wavelengths by Earth’s motion in space and to longer wavelengths in the other direction.

D) Large amounts of matter in the warmer direction have focused the radiation slightly by gravitational lensing, making this direction appear hotter.

65. The effect that makes the cosmic microwave background appear slightly warmer in one direction and cooler in the opposite direction is

A) a basic asymmetry in the background radiation, related to its origin.

B) the Doppler shift caused by motion of the Milky Way Galaxy through space toward the constellation Leo.

C) microwave emission from cool, primordial (pregalactic) clouds of gas and dust in that direction.

D) the presence of large clusters of galaxies in only one direction.

66. The Milky Way Galaxy is moving at about 600 km/s relative to the cosmic microwave background radiation. Why?

A) The Milky Way is being gravitationally attracted by several nearby superclusters of galaxies.

B) This speed is the orbital speed of the Local Group of galaxies around the Virgo cluster.

C) There is no particular reason. All galaxies have “small” random speeds in addition to the universal expansion, and 600 km/s is the speed of the Milky Way.

D) This speed is the orbital speed of the Milky Way Galaxy around the Andromeda Galaxy.

67. The Milky Way Galaxy is moving at about 600 km/s relative to the cosmic microwave background. How does this compare to Earth’s speed in its orbit and to the speed of the solar system in its orbit around the galactic center?

A) This speed is less than both the speed of the solar system and Earth’s orbital speed.

B) This speed is greater than Earth’s orbital speed but less than the solar system’s speed.

C) This speed is greater than the solar system’s speed but less than Earth’s speed in its orbit.

D) This speed is greater than both Earth’s speed in its orbit and the solar system’s speed.

68. When the isotropy of the cosmic microwave radiation was first realized, what effect did this discovery have on the contemporary theories of cosmology?

A) The discovery was a great support for the Big Bang cosmology that predicts this isotropy.

B) The discovery was a great support for the steady-state cosmology that predicts this isotropy.

C) The discovery was a great problem because it was prohibited by all the main cosmologies.

D) The discovery was a puzzle because the Big Bang cosmology does not predict this isotropy, although it does not prohibit it either.

Section: 19-6

69. How many fundamental forces are known in science at the present time under normal conditions?

A) four

B) five

C) three

D) six

70. How many fundamental forces are there in nature at the present time under normal conditions?

A) three: strong, electromagnetic, and gravitational

B) six: color, strong, weak, magnetic, electric, and gravitational

C) four: strong, weak, electromagnetic, and gravitational

D) five: strong, weak, magnetic, electric, and gravitational

71. What is the range of the electromagnetic force (the maximum distance over which it acts)?

A) 10^–15 m (1 femtometer, or roughly the size of a proton)

B) 10^–9 m (1 nanometer, or roughly the size of a hydrogen atom)

C) a few thousand meters, or roughly the size of Earth

D) infinity

72. Which of the four fundamental forces holds the electrons in the atom?

A) strong nuclear force

B) gravitational force

C) weak nuclear force

D) electromagnetic force

73. What is the range of the gravitational force (the maximum distance over which it acts)?

A) 10¹³ m, or roughly the size of the solar system

B) 10²¹ m, or roughly the size of the Milky Way Galaxy

C) infinity

D) 10²⁶ m, or roughly the distance to the farthest quasars

74. The forces of gravity and electromagnetism are long-range forces, extending in principle from their source (mass and electric charge, respectively) to infinity. Why is it that, in the universe, only gravity extends to infinity, whereas electromagnetic forces are much more limited in extent?

A) Gravity and electromagnetism are one and the same force, with electromagnetic effects extending over limited spatial ranges and transforming into gravitational forces at large distances from matter.

B) Electromagnetic forces from charged particles will move other charged particles around to produce a uniform charge distribution and therefore zero electromagnetic forces, whereas gravity concentrates mass and enhances the overall gravity force.

C) Electromagnetic forces from positive charges are canceled by negative charges, whereas there are no negative “masses” to cancel the gravitational force.

D) All atoms are electrically neutral, so in reality the electromagnetic force never reaches beyond the size of an atomic nucleus.

75. Which two forces are infinite in extent?

A) gravity and electromagnetism

B) gravity and the strong nuclear force

C) electromagnetism and the strong nuclear force

D) the two nuclear forces, strong and weak

76. The one physical force that extends farthest in this universe and is NOT canceled out by other effects is the

A) weak nuclear force.

B) strong nuclear force.

C) electromagnetic force.

D) gravitational force.

77. Gravity holds galaxies together. What does the weak nuclear force hold together?

A) leptons (particles including electrons and neutrinos)

B) nothing

C) nuclei

D) quarks inside protons and neutrons

78. When is the weak nuclear force encountered?

A) when a quark is transformed into another quark

B) when a positively charged nucleus repels another positively charged nucleus in the core of a star like the Sun

C) when two quarks interact inside a proton or neutron

D) when an atom absorbs a photon and one of the electrons in the atom is sent into a higher energy level

79. The weak force

A) acted only during the Big Bang and has no known role in the universe at the present time.

B) holds the quarks together inside a proton or neutron.

C) attracts the electrons to the nucleus, holding the atom together.

D) acts during certain kinds of radioactive decay.

80. The physical force that controls the structure of the nucleus and binds together protons and neutrons is the

A) gravitational force.

B) weak nuclear force.

C) electromagnetic force.

D) strong nuclear force.

81. The Theory of Everything is the attempt to describe all four fundamental forces as aspects of a single unified force. The force that has proved to be the MOST difficult to incorporate into such a theory is

A) gravity.

B) electromagnetism.

C) the weak nuclear force.

D) the strong nuclear force.

82. Physicists are attempting to develop a Theory of Everything. What would be unique about such a theory?

A) It would describe both relativistic and nonrelativistic physics.

B) It would include the actions of the electroweak force.

C) It would include the effects of all four fundamental forces.

D) It would predict the end fate of the universe.

83. What is the range of the strong nuclear force compared with the size of a large atomic nucleus, 10^–14 m?

A) the same since it is the strong force that holds the nucleus together

B) 10 times larger than the size of an atomic nucleus

C) infinite; it has no limit

D) 10 times smaller than the size of an atomic nucleus

84. The four physical forces at work in the universe are gravitational, electromagnetic, strong nuclear, and weak nuclear forces. Which two of these are very short-ranged, extending over distances of only about 10^–15 m?

A) strong nuclear and electromagnetic forces

B) electromagnetic and weak nuclear forces

C) strong and weak nuclear forces

D) gravitational and electromagnetic forces

85. What makes gravity the main force determining the large­-scale structure of the universe?

A) Gravity is the strongest of the four fundamental forces.

B) Gravity is the only force that has an infinite range.

C) While the electromagnetic force of a charged particle has an infinite range, it is usually screened (and limited) by the existence of charges of the opposite sign. Ordinary matter always exerts gravity that is attractive.

D) The gravitational force, while short-ranged, is the only force that affects all particles: photons, quarks, charged particles, uncharged particles.

86. What are quarks?

A) particles of zero electric charge and zero mass that are emitted by nuclear reactions in the Sun’s core

B) component particles making up electrons

C) antielectrons (the antimatter form of electrons)

D) component particles making up protons and neutrons

87. What are the particles that make up protons and neutrons?

A) quarks

B) muons

C) gravitons

D) neutrinos

88. In modern particle physics, the proton and the neutron are now thought to be composed of more fundamental particles called

A) photons.

B) gluons.

C) neutrinos.

D) quarks.

89. How many quarks are there in a proton or a neutron?

A) three

B) one

C) six

D) four

90. What is the difference between a proton and a neutron in terms of their constituent quarks?

A) A proton is made of two “up” quarks and a “down” quark, and a neutron is made of two “down” quarks and an “up” quark.

B) A proton is made of two “down” quarks and an “up” quark, and a neutron is made of two “up” quarks and a “down” quark.

C) A proton is made of three “up” quarks, and a neutron is made of three “down” quarks.

D) A proton is made of three “down” quarks, and a neutron is made of three “up” quarks.

91. How do physicists know that the fundamental forces become unified as the energy of particle interactions increases?

A) The theory that says that they do is a very elegant one, and it would be nice if it were right.

B) Experiments have actually been done with high-energy particle accelerators where all four forces have been unified.

C) Although it is not possible to see unification in the laboratory, the theory of unification agrees with many of the observed properties of the universe.

D) Physicists have actually seen the start of the unification in high-energy particle accelerators, where the electromagnetic and weak forces become unified.

92. In what way does the study of the collisions of very high-speed nuclear particles with other matter at particle accelerator laboratories help in the understanding of the early universe?

A) The temperature of the early universe was so high that mutual collisions of particles occurred at energies equivalent to those in nuclear particle accelerators.

B) Nuclear accelerators generate enormous quantities of microwaves similar to those that existed in the early universe, thus allowing the study of the interaction of microwaves with matter.

C) Motions of particles in circular orbits around the original black hole produced by the Big Bang were similar to the particle motions in an accelerator.

D) Collisions in nuclear accelerator laboratories produce large numbers of neutrinos and hence mimic the conditions that were thought to exist in the early universe, where neutrinos occupied most of the space.

93. What is the “electroweak” force?

A) This is a fifth force, distinct from the original four, which helps to explain dark energy.

B) When charged particles of both signs (positive and negative) are closely mixed, the electromagnetic force—generally infinite in extent—is limited in its practical range. Under these circumstances, it is termed electroweak.

C) At sufficiently high temperatures the electromagnetic force and the weak nuclear force are indistinguishable. This composite force is called electroweak.

D) At sufficiently high temperature the strong nuclear force and gravitational force disappear. At even higher temperatures the weak nuclear force and the electromagnetic force combine to form the electroweak force. It is the only force that exists at this temperature.

94. The electromagnetic and weak nuclear forces are predicted to have been indistinguishable at some stage in the early universe. What conditions were required at this stage?

A) extremely high temperatures, producing very energetic collisions between components of matter

B) very low density of matter, such that the weak force became as strong as the electromagnetic force in this region of space

C) very high density of matter, such as the interior of the nucleus of an atom or a neutron star, where particles are so close that weak forces become equivalent to electromagnetic forces

D) extremely low temperatures, where collisions of particles were of low energy and extremely infrequent, such that electromagnetic and weak forces were equivalent

95. At what times during the Big Bang were all four fundamental forces unified?

A) until 10^–24 s, after the start of the Big Bang, when the inflationary era ended

B) during the first 500,000 years from the start of the Big Bang, when the universe was dominated by radiation

C) until 10^–6 s after the start of the Big Bang, when the era of quark confinement ended

D) during the Planck era, up to 10^–43 s, after the start of the Big Bang

96. The Planck era refers to the

A) time at which the expanding universe became transparent to radiation.

B) time of extremely rapid inflation that started 10^–35 s after the universe began.

C) first 10^–43 s of time, when all four fundamental forces are believed to have been united.

D) present age of the universe as given by the Hubble law.

97. The first 10^–43 s of the age of the universe, during which all four fundamental forces are believed to have been united, is called the

A) Planck era.

B) inflationary era.

C) event horizon.

D) Hubble time.

98. Within which time frame from the initial Big Bang do cosmologists believe all four fundamental forces of nature were united into a single force?

A) t = 0 to 10⁶ years, when radiation dominated the universe

B) t = 0 to 1 s, when photons interchanged freely with electron-positron pairs

C) t = 0 to 10^–35 s, when the strong nuclear force “froze out” of the universe

D) t = 0 to 10^–43 s, the Planck time, when gravity “froze out” of the universe

99. Immediately after the cosmological singularity, the four forces were indistinguishable. Then they began to “branch off” and become separate. Which was the last of the four to branch off from the electromagnetic force?

A) gravitation

B) weak force

C) strong force

D) The other forces branched off at the same instant in time.

100. The first 10^–43 seconds of the universe’s existence are called the Planck era. The kind of physics needed to describe that era is

A) Newtonian physics.

B) general relativity.

C) quantum physics.

D) presently unknown.

101. Grand unified theories, or GUTs, predict that at temperatures above 10²⁷ K, the

A) strong and weak forces are indistinguishable from each other, but the other forces are different.

B) strong, weak, electromagnetic, and gravitational forces are indistinguishable from each other.

C) weak and electromagnetic forces are indistinguishable from each other, but the other forces are different.

D) strong, weak, and electromagnetic forces are indistinguishable from each other, but gravity is different.

Section: 19-7

102. Physicist’s equations can describe the evolution of the universe since

A) before the Big Bang.

B) a fraction of a second after the Big Bang.

C) 30,000 years after the Big Bang.

D) 380,000 years after the Big Bang.

103. Which of these statements describes the state of the universe during the inflationary epoch, from approximately 10^–35 s to 10^–33 s after the Big Bang?

A) There were only two distinct forces: the strong, weak, and electromagnetic theories were all unified at that time.

B) There were only three distinct forces: gravity, strong, and electroweak forces.

C) Only quarks existed but they had not yet formed into other elementary particles.

D) Elementary particles such as neutrons and protons existed but they had not yet formed nuclei.

104. Physicists speculate that, early in its history, the universe shifted from a “false vacuum” state to a “true vacuum” state. What is the significance of this transition?

A) The “false vacuum” describes the state of the universe before the Planck time, 10^–43 s. During this time the laws of physics as are currently understood did not apply.

B) The transition was from a high-energy state to a low-energy state. The transition caused the inflationary epoch.

C) The transition was one from a low-energy state to a high-energy state. The consequent intake of energy caused the universe at the end of the inflationary epoch to be much cooler than it had been at the beginning of the inflationary epoch.

D) In its transition to the true vacuum, the universe became trapped in a state of minimum energy, unable to expand or contract.

105. The predicted temperature of the early universe at which the four forces of nature would have been unified is

A) 10²⁷ K.

B) 10³² K.

C) 10⁶ K.

D) 10¹⁵ K.

106. At the start of the Big Bang, all the fundamental forces were unified and behaved like a single force. As the universe expanded and cooled during the Big Bang, which was the first force to become a separate force?

A) strong nuclear force

B) weak nuclear force

C) electromagnetic force

D) gravity

107. When the Big Bang began, all the fundamental forces were unified and behaved like a single force. As the universe expanded and cooled during the Big Bang, when did gravity first become a separate force?

A) at the start of quark confinement, when the universe was about 10^–6 s old

B) at the start of the inflationary period, when the universe was about 10^–35 s old

C) at the end of the inflationary period, when the universe was about 10^–33 s old

D) at the Planck time, when the universe was about 10^–43 s old

108. When the Big Bang began, all the fundamental forces were unified and behaved like a single force. As the universe expanded and cooled during the Big Bang, when did the strong nuclear force first become a separate force?

A) at the start of quark confinement, when the universe was about 10^–6 s old

B) when pair production ceased, when the universe was about 1 s old

C) at the Planck time, when the universe was about 10^–43 s old

D) at the start of the inflationary period, when the universe was about 10^–35 s old

109. Approximately when did the inflationary period begin?

A) when the strong force became a separate force

B) when pair production ceased

C) when gravity became a separate force

D) when the electromagnetic force became a separate force

110. In cosmology, what is the “inflationary period”?

A) the period when the cost of living rose faster than astronomers’ salaries

B) the first 500,000 years of the life of the universe, when matter and radiation interacted vigorously

C) the period of universal expansion from the Big Bang to the present

D) the short period of extremely rapid expansion when the universe was very young

111. What caused the Big Bang and the expansion of the universe?

A) separation of the strong nuclear force as a distinct force

B) decoupling of radiation from matter

C) unknown cause, with expansion starting suddenly and continuing to this time

D) universal repulsive force stronger than the gravitational attraction between matter in the universe

Section: 19-8

112. What is the isotropy (or horizon) problem?

A) The universe is not as uniform as the standard cosmological model predicts it should be.

B) If the universe expanded at the rate originally predicted, then the parts seen in opposite directions would never have been in equilibrium with each other and should not exhibit the degree of isotropy observed.

C) Without modification, the standard theory suggests that an observer on a distant galaxy would not be able to see the Milky Way Galaxy because of the intense cosmological redshift.

D) As astronomers observe galaxies on the “horizon,” the farthest distance obtainable, the standard theory predicts that the recessional velocity should approach the speed of light.

113. If the universe had expanded at its original rate beyond the time when the strong nuclear force became a distinct force (about 10^–35 s after the Big Bang), the isotropy problem would have arisen: Parts of the universe seen in opposite directions would never have been in equilibrium with each other and should not exhibit the degree of isotropy observed in the cosmic microwave background. The proposed solution to this problem is

A) the cosmological redshift.

B) quark confinement.

C) cosmic inflation.

D) the Grand Unified Theory (GUT).

114. What is the “particle horizon”?

A) The particle horizon is the distance beyond which astronomers cannot see because light cannot have traveled any farther in the finite age of the universe.

B) The particle horizon is the distance beyond which astronomers cannot see because no stars or galaxies had yet formed at those distances (because astronomers see back in time as they look out into space).

C) The particle horizon is the distance beyond which signals from Earth radio and television signals cannot have traveled through the universe.

D) The particle horizon is the distance beyond which astronomers cannot see because of absorbing matter in the universe.

115. The cosmic microwave background is found to be extremely uniform throughout space, with only very small fluctuations in intensity. The event that produced this remarkable smoothness in the early universe was

A) the occurrence of the Big Bang everywhere in space at the same time.

B) the start of the production of matter in the universe, which smoothed out the irregularities in space.

C) an exemplar of Heisenberg’s uncertainty principle, which prevented the concentration of radiant energy in localized volumes of space.

D) a sudden but brief period of rapid expansion (cosmic inflation) of the universe during the general expansion of the early universe.

116. The isotropy of the cosmic microwave background radiation (same temperature in all directions) indicates that

A) the universe had an early period of inflation in which regions initially in equilibrium were carried to huge separations from each other.

B) the universe did not begin to expand significantly until after the era of recombination.

C) regions that appear to observers to be on opposite sides of the visible universe are in fact in close contact with each other.

D) the universe has always been dominated by matter.

117. Opposite sides of the universe have the same temperature, yet according to the standard Big Bang theory, these points are too far apart for light to have traveled from one to the other in the age of the universe; that is, they cannot have exchanged heat to even out their temperature. Why, then, do they have the same temperature?

A) Light (and heat) could travel much faster in the early universe, allowing them to exchange heat while the universe was young.

B) The expansion of the universe has always been the same everywhere; therefore, all parts of the universe have the same temperature regardless of whether they have ever exchanged heat or not.

C) It is pure coincidence that opposite sides of the universe have the same temperature.

D) Opposite sides of the universe were originally close together and evened out their temperature, and then a rapid inflation of the universe carried them far apart.

118. During the epoch of cosmic inflation, which of these properties of the universe changed rapidly?

A) temperature

B) pressure

C) symmetries of the four forces

D) the separation of two points in spacetime

119. Why does the observable universe have an “edge”?

A) Astronomers cannot see any farther out into space than the distance that light has traveled over the lifetime of the universe.

B) The density of neutrinos at the “edge” becomes so large that photons cannot penetrate this barrier, and this prevents observers from seeing beyond this point.

C) Absorbing matter prevents observers from seeing beyond a certain distance.

D) There are so many galaxies in the universe that every line of sight eventually hits a galaxy, stopping observers from seeing any farther.

120. The view of the universe from Earth is limited because of what fundamental fact?

A) The cosmological redshift has moved the light from very distant objects out of Earth’s detectable range, making these objects invisible to observers.

B) The matter expanding into space from the Big Bang extends only so far since matter can travel only at or below the speed of light.

C) Light from objects farther away than a certain distance, defined by the travel time of light in the lifetime of the universe, has not yet reached Earth.

D) Intergalactic space contains absorbing material that blocks the view of more distant objects from Earth.

Section: 19-9

121. What order of events do cosmologists predict in the early universe, from earlier to later?

A) Planck time, Big Bang, inflation, formation of protons and neutrons

B) Big Bang, Planck time, formation of protons and neutrons, inflation

C) Big Bang, inflation, Planck time, formation of protons and neutrons

D) Big Bang, Planck time, inflation, formation of protons and neutrons

122. The symmetry breaking that led to the existence of matter is MOST likely associated with which one of the fundamental forces?

A) gravitation

B) electromagnetic force

C) strong nuclear force

D) weak nuclear force

123. The fact that matter exists today is evidence that matter particles outnumber antimatter particles in the universe. This imbalance was caused by

A) matter inflation.

B) symmetry breaking.

C) chirality.

D) the cosmic overshift.

124. Which of these four periods of time describes this universe at present?

A) inflationary period, in which the universe expands rapidly

B) era of recombination, in which electrons and protons combine to form hydrogen atoms

C) the Planck era, in which all four forces of nature are unified

D) period of confinement, in which quarks are unable to travel freely

125. When two high-energy gamma-ray photons collide, the gamma-ray photons could

A) produce a huge number of low-energy photons.

B) disappear completely, leaving nothing behind.

C) disappear, creating a particle-antiparticle pair.

D) disappear, creating two negative electrons.

126. When a gamma-ray photon undergoes pair production, what is one possibility for the final products?

A) an electron and a positron (the antielectron)

B) an electron and an antineutrino

C) a proton, an electron, and an antineutrino

D) two X-ray photons, each with half the energy of the original gamma ray

127. What experimental evidence do physicists have for the direct transformation of energy into matter?

A) increase in the force upon a body when it is accelerated upward (e.g., in an elevator)

B) conversion of gamma-ray photons into electrons and antielectrons (i.e., positrons)

C) blueshift of light because of the motion of its source

D) relativistic increase in mass of an object when it is moving very rapidly

128. What is the period of quark confinement?

A) Because of the very large pressure in early times, all the quarks were confined to a small volume. After the inflationary epoch, the pressure dropped and the quarks were able to spread out to assume the distribution found today.

B) During the period of quark confinement, the energy of the photons was sufficiently high that conglomerations of quarks, such as neutrons and protons, could not exist and quarks were free.

C) During the period of quark confinement, the energy of the photons was sufficiently low that conglomerations of quarks, such as neutrons and protons, could exist without being blasted apart as soon as they were formed.

D) The period of quark confinement was the very early period in the universe when all matter and energy were confined to a region the size of a single quark.

129. During the first millionth of a second in the history of the universe, a hydrogen nucleus could NOT exist. Why not?

A) During this time quarks existed in isolation and so protons did not exist.

B) The electromagnetic force did not exist during this era.

C) Cosmic inflation had pushed protons and electrons too far apart to interact with each other.

D) Protons and antiprotons were continually annihilating each other, and there were no protons to form hydrogen atoms.

130. Within the first second after the Big Bang, the universe was filled with high-energy, gamma-ray photons. These photons created huge numbers of elementary particles and their antiparticles through pair production. What happened to these particles?

A) They are still here. They combined to create the first atoms and molecules that compose the physical universe.

B) The particles are still here as atoms and molecules, but almost all of the antiparticles disappeared.

C) The antiparticles annihilated with normal matter particles to produce more photons.

D) This pair production was followed by the era of cosmic inflation, during which these particles spread out to produce the matter densities that astronomers see today.

131. Why did the formation of particle-antiparticle pairs (such as protons and antiprotons) from gamma rays come to an end in the first second after the start of the Big Bang?

A) Expansion of the universe carried the gamma rays too far apart for them to collide and form particle-antiparticle pairs.

B) The gamma-ray energy decreased as the universe expanded.

C) The universe became transparent to radiation, so gamma rays and particles no longer interacted.

D) The formation of particle-antiparticle pairs exhausted the supply of gamma rays, preventing further formation of particle-antiparticle pairs.

132. During the first one-ten-thousandth of a second (10^–4 s) of the life of the universe, antiprotons were very common. For every billion antiprotons, how many protons were there?

A) exactly 1 billion since protons and antiprotons were created in equal numbers

B) slightly more than 1 billion, thus producing the matter seen today

C) totally unknown number since the early universe was opaque and astronomers cannot see what conditions were like then

D) 10 billion, thus producing the dark matter seen today

Section: 19-10

133. The photons that enter microwave detectors today (and that astronomers interpret as the cosmic microwave background radiation) had their last interaction with matter

A) at the instant of the Big Bang.

B) during the inflationary epoch.

C) during the era of recombination.

D) at the time of symmetry breaking.

134. When the universe was about 30,000 years old, the radiation density had decreased to the point where it was equal to the matter density. Yet it was only many years later that atoms formed. What happened to make possible the formation of atoms?

A) The average photon energy decreased until it could no longer ionize hydrogen.

B) Radiation pressure decreased until it was equal to the inward pressure due to gravity.

C) The temperature reached the point where the reaction 4 ¹H  ⁴He could proceed.

D) Neutrons decayed to produce protons and electrons, the building blocks of atoms.

135. If astronomers could have turned a receiver toward the heavens in hopes of observing the cosmic microwave radiation 4.5 billion years ago, at the time the solar nebula was forming, what would they have found?

A) They would have seen nothing since the cosmic microwave background had not yet been created.

B) They would have seen the same blackbody curve seen now, with a peak intensity at a wavelength of about 1 mm.

C) They would have seen a blackbody curve of the same shape seen now but with a peak intensity at a wavelength shorter than 1 mm.

D) They would have seen a blackbody curve of the same shape seen now but with a peak intensity at a wavelength longer than 1 mm.

136. As astronomers look at more distant regions of space, they see the regions as they existed at earlier times, but their farthest views are blocked by a “wall” beyond which the universe is opaque. What event occurred at the time marked by this wall?

A) Quarks combined to form neutrons and protons.

B) Electrons and protons combined to form neutral hydrogen atoms.

C) Protons combined with neutrons to form helium nuclei.

D) Gravity froze out as a separate force.

137. How was MOST of the helium in the universe created?

A) by nuclear reactions in the cores of stars and then thrown out into space by supernovae

B) by high-energy processes during the collapse of pregalactic clouds during the formation of galaxies

C) by nuclear reactions during the first 3 minutes following the Big Bang

D) by the collision of cosmic rays with hydrogen nuclei in interstellar gas clouds

138. During what time interval was helium created in the Big Bang?

A) during the first 10^–43 s

B) during the first 10^–6 s

C) during the first 500,000 years

D) during the first 3 minutes

139. Which element(s) was (were) created during the Big Bang?

A) hydrogen and helium

B) hydrogen, helium, lithium, and beryllium

C) hydrogen

D) hydrogen, helium, and lithium

140. At about what time after the Big Bang did the universe pass through the transition from being radiation-dominated to being matter-dominated?

A) at the Planck time, 10^–43 s

B) about 15 billion years, as stars began to form

C) 500,000 years

D) 30,000 years

141. In the present universe, the “mass density” of radiation, using Einstein’s equation E = mc² for the photons, is

A) essentially zero since there is very little radiation left in the universe at the present time.

B) much greater than the mass density of matter leading to a radiation-dominated universe.

C) equal to the mass density of matter, since these parameters have remained balanced throughout the evolution of the universe as a consequence of the equipartition of energy.

D) much less than the mass density of matter, leading to a matter-dominated universe.

142. What significant event occurred about 400,000 years after the start of the Big Bang?

A) All of the galaxies seen today formed.

B) Electrons and nuclei combined to form neutral atoms.

C) Quarks became confined.

D) The formation of helium stopped.

143. In the present theory of the Big Bang, what significant event occurred at about 400,000 years after the universe started expanding?

A) The primordial helium in the universe was produced.

B) The universe became transparent to neutrinos.

C) The temperature of the cosmic background radiation had cooled to its present level of about 3 K.

D) The universe became transparent to photons.

144. Helium nuclei began permanent formation in the early universe when the temperature dropped to the point where the gamma radiation no longer had enough energy to break nuclei apart as soon as they formed. Did this helium production cease? If so, why?

A) Helium production at the site of the Big Bang has continued, although the rate has decreased.

B) Helium production did not cease until millions of years later when the density of the primordial universe became too low to support nuclear fusion.

C) The production of helium in the early universe ceased after a few minutes, but only because the available energy and particles were being used up in the production of heavier nuclei like carbon, oxygen, and silicon.

D) After a few minutes the early universe cooled to the point where protons no longer had enough energy to overcome the repulsion of the electromagnetic force and come close enough to each other to undergo nuclear fusion.

145. Which of these statements correctly describes the universe for the entire first 380,000 years of its life?

A) The universe was filled with free quarks (not confined inside neutrons or protons).

B) All the fundamental forces of nature were unified into one force.

C) The universe was filled with a sea of nuclear particles undergoing violent reactions.

D) The universe was opaque.

146. At an age of 380,000 years, the temperature of the universe had fallen to 3000 K, and electrons could then combine with protons to produce neutral hydrogen gas. What major transition took place in the universe at this time?

A) The present laws of physics began to apply for the first time.

B) The universe suddenly lost its electrical charge and became neutral.

C) Nuclear fusion no longer occurred below this temperature.

D) The universe became transparent to light for the first time.

147. Which of these statements does NOT correctly describe the universe at the era of recombination?

A) The temperature of the universe was about 3 K.

B) The universe was about 380,000 years old.

C) The universe became transparent to radiation.

D) Electrons and protons combined to form neutral hydrogen atoms.

148. What was the temperature of the universe at the time the universe became transparent to radiation?

A) 300,000 K

B) 30 billion K

C) 3 K

D) 3000 K

149. The conditions in the early universe at the time matter became decoupled from radiation were (in terms of age t, peak background wavelength , and equivalent temperature T)

A) t = 2  10¹¹ years,  = 1 mm, microwaves, T = 3 K

B) The universe was never radiation-dominated since matter has always dominated by generating the gravitational field into which the universe has expanded.

C) t = 3 minutes,  = 0.3 m, near UV, T = 10⁷ K

D) t = 380,000 years,  = 1 m, infrared, T = 3000 K

150. When did the universe cool to a temperature of 3 K?

A) 1 second after the start of the Big Bang, when pair production ceased

B) 380,000 years after the Big Bang, when the universe became transparent to radiation

C) 3 minutes after the start of the Big Bang, when primordial nuclear reactions ceased

D) very recently

151. At what time did the universe cool to a temperature of about 3 K?

A) end of the inflationary era

B) end of the Planck time

C) very recently

D) era of recombination

152. Because of the travel time of light, astronomers see more distant parts of the universe as they were when the universe was younger, but they cannot see back “into” times when the universe was opaque and light could not travel freely. Using photons, then, what is the furthest back in time that astronomers can when they look out into the universe?

A) 1/1 millionth seconds after the start of the Big Bang

B) 380,000 years after the start of the Big Bang

C) 3 minutes after the start of the Big Bang

D) 3000 years after the start of the Big Bang

153. What is the ratio of cosmic microwave background photons to hydrogen atoms, on average, in the universe?

A) about 1 billion to 1

B) about 1 million to 1

C) about 1 to 100—very few microwave photons

D) about 1 to 1—equal numbers of photons and hydrogen atoms

154. Even though cosmic microwave background photons outnumber hydrogen atoms by about 1 billion to 1 in the universe, the universe is still considered to be matter-dominated because the

A) photon energies are extremely small.

B) nature of the photons is such that they interact with nothing as they pass through the universe.

C) photons, while collectively carrying a large amount of energy, do not carry an equivalent amount of momentum and hence play little role in collisions with matter.

D) photons have no rest mass and hence can generate no gravity.

155. Cosmologists presently consider the universe to be matter dominated. In this stage the number of photons

A) vastly outnumbers the number of particles.

B) is vastly outnumbered by the number of particles.

C) is greatly outnumbered by the number of particles, but is less than the number of particles plus antiparticles.

D) is close to being equal to the number of particles.

Section: 19-11

156. According to the current understanding of cosmology, what is the origin of the foamy structure of the universe, with clusters of galaxies distributed on the perimeters of voids?

A) The early epoch of star formation caused the clusters of galaxies to repel one another, and the present distribution was the result.

B) The matter in the universe is now believed to be very smoothly distributed, just as it was after the Big Bang. What astronomers see as voids is really dark matter.

C) Quantum fluctuations in the early universe were greatly exaggerated during the epoch of inflation to become the uneven distribution astronomers now observe.

D) The universe was formed “clumpy” in the Big Bang, and the clumpiness persists to the present day.

157. During the formation of the universe, where did the density enhancements come from that subsequently collapsed to form superclusters of galaxies?

A) turbulence during the time when the universe was opaque, which left denser regions that were free to collapse when the universe became transparent

B) quantum fluctuations in the density of matter during the early Big Bang, which were later expanded by inflation

C) spatial variations in the rate heating from pair annihilation after the first second of the Big Bang

D) random gravitational fragmentation of the matter in the universe after the era of recombination

158. The very small detected irregularities in the uniformity of the cosmic microwave background are considered to be very important in the study of the evolution of the universe because they

A) show that the Big Bang explosion was very nonuniform.

B) were the seeds of supermassive black holes around which the earliest superclusters of galaxies then formed.

C) are thought to have led to the development of the present concentrations of matter and energy in superclusters of galaxies.

D) are thought to contain most of the elusive “missing matter” in the form of energy concentration in the universe.

159. If the early universe had been perfectly uniform, galaxies could not have formed. As it is, cosmologists believe the variations observed in the cosmic microwave background which were sufficient to produce the galaxies and other structures now seen were only about 3 parts in

A) 10.

B) 1000.

C) 100,000.

D) 10 million.

160. How do astronomers measure the relative amounts of dark matter and luminous matter in the early universe?

A) The same way they do in the present universe: rotation curves, gravitational lensing, etc.

B) By measuring the sizes of the peaks created by acoustic waves in the cosmic microwave background.

C) By measuring the relative amplitudes in the acoustic overtone waves in the cosmic microwave background.

D) By measuring the relative frequencies in the acoustic overtone waves in the cosmic microwave background.

161. What designation do astronomers give to the first stars?

A) Population 0

B) Population I

C) Population II

D) Population III

162. Which one of these is NOT believed to be a characteristic of Population III stars?

A) They were composed of almost pure hydrogen.

B) They formed from small clouds of gas so are less massive than the Sun.

C) They were formed soon after the Big Bang.

D) Most have long since disappeared.

163. How do the youngest, most distant galaxies compare with the older galaxies astronomers view nearby today?

A) The youngest galaxies are bluer and brighter than the older ones.

B) The youngest galaxies have the same color as the older ones but are significantly brighter.

C) There is no observable difference between the youngest galaxies and the older ones.

D) The youngest galaxies are redder and fainter than the older ones.

164. Why are the youngest galaxies that are seen in distant parts of the universe bluer and brighter than the older, closer galaxies observed?

A) The red light has been partially absorbed by gas and dust between Earth and the galaxies.

B) Young galaxies have more dust, and dust scatters blue light better than red.

C) The light from the most distant galaxies has been bent by gravitational lenses and focused on Earth.

D) Young galaxies have a burst of star formation that produces many hot, bright, blue stars.

165. For how long did vigorous star formation last in elliptical galaxies?

A) continuously throughout the galaxy’s life, right to the present day

B) about the first million years of the galaxy’s life

C) about the first billion years of the galaxy’s life

D) about the first 8–10 billion years of the galaxy’s life

166. For how long did continuous star formation last in spiral galaxies?

A) about the first 8–10 billion years of the galaxy’s life

B) about the first billion years of the galaxy’s life

C) about the first million years of the galaxy’s life

D) continuously throughout the galaxy’s life, right to the present day

167. A comparison of an elliptical galaxy and a spiral galaxy of the same mass a billion years after both began formation would show that

A) they should each have about the same number of stars.

B) the elliptical galaxy will have formed more stars than the spiral galaxy.

C) the spiral galaxy will have formed more stars than the elliptical.

D) neither galaxy will have formed any stars during the first billion years.

168. The cosmic microwave background shows a pattern of sound waves initially generated by

A) recombination.

B) symmetry breaking.

C) nucleosynthesis.

D) inflation.

169. Many of the small temperature fluctuations in the cosmic microwave background are a result of

A) galaxy formation.

B) symmetry breaking.

C) sound waves.

D) polarization.

170. A star with no elements heavier than lithium is referred to as

A) Population 0.

B) Population I.

C) Population II.

D) Population III.

171. During which period in the universe’s history did its galaxies undergo the MOST vigorous period of star formation?

A) the first 500 million years after the Big Bang

B) from about 500 million to 2.5 million years after the Big Bang

C) the past four billion years

D) the past billion years

172. The “Little Cub” galaxy is a small elliptical galaxy near the Milky Way that is forming stars vigorously. It is unusual because

A) most ellipticals formed their stars long ago, with little recent star formation.

B) There are very few ellipticals near the Milky Way.

C) At the present day, ellipticals are nearly all more massive than the Milky Way.

D) The Milky Way should have accreted all nearby gas clouds.

Section: 19-12

173. What appears to be the relationship between the distribution of dark matter and the distribution of luminous matter?

A) There seems to be no correlation at all.

B) There seems to be a separate distribution of dark matter—dark-matter galaxy clusters, voids in the dark matter, and so on. But these formations all occur in regions of space far from luminous matter.

C) The distribution of dark matter seems to coincide with the distribution of luminous matter.

D) The distribution of dark matter seems to be just the reverse of the distribution of luminous matter: Dark-matter galaxy clusters occur in the voids of luminous matter; luminous galaxy clusters occur in the voids of dark matter.

174. In a primordial, pregalactic gas cloud, what is believed to have been the MOST important condition that caused the cloud to become a spiral galaxy (as opposed to some other type of galaxy)?

A) The cloud started off with a lot of dust and heavy elements.

B) The initial rate of star formation was high.

C) The cloud started off flattened and disk-shaped before it collapsed.

D) The initial rate of star formation was low.

175. In a primordial, pregalactic gas cloud, what is believed to have been the MOST important condition that caused the cloud to become an elliptical galaxy (as opposed to some other type of galaxy)?

A) The initial rate of star formation was high.

B) The cloud started off with a lot of dust and heavy elements.

C) The cloud started off flattened and disk-shaped before it collapsed.

D) The initial rate of star formation was low.

176. Does the initial rate of star formation have any effect on the type of galaxy that is formed from a pregalactic gas cloud?

A) No. The initial rate of star formation determines the size of the galaxy. A high initial rate of star formation means that many stars have formed a large galaxy before the remaining gas and dust dissipate and star formation ceases. But a galaxy of any type is equally likely to form.

B) Yes. A high initial rate of star formation uses up the gas and dust before it has a chance to settle into a disk. This results in an elliptical galaxy.

C) Yes. A high initial rate of star formation produces an early generation of stars that evolve and send out more gas and dust to form a disk and to perpetuate star formation. So, this results in a spiral galaxy.

D) Yes. An initially high rate of star formation produces many high-mass stars that explode and send out shock waves to trigger the formation of other stars. This chaotic situation results in an irregular galaxy.

177. Astronomers can determine the amount of mass in a cluster of galaxies by observing its gravitational interactions. How much of this mass is luminous matter?

A) 1–2%

B) 10–20%

C) 50%

D) 80–90%

Section: 19-13

178. Why would cosmologists expect the rate of expansion of the universe to be slowing down?

A) The gravitational pull of all objects in the universe on each other would lead to slowdown.

B) All expansions after explosions naturally slow down with time.

C) Galaxies feel a kind of friction as they move through space, which slows them down.

D) The greater the distance between two objects (such as galaxies or superclusters), the harder it is to push them farther apart.

179. What is “dark energy”?

A) energy associated with the matter that has fallen into a black hole

B) matter-energy needed to bridge the gap between the energy seen or inferred and the energy needed to make the universe flat

C) energy associated with dark matter

D) an unusual form of energy that creates a universewide repulsive force

180. Suppose the universe contained only “normal” matter, like hydrogen, helium, etc. How would the expansion rate of the universe change with time?

A) It would speed up.

B) It would slow down initially but then speed up when the matter pressure became high enough.

C) It would slow down.

D) It is impossible to tell, because it depends on the density of the matter.

181. Which parameter of the present universe is considered to be critical in determining the ultimate fate of the universe?

A) amount of mass in black holes in the universe

B) number of neutrinos in the universe

C) amount of matter and energy in the universe

D) number of photons of radiation in the universe

182. The future of the overall universe, in terms of its ultimate evolution and whether it will expand forever or eventually contract again, depends MOST directly on which of these parameters?

A) present volume of the universe

B) amount of matter and energy within the universe

C) intensity of cosmic microwave background radiation

D) temperature of the gas within the universe

Section: 19-14

183. What condition is necessary for the universe to be closed?

A) The universe must have no mass in it.

B) The density of the universe must be large.

C) The density of the universe must be small.

D) The cosmological constant must be large.

184. Consider a universe without dark energy (i.e., all the energy in the universe produces an attractive gravitational force). What will happen if the universe is closed?

A) The universe will expand past its maximum size, then fragment into miniuniverses.

B) The universe will eventually fall back in on itself, heading toward a “Big Crunch.”

C) The universe will expand forever.

D) The universe will reach a maximum size and remain there, like a balloon being blown up.

185. Two light beams that are initially parallel to each other will eventually converge in what kind of universe?

A) flat only

B) hyperbolic only

C) either flat or hyperbolic

D) closed

186. Which of these universe scenarios will end in the universe stopping its expansion and recollapsing at some time in the future?

A) open universe

B) Newtonian universe

C) closed universe

D) flat universe

187. At the present time

A) most astronomers believe the universe has a spherical geometry (positive curvature).

B) most astronomers believe the universe has a flat geometry (zero curvature).

C) most astronomers believe the universe has a hyperbolic geometry (negative curvature).

D) there is no consensus among astronomers about the geometry of the universe.

188. What kind of curvature (geometry of space) does the universe have if the universe is open?

A) hyperbolic

B) parabolic

C) spherical

D) flat

189. What kind of curvature (geometry of space) does the universe have if the universe is just on the boundary between being open and being closed?

A) parabolic

B) hyperbolic

C) spherical

D) flat

190. What kind of curvature (geometry of space) does the universe have if the universe is closed?

A) parabolic

B) flat

C) hyperbolic

D) spherical

191. If space were flat and there were no “dark energy,” what would be the future of the universe?

A) The universe would expand forever, but only barely; if it had any more matter in it than it does, it would eventually stop expanding and start to collapse.

B) The universe would expand to a maximum size and then collapse into a Big Crunch.

C) The future of the universe is not related to the geometry of space.

D) The universe would have more than enough energy to expand forever, not stopping even when infinite time has elapsed.

192. If the geometry of space is hyperbolic, what is the future of the universe?

A) The universe will expand to a maximum size and then collapse into a Big Crunch.

B) The universe will expand forever, but only barely; if it has any more matter in it than it does, it will eventually stop expanding and start to collapse.

C) The future of the universe is not related to the geometry of space.

D) The universe will expand forever.

193. If the geometry of space is spherical and if the universe has no dark energy, what is the future of the universe?

A) The future of the universe is not related to the geometry of space.

B) The universe will expand forever, not stopping even when infinite time has elapsed.

C) The universe will hardly expand forever; if it has any more matter in it than it does, it will eventually stop expanding and start to collapse.

D) The universe will expand to a maximum size and then collapse into a Big Crunch.

194. If space has a hyperbolic geometry (open universe), what will happen to two initially parallel laser beams as they traverse billions of light-years of space?

A) The beams will gradually diverge (move apart).

B) The beams will remain parallel.

C) The beams will gradually diverge (move apart) to a maximum separation, then gradually approach and cross.

D) The beams will gradually approach each other and eventually cross.

195. Measurement of the sizes of hot spots in the temperature distribution of the cosmic microwave background suggests that the universe is flat. Yet the combined density of all the normal matter seen, plus the dark matter inferred, is only about one-quarter of the critical density. What conclusion have cosmologists drawn from this discrepancy?

A) The universe is not flat after all.

B) There must be a large amount of luminous matter and radiation in the universe, which cosmologists have not yet found.

C) The universe is smaller than cosmologists have been assuming and thus the measured density is larger.

D) There is an entity to which cosmologists have given the name “dark energy” that contributes to the total density but that cannot be detected by the usual methods.

196. How did cosmic inflation affect the geometry of the universe?

A) Inflation made the expansion rate so large that the gravity of normal matter was not enough to slow it down, even today. Thus the universe still has a hyperbolic geometry.

B) By expanding it so much, inflation spread out the matter so much that the geometry became hyperbolic.

C) By expanding it so much, inflation forced the universe to be flat.

D) Inflation did not affect the geometry of the universe.

197. What observations have MOST clearly revealed the geometry of the universe?

A) the brightnesses of supernovae

B) the apparent sizes of features in the cosmic microwave background

C) gravitational­-lensing near galaxy clusters

D) galaxy rotation curves

Section: 19-15

198. How does the observed amount of visible matter in the universe compare with the amount required to make the geometry flat?

A) The observed amount of visible matter is about 1/3 of the amount needed.

B) The observed amount of visible matter equals the amount needed, to within observational uncertainty.

C) The observed amount of visible matter is about twice the amount needed.

D) The observed amount of visible matter is about 1/25 of the amount needed.

199. Which of these accurately characterizes the current understanding of the expansion of the universe?

A) The universe is expanding too slowly to overcome its mutual gravitational attraction, and thus it will eventually stop expanding and eventually collapse.

B) The universe is expanding at exactly its escape speed. Thus it will continue to slow forever and stop at an infinite time in the future.

C) The universe is expanding at a rate greater than its escape speed. Although the expansion is slowing down, it will continue to expand forever without ever stopping.

D) None of these are correct.

200. How does the observed total amount of matter in the universe, including dark matter, compare with the amount of matter required to make its geometry flat?

A) The observed total amount of matter equals the amount needed, to within observational uncertainty.

B) The observed total amount of matter is about 1/3 the amount needed.

C) The observed total amount of matter is about twice the amount needed.

D) The observed total amount of matter is about 1/25 of the amount needed.

201. Which of these cosmological problems is “dark energy” believed to solve?

A) Why is the universe’s expansion accelerating?

B) Why did the universe suddenly inflate during the Big Bang?

C) Why is the temperature of the cosmic background radiation so smooth (isotropic) around the sky?

D) Why is the night sky dark?

202. Measurement of structure in the cosmic microwave background radiation has recently indicated that the universe is flat (on the cusp between a closed and an open universe), but the measured density of detected matter and radiation is only 20–40% of the critical density required for a flat universe. In what form is the other 60–80% of the “matter” thought to be?

A) myriads of small primordial black holes whose gravitational effects are spread throughout the universe and which emit no radiation

B) dark energy emitting no visible radiation

C) antimatter, which generates a negative gravitational effect and emits radiation only if it meets matter and is annihilated

D) neutrinos, which have very little rest mass and are very difficult to detect, but are very abundant

203. What is the difference between dark matter and dark energy?

A) Dark matter exists today, whereas dark energy existed only until the strong nuclear force became a separate force at the Planck time.

B) Dark energy is the energy emitted by dark matter, much as luminous energy (light) is emitted by visible matter.

C) There is no difference: Energy has mass by E = mc², and dark matter and dark energy are two names for the same effect.

D) Dark matter is attractive and slows the universal expansion, whereas dark energy is repulsive and accelerates the universal expansion.

204. What method is being used to discover whether this is an unbounded universe, in which expansion will continue forever, or a bounded universe, in which expansion will eventually turn into contraction and lead to recollapse?

A) careful monitoring of the Moon–Earth distance to detect the slowdown of the expansion of the universe

B) measurement of the deviation from uniformity of the cosmic background radiation

C) measurement of the brightnesses of distant supernovae

D) measurement of the bending of light by distant galaxies as it follows the curvature of space

205. Which of these statements correctly describes cosmologists’ current state of knowledge about the future expansion of the universe?

A) The amount of matter (both visible and dark) is only 1/3 the amount needed to halt expansion, so the expansion will continue to slow down but will never stop.

B) The amount of matter (both visible and dark) is only 1/3 the amount needed to halt expansion, and there is strong evidence of a previously unknown form of energy that acts to accelerate the expansion.

C) The amount of matter (both visible and dark) is exactly the right amount needed to halt expansion after an infinite amount of time has gone by.

D) The amount of matter (both visible and dark) is 3 times the amount needed to halt expansion, so the expansion will eventually stop and the universe will begin to collapse.

206. Recent results from very bright supernovae in very distant galaxies indicate that the expansion of the universe

A) has now stopped and the universe will shortly begin to contract again toward a Big Crunch.

B) is decelerating (slowing down).

C) is continuing at a constant rate and has done so since just after the Big Bang.

D) is accelerating (speeding up).

207. On the basis of recent results from very bright and very distant Type Ia supernovae, which of these statements BEST characterizes the past and present motion of clusters of galaxies?

A) The universe is “flat,” with a decelerating rate of expansion.

B) The universe is “open,” with a constant rate of expansion given by the Hubble law.

C) The universe is “closed,” with a decelerating rate of expansion.

D) The universe is “flat,” with an accelerating rate of expansion.

208. If the cosmological constant exists, what physical effect is believed to create it?

A) New matter is constantly being created in the expanding voids between superclusters of galaxies, and the energy released by this creation accelerates the expansion of the universe.

B) No one knows. The cosmological constant has been hypothesized to exist because astronomers see the universal expansion accelerating, but its source is still a mystery.

C) Energy released by nuclear reactions in stars and then emitted into space is heating the universe and accelerating its expansion.

D) Virtual particles that constantly appear and disappear add energy to empty space, and this vacuum energy provides a constant repulsive force that accelerates the universe.

209. How does the cosmological constant differ from quintessence?

A) The cosmological constant is a specific physical effect that can be described mathematically, whereas quintessence is the total of all indefinable properties that make the universe what it is at any given time.

B) The cosmological constant provides an accelerating force in the universal expansion, whereas quintessence provides a decelerating term; it is the balance between the cosmological constant and quintessence that determines whether the expansion accelerates or decelerates.

C) There is essentially no difference; basically, quintessence is the modern name for the cosmological constant.

D) The cosmological constant provides a constant accelerating force in the universal expansion, whereas quintessence can change as the expansion proceeds.

210. Einstein introduced a “cosmological constant” into his formulation of the structure of the universe as described by his general theory of relativity. How did he envision that the cosmological constant would manifest itself?

A) as many “white holes” that would contribute matter to an expanding universe to maintain constant density, as required by the cosmological principle—a continuous-creation universe

B) as an extra “gravity” that would hold the universe against continuous expansion

C) as antimatter that, by annihilating real matter, would translate matter into energy, thereby maintaining a constant mass density in a condensing universe

D) as a form of energy that, on its own, would make the universe expand—a form of antigravity

211. Why is flatness a problem in cosmology?

A) The total amount of known matter (even if dark matter is included) is not enough to make the universe flat.

B) The universe appears to be flat to within observational error, yet the universe is expanding, and it is impossible for an expanding universe to be flat.

C) The universe appears to have a hyperbolic geometry to within observational error, yet the universe is expanding, and expanding universes have to be flat.

D) Matter is known to create bumps in the geometry of spacetime; therefore, the universe cannot be flat.

212. The 2003 observation of a very distant galaxy, the light from which passed through a less distant galaxy cluster on its way to Earth, showed an overall blueshift. What is the significance?

A) The observation shows that the most distant galaxies have already started to fall back to the center of the universe (the Big Crunch), and their light experiences a Doppler blueshift.

B) The mass density at Earth’s location is larger than the mass density in the vicinity of the very distant galaxy, so light experiences a gravitational blueshift as it moves toward Earth.

C) The apparent blueshift is not unusual. Most distant galaxies look blue because of the predominance of blue stars in the early universe.

D) The gravitational blueshift experienced by the light approaching the cluster (at an earlier time) outweighed the gravitational redshift it experienced (at a later time) while moving away from the cluster. This suggests an outward cosmological acceleration.

213. What is an advantage of quintessence over the cosmological constant as a description of dark energy?

A) Einstein’s reasoning in rejecting the cosmological constant still seems to be true today.

B) The cosmological constant would have to have, from the beginning of the universe, the right value to give a flat universe now: an amazing coincidence. The quintessence model does not have this requirement.

C) Quintessence is a well-established concept in physics, going back to ancient times, whereas the cosmological constant is a relatively new and untested concept.

D) The quintessence model predicts the correct age for the universe; the cosmological constant does not.

214. As currently understood, what happens to the energy density of matter and the energy density of a cosmological constant as the universe expands?

A) Both the density of matter and the density of a cosmological constant increase as the universe expands.

B) The matter density decreases, but the density of the cosmological constant increases.

C) The matter density decreases, but the density of the cosmological constant remains constant.

D) Both the density of matter and the density of a cosmological constant decrease as the universe expands.

215. What do superstring theories try to explain?

A) forces holding superclusters of galaxies together

B) apparent acceleration of the expansion of the universe

C) behavior of matter during the Planck time when all four fundamental forces were unified

D) behavior of matter during the inflationary epoch

216. Which of these is NOT predicted by superstring theories?

A) The universe cannot have a negative curvature.

B) Spacetime may not be entirely smooth.

C) Dark matter may include particles predicted by string theory.

D) The basic theory of the universe, of which string theories are a modification, is general relativity.

217. At the present time cosmologists believe the structure and behavior of the universe are dominated by

A) radiation.

B) ordinary matter.

C) dark matter.

D) dark energy.