Deck 26: Exploring the Early Universe

Question 1

The universe began in the Big Bang. When did the first stars and galaxies begin to appear?

A)immediately, as they were a direct product of the Big Bang
B)about 380,000 years later
C)about 400 million years later
D)not until about 3.2 billion years later

Answer 1

C

Question 2

The universe began in the Big Bang. When did were the first atoms formed?

A)immediately, as they were a direct product of the Big Bang
B)about 380,000 years later
C)about 400 million years later
D)not until about 3.2 billion years later

Answer 2

B

Question 3

The flatness problem in cosmology asks the question:

A)why is the sky dark at night?
B)why was the density of the universe so close to the critical density just after the Big Bang?
C)why are the four forces (strong, weak, electromagnetic, and gravitational) not unified in the present-day universe?
D)why is the temperature of the cosmic background radiation so smooth (isotropic) around the sky?

Answer 3

D

Question 4

Which of the following statements correctly describes the "flatness problem" in cosmology?

A)Observations of the distant universe indicate that the universe is at least moderately flat, yet matter creates lumps in the geometry of spacetime. Therefore it is hard to account for the observed flatness.
B)The density of the universe must have been equal to the critical density to a precision of 50 decimal places in order for us to see the universe we see today. This astounding flatness is hard to account for.
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)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.

Question 5

If the average mass density ρ₀ had been less than the critical density ρ_c in the early universe, what might have been the likely result?

A)The universe would have expanded very rapidly, and matter would not have clumped together to form galaxies.
B)The universe's mass would have clumped together in a relatively short time and collapsed to a singularity like that from which it emerged.
C)Cosmic inflation would not have occurred.
D)The cosmic microwave background radiation would not have been created.

Question 6

If the average mass density ρ₀ had been greater than the critical density ρ_c in the early universe, what might have been the likely result?

A)The universe would have expanded very rapidly, and matter would not have clumped together to form galaxies.
B)The universe's mass would have clumped together in a relatively short time and collapsed to a singularity like that from which it emerged.
C)Cosmic inflation would not have occurred.
D)The cosmic microwave background radiation would not have been created.

Question 7

When did the inflationary epoch occur?

A)during the Planck time-that is, during the first 10^-43 seconds after the cosmological singularity
B)immediately after the Planck time
C)throughout the first 3 minutes
D)throughout the first 380,000 years

Question 8

The inflationary epoch accomplished all of the following EXCEPT one. Which is the EXCEPTION?

A)It took whatever curvature the early universe had and flattened it.
B)The epoch allowed the early, preinflationary universe to be very small and thus capable of thermal equalization.
C)It permitted matter to move faster than the speed of light for a brief period.
D)It forced the observed density of the universe to be equal to the critical density to great precision.

Question 9

The proposed inflationary epoch would explain many features of the observable universe. But can inflation itself be tested?

A)No, unfortunately there is no direct test for inflation.
B)Yes, the temperature of the cosmic microwave background provides a test of cosmic inflation.
C)Yes, the uniformity of the cosmic microwave background provides a test of cosmic inflation.
D)Yes, the polarization of the cosmic microwave background provides a test of cosmic inflation.

Question 10

During the period of cosmic inflation, the universe underwent an extremely rapid expansion, increasing its size by a factor of 10⁵⁰ in a time of perhaps 10^-32 seconds. Did this action violate the postulate of special relativity that nothing can travel faster than the speed of light?

A)Yes, but this was before the Planck time, and the laws of physics as we presently understand them, including the laws of special relativity, did not yet apply.
B)No. The universe was so small at the beginning of the inflationary epoch that even an expansion by 50 orders of magnitude did not take it far enough in 10^-32 seconds to exceed the speed of light.
C)The postulate of special relativity was not violated because it was the dimensions of space that were expanding. The postulate applies to objects moving through space.
D)The postulate of special relativity was not violated because the curvature of space was so extreme at this time that the distance objects moved through during this 10^-32 seconds was foreshortened so that the speed was less than the speed of light.

Question 11

It has been shown that above 100 GeV in energy, intermediate vector bosons behave like photons. What is the significance of this in the early universe?

A)Light (the cosmic microwave background radiation) was emitted by the weak interaction.
B)Above this energy matter behaved like radiation: the radiation-dominated era of the early universe.
C)The weak and electromagnetic interactions were unified.
D)The strong force separated from the electromagnetic force.

Question 12

The inflationary epoch required an enormous amount of energy. What was the source of this energy?

A)The universe cooled as it expanded, releasing energy.
B)The universe made the transition from the false vacuum to the true vacuum, releasing energy.
C)The particles gained energy in the form of mass from the Higgs field.
D)Gravitons, released by the newly created gravitational field, provided energy.

Question 13

Quantum electrodynamics describes forces in terms of the exchange of particles. The electromagnetic force, for example, involves the exchange of virtual photons. The weak force involves the exchange of:

A)gluons.
B)intermediate vector bosons.
C)gravitons.
D)virtual neutrinos.

Question 14

In modern quantum physics, forces are manifested through the exchange of particles. Which one of the following is NOT one of these exchanged particles?

A)gluons
B)virtual photons
C)gravitons
D)protons

Question 15

When the strong force is exerted between particles, these particles exchange:

A)gluons.
B)intermediate vector bosons.
C)gravitons.
D)virtual photons.

Question 16

The Higgs particle is particularly important because other particles, through their interaction with the Higgs field, gain:

A)energy.
B)mass.
C)charge.
D)strangeness.

Question 17

What does the theory of quantum electrodynamics describe?

A)the change of one type of quark into another type of quark into another type of quark by the weak force
B)the interactions of charged particles by the exchange of virtual photons
C)the bonding between quarks by the exchange of gluons
D)the interactions of nuclear particles by the strong force

Question 18

A supergrand unified theory (or theory of everything) would describe the physics of a universe in which what forces are unified?

A)all four
B)everything except gravity
C)the electromagnetic and weak forces
D)the strong and weak forces

Question 19

Which of the four fundamental forces holds the protons in the nucleus of an atom together?

A)the gravitational force
B)the weak nuclear force
C)the electromagnetic force
D)the strong nuclear force

Question 20

If gravity holds galaxies together and the electromagnetic force holds atoms together, what does the weak nuclear force hold together?

A)nuclei
B)nothing
C)the quarks inside protons and neutrons
D)leptons (particles like electrons and neutrinos)

Question 21

Tritium (³H) is a heavy isotope of hydrogen, composed of one proton and two neutrons. Tritium decays into ³He, a light isotope of helium, composed of two protons and one neutron, through the reaction . ${ }^{3} H \rightarrow{ }^{3} H e+\bar{e}+\bar{v}$ The production of an electron and an antineutrino in this decay shows that the reaction was governed by the:

A)electromagnetic force.
B)gravitational force.
C)strong force.
D)weak force.

Question 22

What is the range of the strong nuclear force (the maximum distance over which it acts)?

A)10^-9 m (1 nm, or roughly the size of a hydrogen atom)
B)infinity
C)a few thousand meters, or roughly the size of Earth)
D)10^-15 m (1 fm, or roughly the size of a proton)

Question 23

What is the range of the strong nuclear force compared to the size of the nucleus, 10^-14 m?

A)The range of the strong nuclear force is 10 times smaller than the size of an atomic nucleus.
B)The range of the strong nuclear force is infinite; it has no limit.
C)The range of the strong nuclear force is the same as the size of the nucleus.
D)The range of the strong nuclear force is 10 times larger than the size of an atomic nucleus.

Question 24

Within which time interval from the initial Big Bang do we believe all four fundamental forces of nature were united into a single force?

A)t = 0 to 10^-43 second, the Planck time, when gravity "froze out" of the universe
B)t = 0 to 10⁶ years, when radiation dominated the universe
C)t = 0 to 10^-35 second, when the strong nuclear force "froze out" of the universe
D)t = 0 to 1 second, when photons interchanged freely with electron-positron pairs

Question 25

During the epoch of cosmic inflation, the universe expanded very rapidly. What was the effect on the temperature of the universe?

A)As the universe expanded rapidly, it cooled rapidly. Thereafter it expanded and cooled at a more moderate rate.
B)Because of the incredible speed of the expansion, there was no time for particles to exchange energy. Thus, the temperature remained constant during the expansion.
C)Because of the tremendous amount of energy available, the temperature actually rose during the expansion.
D)The temperature decreased during the expansion and then rose again to essentially the value it had initially.

Question 26

In modern quantum physics, the Heisenberg uncertainty principle states that the more precisely you know a particle's:

A)velocity, the more certain you are of its position in space.
B)mass, the less certain you are of its size.
C)position, the more certain you are of its speed and motion.
D)position, the less certain you are of its speed and motion.

Question 27

Particle-antiparticle pairs are coming into existence all the time in the space around (and inside!) us. According to the Heisenberg uncertainty principle, the more massive the particles, the:

A)shorter the time that they can exist.
B)less precisely we know their position.
C)longer the time that they can exist.
D)more precisely we know their position.

Question 28

According to the Heisenberg uncertainly principle, can matter spontaneously come into existence without having been created from energy?

A)No, never. Matter is a form of energy, and the spontaneous creation of matter would violate conservation of energy.
B)Yes, but only for extremely short times.
C)Yes, but only if the particles created are electrically neutral.
D)Yes, but only if an equal amount of matter disappears from some other part of the universe.

Question 29

The mass of a proton is 1.67 × 10^-27 kg. For what maximum length of time could a proton-antiproton pair spontaneously come into existence, without violating any laws of physics such as conservation of energy?

A)3.5 × 10^-25 s
B)1.1 × 10^-16 s
C)2.2 × 10^-16 s
D)7.0 × 10^-27 s

Question 30

What is a virtual particle?

A)a collection of particles that acts like a single particle
B)any particle that has no mass and is electrically neutral
C)a particle that can never have any detectable effect whatsoever on the real universe
D)a particle that exists for such a short time interval that we cannot detect it by direct measurement

Question 31

A particle and an antiparticle have:

A)the same mass and the same electric charge.
B)the same mass and equal electric charges of opposite sign.
C)equal masses of opposite sign and the same electric charge.
D)equal masses of opposite sign and equal charges of opposite sign.

Question 32

The constant creation and annihilation of virtual pairs of particles and antiparticles causes a slight change in the wavelengths of the hydrogen spectral lines. This effect is called the:

A)Wolf effect.
B)Rutherford shift.
C)Lamb shift.
D)Einstein effect.

Question 33

What is the difference between the virtual electron and virtual positron created spontaneously in the vacuum and the electron-positron pair created from a high-energy gamma ray?

A)They are exactly the same.
B)The particles created by the gamma ray are real. They can last forever if they do not annihilate. The virtual particles must recombine to become part of the vacuum within a very short time. Only under very special circumstances can they become real.
C)The particles created from the vacuum are real. They can last forever if they do not annihilate. The particles created from the gamma ray must recombine within a very short time. Only under very special circumstances can they become real.
D)The virtual particles have positive and negative mass, whereas the particles created by the gamma ray have positive mass.

Question 34

Energy and mass are equivalent according to Einstein, and both can be measured in electron volts. (1 eV is the energy acquired by an electron when accelerated through 1 volt.) If the rest mass of an electron is 511 keV, what is the threshold energy of a gamma-ray photon in order that it can undergo electron pair production?

A)1.533 MeV
B)1.022 MeV
C)0 eV, because it produces a particle-antiparticle pair
D)511 keV

Question 35

Suppose the inflationary epoch lasted 10^-32 seconds. How much mass could have been created in a virtual pair during this time without violating the law of conservation of energy?

A)2 × 10^-27 kg, about the mass of one hydrogen atom
B)10^-19 kg
C)about 1 kg
D)2 × 10³⁰ kg, about the mass of the Sun

Question 36

In the present universe there is an imbalance between particles and antiparticles. How did this originate?

A)It is believed that there are slight differences in mass between matter and antimatter. Thus the inflationary epoch, which gave mass and energy to the universe, created an imbalance of particles.
B)It is believed that there are slight differences in electric charge between matter and antimatter. Thus the inflationary epoch, which gave mass and energy to the universe, created an imbalance of particles.
C)We see only a small part of the universe. The missing antimatter is elsewhere, repelled by the negative mass of normal matter early in the history of the universe.
D)This question is an unsolved mystery of present-day cosmology.

Question 37

As the temperature in the early universe decreased, pair production reactions were confined to particles of smaller and smaller mass while annihilation reactions were unaffected. Why this difference?

A)Particles and antiparticles contain a certain amount of mass-energy independent of their temperature whereas the energy of a photon depends on its temperature.
B)Photons contain a certain amount of mass-energy independent of their temperature whereas the energy of particles and antiparticles depend on its temperature.
C)When particles collide, the interaction raises their temperature, but this does not happen when gamma-ray photons interact.
D)When gamma-ray photons collide, the interaction raises their temperature, but this does not happen when particles interact.

Question 38

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

A)an unknown number, because the early universe was opaque and we cannot see what conditions were like then
B)1 billion and 1, thus producing the matter we see today
C)10 billion, thus producing the dark matter we see today
D)exactly 1 billion, because protons and antiprotons were created in equal numbers

Question 39

What is the situation with respect to matter and antimatter in the universe today?

A)There is perfect symmetry and balance between matter and antimatter.
B)There is a slight imbalance between matter and antimatter, with about one matter particle in excess for every billion matter-antimatter pairs.
C)There is far more matter than antimatter in the universe.
D)There is far more antimatter than matter in the universe.

Question 40

The first 380,000 years of our universe, before matter and radiation decoupled, were dominated by the primordial fireball filled with an immense quantity of radiant energy. What was the source of these photons?

A)They were created during the cosmic singularity and bounced around until the universe became transparent to radiation.
B)They were given off when the free electrons were captured by protons to form the first hydrogen atoms.
C)As the universe expanded and the radiation cooled, photons no longer had enough energy to create particles by pair production. But particles continued to annihilate and produce additional photons.
D)The "fireball" that we see in the distant past is really a reflection of all the radiation produced before that time bouncing back from the universe in its early opaque state.

Question 41

At what point in the age of the universe did it cool to a temperature of about 3 K?

A)at the end of the quark confinement
B)at the end of the deuterium bottleneck
C)very recently
D)when the temperature reached the proton threshold

Question 42

After about 2 seconds, the early universe contained almost no neutrons. But protons remained. Why this difference?

A)Neutrons are much more massive than protons. After the first 2 seconds the temperature of the universe had fallen below the threshold for pair production of neutrons but not of protons.
B)Neutrons, being uncharged, can get close to other particles and annihilate them. Protons, being charged, cannot get close to other particles.
C)Free neutrons decay spontaneously. Free protons are stable.
D)Virtual proton-antiproton pairs were produced during the inflationary period, but neutral neutrons have no antiparticles and thus were not pair produced.

Question 43

Why is it that we find no free (unbound) neutrons floating around in the universe today, whereas free protons and electrons exist in profusion?

A)Free neutrons decay spontaneously to produce protons, electrons, and neutrinos.
B)The free neutrons interact quickly with the free electrons to produce antiprotons, so there are few neutrons left.
C)The original Big Bang produced only charged particles; hence neutrons were not produced, and those that now exist in the nuclei of atoms have come from proton decay.
D)Free neutrons react readily with free protons to produce high-energy photons, or γ radiation.

Question 44

Arrange the following particles in order of increasing threshold temperature.

A)electron, proton, neutrino, and neutron
B)neutrino, neutron, electron, and proton
C)neutrino, electron, proton, and neutron
D)proton, electron, neutron, and neutrino

Question 45

What is the deuterium bottleneck?

A)Deuterium had to form before helium could form, but deuterium is easily destroyed, thus preventing the formation of helium.
B)Deuterium absorbs neutrons efficiently, thus producing heavier and heavier isotopes of hydrogen instead of heavier elements such as helium.
C)Deuterium had to form before helium could form, but deuterium is almost impossible to create, thus preventing the formation of helium.
D)Helium is used up in the formation of deuterium. However, deuterium is difficult to create, thus leaving us with large amounts of helium.

Question 46

Where and how was most of the helium in the universe created?

A)by nuclear reactions in the cores of stars, and was then thrown out into space by supernovae
B)by the collision of cosmic rays with hydrogen nuclei in interstellar gas clouds
C)by high-energy processes during the collapse of protogalactic clouds during the formation of galaxies
D)by nuclear reactions during the Big Bang

Question 47

During what time was helium created in the Big Bang?

A)during the first 300,000 years
B)during the first 10^-43 second
C)during the first 10^-6 second
D)during the first 15 minutes

Question 48

In the present-day universe there are about 10 hydrogen atoms (¹H) to each helium atom (⁴He). Considering just hydrogen and helium (the vast majority of matter), what is the ratio of neutrons to protons?

A)1:10
B)1:6
C)1:4
D)1:1

Question 49

The cosmic microwave background of photons is observed to have a temperature of about 3 K. What is believed to be the present temperature of the cosmic neutrino-antineutrino background?

A)less than 3 K-the microwave background is warmer because it received extra energy from the annihilation reaction that produced it.
B)3 K-the expansion of the universe is the same for all particles in it.
C)4 K-the universe was hotter when it became transparent to neutrinos.
D)1,000,000 K-the neutrinos do not interact with matter and therefore have not been redshifted.

Question 50

Would you expect the neutrino-antineutrino background to be at a temperature above or below the 2.75 K of the cosmic microwave background? Why?

A)The neutrino-antineutrino background "escaped" from the primordial fireball long before the CMB. Thus, it has been cooling for a longer time and it should be at a lower temperature.
B)The neutrino-antineutrino background is constantly undergoing annihilation, which produces high-energy gamma rays and heat. Thus, the temperature should be higher than the CMB.
C)The threshold temperature of the neutrino is the highest of any pair of production particles. Thus, the temperature should be higher than the CMB.
D)The neutrino-antineutrino background was produced at an earlier, hotter time in the history of the universe. Thus, this background should be hotter than the CMB.

Question 51

How was the "deuterium bottleneck" removed so that helium could form?

A)With the formation of the first stars, nucleosynthesis had a confined, uninterrupted space in which to produce both deuterium and hydrogen.
B)The formation of deuterium requires the strong force, and this did not separate out from the other forces until sometime after the cosmic singularity. When it did, the deuterium bottleneck ended.
C)As the universe expanded, its photons cooled, lost energy, and eventually became unable to disrupt deuterium.
D)As the universe expanded, the protons and neutrons cooled, lost energy, and were eventually able to stick together without bouncing apart.

Question 52

The early universe had a large flux of neutrinos and antineutrinos. What happened to them?

A)They annihilated to produce protons and antiprotons.
B)They are still present as a cosmic background.
C)They were absorbed by matter, giving it some of the additional energy required for inflationary expansion.
D)They were absorbed by neutrons to form protons and electrons.

Question 53

The neutrino has been shown to have a small mass. What difference does that make in determining its speed?

A) Because it is a particle with mass, its speed is governed by the temperature of its surroundings because it will always be in thermal equilibrium.
B) Because it is a particle with mass, its speed must be less than the speed of light, c.
C) Its speed must be faster than the speed of light, because this is the only way that a lepton can have mass.
D) Because of its nature, a neutrino can only exist when it is traveling at the speed of light, just like photons of electromagnetic radiation.

Question 54

Why are we so sure the early universe MUST have had some fluctuations in density?

A)The existence of galaxies shows empirically that these density fluctuations must have been present. Theoretically the early universe was expected to be perfectly smooth.
B)These fluctuations are required by the Heisenberg uncertainty principle.
C)Mutual gravitational attraction will always lead to lumpiness, even in a distribution that is perfectly uniform.
D)This is related to the imbalance of matter and antimatter. This led to nonuniform annihilation reactions, and gravitational attraction led to collapse and lumpiness.

Question 55

To what does the phrase "Jeans length" refer?

A)the maximum distance over which a temperature fluctuation can occur in a "connected" universe
B)the minimum diameter of a density fluctuation that can collapse gravitationally to form a galaxy or other astronomical object
C)the minimum distance over which a temperature fluctuation can occur in a "connected" universe
D)the maximum diameter of a density fluctuation that can collapse gravitationally to form a galaxy or other astronomical object

Question 56

Suppose the Jeans length in a large interstellar cloud at a temperature of 20 K is 0.8 ly. If the cloud gradually heats up to 40 K while its density remains constant, what would happen to the Jeans length?

A)The Jeans length would decrease to 0.4 ly.
B)The Jeans length would increase to 1.6 ly.
C)The Jeans length would increase to 1.1 ly.
D)The Jeans length would decrease to 0.6 ly.

Question 57

The era of recombination saw the formation of the first permanent atoms, paving the way for the formation of larger structures. At this time the mass contained in a volume 1 Jeans length on a side was about 500,000 M. ${\odot}$ This suggests the mass of a typical structure formed early in the universe. What familiar object has this mass?

A)a spiral galaxy
B)a Population III star
C)an open cluster
D)a globular cluster

Question 58

The formula for the Jeans length is , where m is the single particle mass and ρ_m is the density. Population III stars formed in the early universe where the material consisted almost entirely of hydrogen and helium rather than material enriched by heavy elements. Thus, for the formation of Population III stars, L_J was:

A)small because ρ_m was large.
B)small because m was large.
C)large because m was small.
D)unaffected by the composition of the material.

Question 59

Consider a density fluctuation around the era of recombination when the universe had a temperature of 3000 K. If the density fluctuation is about 10 ly across, will the matter within that fluctuation begin clumping together?

A)The Jeans length for these numbers is about 100 ly. Matter will not begin clumping together because there is not enough mass in the fluctuation of 10 ly to overcome the gas pressure.
B)The Jeans length for these numbers is about 100 ly. Matter will begin clumping together since this is within the Jeans limit.
C)The Jeans length for these numbers is about 1 ly. Matter will not begin clumping together because there is not enough mass in the fluctuation to overcome the gas pressure.
D)The Jeans length for these numbers is about 1 ly. Matter will begin clumping together since this is within the Jeans limit.

Question 60

The early universe was characterized by a much lower metal content than the present universe. What difference did that make to the process of star formation?

A)The first generation of stars had lower metallicity than the present generation, but in each era the formation of large stars and small stars was equally likely.
B)It is easier to make a gas and dust cloud collapse when the metal content is low. Thus, even small stars could be formed in the early universe, but large stars were a rarity.
C)It is harder to make a gas and dust cloud collapse when the metal content is low. Thus, even small stars could be formed in the early universe, but large stars were a rarity.
D)It is harder to make a gas and dust cloud collapse when the metal content is low. Thus, large stars were more likely to be formed in the early universe, but small stars were a rarity.

Question 61

Detailed examination of the cosmic microwave background suggests that the material from which it scattered had been re-ionized. What do we believe to be the source of this reionization?

A)The cosmic microwave background itself caused the reionization of hydrogen and helium.
B)Large, hot Population III stars emitted enough high-energy radiation during their lifetimes to cause reionization.
C)The intense background of neutrinos and antineutrinos released much earlier than the cosmic microwave background radiation had enough energy to cause reionization.
D)The explosions of giant Population III stars would cause a shower of high-speed particles that could result in reionization.

Question 62

What do we expect for the lifetime of Population III stars?

A)These stars were almost pure hydrogen and helium, and such stars use only the hydrogen fusion reaction. Therefore, we expect their lifetimes were long.
B)These stars were characterized by almost zero metallicity, and such stars require large masses to collapse. With these large masses, we expect that their lifetimes were long.
C)These stars were characterized by almost zero metallicity, and such stars require large masses and consequently high central temperatures to collapse. With high central temperatures, we expect their lifetimes were short.
D)Since they were formed as early as 400 million years after the Big Bang, and we still find large numbers of them currently in existence, we know that their lifetimes are extremely long.

Question 63

Which one of the following statements correctly describes a major difference between galaxy formation with cold dark matter and galaxy formation with hot dark matter?

A)In both cases, galaxies form by the breakup of larger objects, but the breakup occurs faster if the dark matter is hot than if it is cold.
B)With cold dark matter, galaxies form by the breakup of larger objects, whereas with hot dark matter galaxies form through the merger of smaller objects.
C)In both cases, galaxies form through the merger of smaller objects, but the merger occurs faster if the dark matter is cold than if it is hot.
D)With cold dark matter, galaxies form through the merger of smaller objects, whereas with hot dark matter, galaxies form by the breakup of larger objects.

Question 64

Consider a large cloud of hydrogen gas, all at a uniform temperature. How do density fluctuations within this cloud affect its tendency to collapse gravitationally?

A)In those regions where the density is larger the gas pressure will also be larger, and the gas will be less likely to collapse.
B)In those regions where the density is larger the Jeans length is smaller, meaning that smaller regions of the gas are likely to collapse.
C)The Jeans length must be calculated for the entire cloud. Density fluctuations within the cloud have no effect.
D)The likelihood of gravitational collapse cannot be calculated from the information given.

Question 65

When we apply the Jeans length criterion to the conditions of the early universe, it is found that the structures most likely to have formed first are:

A)long filaments of galaxies.
B)individual spiral galaxies.
C)individual elliptical galaxies.
D)globular clusters.

Question 66

Models of the evolution of structure in the early universe fall into the categories of "bottom-up" formation (stars $\rightarrow$ superclusters) or "top-down" formation (large sheets $\rightarrow$ galaxies and stars). What parameter in the assumptions for the model is most important in determining which one of these scenarios results?

A)whether dark matter is hot or cold, cold dark matter resulting in the top-down scenario
B)whether dark matter is hot or cold, hot dark matter resulting in the top-down scenario
C)whether neutrinos have mass, massless neutrinos resulting in the top-down scenario
D)whether the early universe had an abundance or a scarcity of metals, an abundance resulting in the bottom-up scenario

Question 67

All of the following are attributes of string theory EXCEPT one. Which is the EXCEPTION?

A)Some of the dimensions of spacetime are hidden.
B)Massive particles are predicted-perhaps giving a clue to dark matter.
C)String theories can include quantum gravity.
D)String theory has been discredited in favor of M-theory.

Question 68

In what ways do the Kaluza-Klein multidimensional theories simplify our picture of the universe? Three of the following statements are correct. Which one is NOT a simplification of Kaluza-Klein theory?

A)In three space dimensions the trajectory of an object influenced by gravity appears straight, and in the multidimensional space of the Kaluza-Klein theory it appears straight.
B)Many of the additional dimensions cancel out, leaving us with a simpler system than we had originally.
C)Some dimensions in the multidimensional theory may not be observable because they are coiled tightly.
D)We may be able to account for the existence of dark matter and dark energy.

Question 69

Each of the following is an attribute of the Kaluza-Klein theories EXCEPT one. Which one is the EXCEPTION?

A)The theories have at least four space dimensions.
B)The dimensions we cannot see are curled tightly into loops.
C)Different loop vibrations correspond to particles with different masses.
D)In the new theories, gravitation no longer has an effect on space.

Question 70

In 1919, Theodor Kaluza introduced a five-dimensional spacetime. In 1995 Edward Witten extended this to 11 dimensions. Was this necessary? Why?

A)No, this was not really necessary. Witten was attempting to recast the theory to make its connections with general relativity more apparent.
B)Yes, this was necessary. Mistakes had been found in the earlier theory, and additional dimensions seemed the easiest way to fix these.
C)Yes, this was necessary. Between Kaluza and Witten, two additional forces had been discovered, and the natural solution was to add more dimensions to the theory.
D)Yes, this was necessary. Witten's extra dimensions were specifically an attempt to account for dark matter.

Question 71

In the string theory of Edward Witten:

A)there are 11 dimensions, including two time dimensions (essentially "forward" and "backward").
B)all of the dimensions are coiled up tightly like coiled strings.
C)the 11 dimensions include several dimensions that ought to be observable at very low temperatures.
D)seven dimensions are coiled tightly like coiled strings.

Question 72

Why might it be impossible for us to observe the very massive particles predicted by string theory?

A)They are not actually particles, but just points in space.
B)Extremely high energies are needed to create these particles.
C)The fundamental strings that make up these particles vibrate at frequencies we cannot detect.
D)The particles do not exist in three-dimensional space.