Question 1

At present, the energy density of matter greatly exceeds that of radiation. But in the early universe, radiation energy was dominant. Why was this?
A)Dark matter had not yet formed to add to the matter energy density.
B)The radiation temperature was high enough to prevent the transformation of radiation into matter; photon Y matter + antimatter.
C)Radiation was at a much greater temperature (and energy) and density than now.
D)Large numbers of antiparticles existed and combined with matter particles to form photons.

Accepted Answer

In the early universe, the temperature and energy density of radiation were much higher than they are now, which made radiation energy dominant over matter energy.

Question 2

At about what time after the Big Bang did the universe pass through the transition from a radiation-dominated to a matter-dominated universe? A)about 15 billion years, as stars began to form B)24,000 years C)380,000 years D)at the Planck time, 10^-43 s

Accepted Answer

The transition from a radiation-dominated to a matter-dominated universe occurred approximately 24,000 years after the Big Bang. This transition marks the era when the density of matter surpassed the density of radiation, leading to the universe's structure being more influenced by matter.

Question 3

The ionization energy of a hydrogen atom, that is the energy required to remove the electron from the atom completely, is 13.6 electron volts. Suppose it were 25 electron volts instead. What difference would this make to cosmology?
A)None. Individual atoms are much too small to have any effect on the evolution of the universe.
B)The cosmic microwave background radiation would be hotter than 2.7 kelvins.
C)The cosmic microwave background radiation would be cooler than 2.7 kelvins.
D)There would be no cosmic microwave background radiation.

Accepted Answer

The ionization energy of hydrogen affects the temperature at which hydrogen atoms can form in the early universe. If the ionization energy were higher, it would take more energy (or a higher temperature) for electrons to escape hydrogen atoms, meaning the universe would have to cool down more before atoms could form. This would delay the release of photons that create the cosmic microwave background radiation, leading to a hotter cosmic microwave background than the observed 2.7 kelvins.

Question 4

Suppose the ionization energy of hydrogen were larger than 13.6 eV and the decoupling of radiation and matter had occurred earlier, say at z = 2000. What would the temperature have been at that time?
A)2500 K
B)3000 K
C)3500 K
D)5500 K

Accepted Answer

The temperature at the time of decoupling is related to the redshift z by the formula T = T0 * (1 + z), where T0 is the current temperature of the cosmic microwave background, approximately 2.7 K. At z = 2000, the temperature would be 2.7 K * 2000 = 5400 K, which is closest to 5500 K.

Question 5

The conditions in the early universe, when the universe suddenly became transparent to radiation, in terms of age t, peak background wavelength λ, and equivalent temperature T, were: A)t = 3 minutes, λ = 0.3 μm, near UV, and T = 10⁷ K. B)t = 380,000 years, λ = 1 μm, infrared, and T = 3000 K. C)t = 3 × 10¹¹ years, λ = 1 mm, microwaves, and T = 3 K. D)The universe was never dominated by radiation, because matter has always dominated by generating the gravitational field into which the universe has expanded.

Accepted Answer

This is known as the cosmic microwave background radiation (CMB) era, which occurred when the universe was about 380,000 years old. The peak background wavelength was around 1 μm, in the infrared part of the spectrum, with an equivalent temperature of about 3000 K. This is the background radiation that we can observe today and is often considered strong evidence for the Big Bang theory. Option A refers to the nucleosynthesis era, which occurred earlier at around 3 minutes after the Big Bang, while option C refers to the recombination era, which occurred later at around 300,000 years after the Big Bang. Option D is incorrect because radiation did dominate the early universe, particularly during the CMB era.

Question 6

As we look at more distant regions of space, we see those regions as they existed at earlier times, but our farthest views are blocked by a "wall"-beyond which the universe is opaque. What event occurred at the time marked by this wall?
A)Electrons and protons combined to form neutral hydrogen atoms, making the universe transparent for the first time.
B)Quarks combined to form neutrons and protons, removing $\gamma$ -ray absorbers for the first time.
C)Gravity froze out as a separate force.
D)Electrons and protons combined to form helium atoms, removing major absorbers of electromagnetic radiation from the universe for the first time.

Accepted Answer

The wall that blocks our farthest views of the universe is known as the cosmic microwave background radiation (CMBR), which was created when the universe became transparent for the first time. Before this event, the universe was filled with a hot, dense, ionized plasma that absorbed all electromagnetic radiation. As the universe expanded and cooled, electrons and protons combined to form neutral hydrogen atoms, which no longer interacted with this radiation, making the universe transparent for the first time. The CMBR is the afterglow of this event and provides important evidence for the Big Bang theory.

Question 7

Figure 25-13 of Universe, 11th ed., shows temperature variations in the cosmic microwave background. Dark areas in the image correspond to regions where the photons were gravitationally redshifted when they left the early universe. Suppose we point a telescope at one of these regions now. What would we expect to see?
A)nothing, because the early universe no longer exists
B)a fuzzy ball, because the primordial fireball is as far back as we can see
C)a red glow, because of the gravitational redshift
D)some structure, perhaps a cluster of galaxies, because such regions indicate clumps of larger density in the early universe

Accepted Answer

Regions in the cosmic microwave background that are darker indicate areas of higher density in the early universe. These denser regions eventually led to the formation of structures such as galaxies and clusters of galaxies due to gravitational attraction. Thus, pointing a telescope at one of these regions today would likely reveal some form of cosmic structure, like a cluster of galaxies.

Question 8

When the universe was about 24,000 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 ¹H $ \rightarrow $ ⁴He could proceed. D)Neutrons decayed to produce protons and electrons, the building blocks of atoms.

Accepted Answer

Atoms are formed when neutral atoms are able to exist, meaning that the electrons are no longer stripped from the atoms due to high energy levels of photons. When the radiation density decreases to the point where it is equal to the matter density, the average photon energy has decreased to the point where it can no longer ionize hydrogen, allowing neutral atoms to form.

Question 9

What condition is necessary for the universe to be unbounded (infinite in extent)?
A)The density of the universe must be equal to or less than some critical value.
B)The density of the universe must be equal to or greater than some critical value.
C)The universe must have no mass in it.
D)The density of the universe must be exactly equal to some critical value.

Accepted Answer

If the density of the universe is equal to or less than some critical value, the universe is expanding at a rate that is greater than or equal to the escape velocity, which means it is unbounded and infinite in extent. If the density is greater than this critical value, the universe would eventually collapse in on itself due to its own gravity, making it bounded and of finite extent.

Question 10

What will happen if the universe is unbounded?
A)The universe will expand forever.
B)The universe will expand to some maximum size and then fragment into mini-universes.
C)The universe will reach a maximum size and stay there, like a balloon that has been inflated.
D)The universe will eventually fall back in on itself, heading toward a recollapse.

Accepted Answer

If the universe is unbounded, it means there are no spatial boundaries to the universe. In this case, there is no "edge" to the universe and therefore no limit to how far it can expand. Current observations and measurements of the universe suggest that it is in fact unbounded and will continue to expand indefinitely.

Question 11

What will happen if the universe is bounded?
A)The universe will expand to some maximum size and then fragment into mini-universes.
B)The universe will eventually fall back in on itself, heading toward a recollapse.
C)The universe will reach a maximum size and stay there, like a balloon that has been inflated.
D)The universe will expand forever.

Accepted Answer

If the universe is bounded, it means that it has a finite amount of space. In this case, the gravitational forces present in the universe will eventually cause it to stop expanding and to start contracting. This contraction will continue until all matter in the universe is compressed into a single point, a scenario known as the Big Crunch. Therefore, option B is the correct answer.

Question 12

In cosmology, to what does the phrase "critical density" refer?
A)the density of the universe above which matter is ionized and the universe is opaque
B)the density of the universe above which the universe is bounded and below which it is unbounded
C)the density of the universe below which the universe will stop expanding
D)the density of the universe below which the universe is bounded and above which it is unbounded

Accepted Answer

Critical density refers to the density of the universe above which the universe is bounded (closed) and below which it is unbounded (open). This density is also called the density parameter and is used to determine the ultimate fate of the universe (i.e. whether it will continue to expand indefinitely or eventually collapse).

Question 13

How does the measured mass density of matter in the universe, including the hypothesized dark matter, compare to the critical density required to just close the universe?
A)The measured density is about 5 times the critical density.
B)The measured density is equal to the critical density to within the measurement uncertainty.
C)The measured density is less than 0.1 of the critical density.
D)The measured density is between 0.2 and 0.4 of the critical density.

Accepted Answer

The measured mass density of matter in the universe, including the hypothesized dark matter, is estimated to be between 0.2 and 0.4 times the critical density required to just close the universe. This means that the universe is open, and will continue to expand indefinitely.

Question 14

If space has a hyperbolic geometry (unbounded universe), what will happen to two initially parallel flashlight beams as they traverse billions of light years of space?
A)They will remain parallel.
B)They will gradually diverge (move apart) to a maximum separation, and then gradually converge and cross.
C)They will gradually converge (move together) and eventually cross.
D)They will gradually diverge (move apart).

Accepted Answer

In a hyperbolic geometry, parallel lines (or in this case, the paths of light beams) will gradually diverge from each other as they extend into the distance. This reflects the nature of hyperbolic space, where the geometry is such that the rules of Euclidean geometry (like the parallel postulate) do not apply in the same way, leading to diverging paths instead of parallel ones.

Question 15

Measurement of structure in the cosmic microwave background radiation has recently indicated that we live in a flat universe between a closed and an open universe, and yet the measured density of detected matter and radiation is only 20% to 40% of the critical density required for a flat universe. In what form is the other 60% to 80% of the "matter" likely to be?
A)antimatter, which generates a negative gravitational effect and emits radiation only if it meets matter and is annihilated
B)neutrinos, which have very little rest mass and are very difficult to detect, but are very abundant
C)large numbers of small primordial black holes, whose gravitational effects are spread throughout the universe and that emit no radiation
D)dark energy, emitting no radiation and generating no detectable gravitational effects

Accepted Answer

The missing mass is likely in the form of dark energy, which is believed to be responsible for the observed accelerating expansion of the universe. Dark energy does not emit radiation and its gravitational effects are difficult to detect, which is why it has not been directly observed.

Question 16

Einstein introduced a "cosmological constant" into his formulation of the structure of the universe on the basis of the general theory of relativity. How did he envision that this cosmological constant would manifest itself?
A)as antimatter that, by annihilating with real matter, would translate matter into energy, thereby maintaining a constant mass density in an condensing universe
B)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
C)as a form of energy that, on its own, would make the universe expand-a form of antigravity
D)as an extra "gravity" that would hold the universe against continuous expansion

Accepted Answer

The cosmological constant is a form of energy that, on its own, would make the universe expand-a form of antigravity.

Question 17

The future of our universe-continuous expansion or eventual contraction-can be determined by observing the rate at which cosmological expansion is changing because of gravitational attraction between masses in the universe. How would this effect of deceleration be measured in terms of the Hubble relationship between speed of recession (graphed vertically) and distance (graphed horizontally)?
A)The straight line will become slightly curved and steeper at the largest distances.
B)The straight line will be found to be slightly curved and steeper at the shortest distances, but otherwise it will be straight.
C)The straight line will become slightly curved and less steep at the largest distances.
D)The straight-line slope will become zero at some distance and change to the opposite slope beyond this distance.

Accepted Answer

The answer of The future of our universe-continuous expansion or...

Question 18

It is not yet known whether we live in an unbounded universe that will expand forever or in a bounded universe that will eventually recollapse. How are astronomers trying to settle this question? A)by accurate measurement of the speed of a galaxy of known distance at two different times, to measure the deceleration directly B)by measuring the curvature of space by tracing photon paths to Earth from distant galaxies C)by determining how much galaxy speeds depart from a straight-line Hubble relationship, v = H₀ d, at large distances D)by careful observations of the size of the irregularities in the cosmic microwave background radiation

Accepted Answer

The answer of It is not yet known whether we...

Question 19

If there is no dark energy, what is the future of the universe? (See Figure 25-18 of Universe, 11th ed.)

A)The future of the universe is not related to the geometry of space.
B)It will expand forever, not stopping even after infinite time, regardless of how much matter is in the universe.
C)It will expand to a maximum size and then recollapse.
D)It will barely expand forever, reaching zero expansion speed after infinite time.

Accepted Answer

If there is no dark energy, the fate of the universe is determined by the amount of matter in the universe and the geometry of space. If the amount of matter is high enough, gravity will eventually overcome the expansion and cause the universe to recollapse. This is known as the Big Crunch scenario, which is depicted in Figure 25-18 as the "closed universe" model.

Question 20

If there is dark energy in the universe with a positive energy density, what is the future of the universe? (See Figure 25-18 of Universe, 11th ed.(in the text))
A)It will expand forever at a rate that depends upon how much matter there is in the universe.
B)It will expand forever at an accelerating rate, regardless of how much matter there is in the universe.
C)The future of the universe is not related to the geometry of space.
D)It will expand to a maximum size and then recollapse.

Accepted Answer

The answer of If there is dark energy in the...

Quiz 25: Cosmology: The Origin and Evolution of the Universe

At present, the energy density of matter greatly exceeds that of radiation. But in the early universe, radiation energy was dominant. Why was this?

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

The ionization energy of a hydrogen atom, that is the energy required to remove the electron from the atom completely, is 13.6 electron volts. Suppose it were 25 electron volts instead. What difference would this make to cosmology?

Suppose the ionization energy of hydrogen were larger than 13.6 eV and the decoupling of radiation and matter had occurred earlier, say at z = 2000. What would the temperature have been at that time?

The conditions in the early universe, when the universe suddenly became transparent to radiation, in terms of age t, peak background wavelength λ, and equivalent temperature T, were:

As we look at more distant regions of space, we see those regions as they existed at earlier times, but our farthest views are blocked by a "wall"-beyond which the universe is opaque. What event occurred at the time marked by this wall?

When the universe was about 24,000 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?

What condition is necessary for the universe to be unbounded (infinite in extent) ?

What will happen if the universe is unbounded?

What will happen if the universe is bounded?

In cosmology, to what does the phrase "critical density" refer?

How does the measured mass density of matter in the universe, including the hypothesized dark matter, compare to the critical density required to just close the universe?

If space has a hyperbolic geometry (unbounded universe) , what will happen to two initially parallel flashlight beams as they traverse billions of light years of space?

Einstein introduced a "cosmological constant" into his formulation of the structure of the universe on the basis of the general theory of relativity. How did he envision that this cosmological constant would manifest itself?

It is not yet known whether we live in an unbounded universe that will expand forever or in a bounded universe that will eventually recollapse. How are astronomers trying to settle this question?

If there is no dark energy, what is the future of the universe? (See Figure 25-18 of Universe, 11th ed.)

If there is dark energy in the universe with a positive energy density, what is the future of the universe? (See Figure 25-18 of Universe, 11th ed.(in the text) )