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?

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?

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?

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?

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:

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?

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?

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?

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)?

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?

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?

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?

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?

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?

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?

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?

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)?

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?

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.)

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))

Accepted Answer

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

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