Question 1

If the universe is accelerating, it will expand forever.

Accepted Answer

If the universe's expansion is accelerating, it means the rate of expansion is increasing over time. This implies that the force driving the expansion (dark energy) is stronger than the force of gravity, which would otherwise slow down the expansion. The result of this acceleration is that the universe will expand forever, without ever collapsing back in on itself.

Question 2

The actual matter density of the universe, accounting for all of the luminous matter and all of the dark matter known to exist in galaxies and clusters, is what fraction of the critical density?

Accepted Answer

The actual matter density of the universe, including both luminous and dark matter, is approximately 25 percent of the critical density. This means that matter alone is not enough to reach the critical density needed for a flat universe, and dark energy is believed to make up the rest.

Question 3

Gravitational Lensing: Summarize the observational evidence that a gravitational field can deflect the path of light. In figures such as 16.8 on page 451, how would you prove to a skeptic that the distorted images indicated by the arrows are indeed multiple images of a single gravitationally-lensed galaxy. Muster as much observational evidence as you can.

Accepted Answer

The answer of Gravitational Lensing: Summarize the observational evidence that...

Question 4

A galaxy with a lot of dark matter would have a high mass-to-light ratio compared to the Sun.

Accepted Answer

Dark matter has mass but does not emit light, so a galaxy with a lot of dark matter would have a higher total mass compared to the amount of light it emits, resulting in a higher mass-to-light ratio than the Sun.

Question 5

Some galaxy clusters have not finished forming and are still attracting new members today.

Accepted Answer

Galaxy clusters are constantly evolving and can continue to attract new members through a process known as accretion. This can occur through gravitational interactions with smaller nearby galaxies or through the merging of two or more galaxy clusters. Therefore, some galaxy clusters are still in the process of forming and growing even today.

Question 6

Which model of the universe gives the youngest age for its present size and expansion rate?

Accepted Answer

The answer of Which model of the universe gives the...

Question 7

If we were to learn that the universe is a re-collapsing universe, then we must have misinterpreted Hubble's Law because the universe must be contracting, rather than expanding as generally believed.

Accepted Answer

Hubble's Law itself is an observation that galaxies are moving away from us at speeds proportional to their distances, indicating the universe is expanding. The concept of a re-collapsing universe doesn't necessarily contradict Hubble's Law but suggests that at some point in the future, the expansion could reverse into a contraction due to gravitational forces overcoming the expansion. This does not imply a current contraction or a misinterpretation of Hubble's Law.

Question 8

Which of the following best describes how galaxies are distributed on large scales in the universe?

Accepted Answer

The distribution of galaxies on large scales in the universe appears to form chains and sheets that surround great voids, rather than being uniformly or randomly distributed. This pattern is known as the large-scale structure of the universe and is evident in surveys like the Sloan Digital Sky Survey. While there is a hierarchy of structures (clusters, superclusters, etc.), the overall distribution appears to follow a web-like pattern. Option E is not accurate as there is no center of the universe.

Question 9

An Accelerating Universe? Current evidence that the expansion of the universe is accelerating is based entirely upon observations of distant white dwarf supernova. List all of the assumptions that you can think of that are made in using white dwarf supernova as distance indicators. Is it conceivable that any of these assumptions may be incorrect?

Accepted Answer

The answer of An Accelerating Universe? Current evidence that the...

Question 10

How do we know that there can only be insignificant amounts of dark matter in our solar system?

Accepted Answer

The answer of How do we know that there can...

Question 11

How do astronomers create three-dimensional maps of the universe?

Accepted Answer

Astronomers create three-dimensional maps of the universe by using a galaxy's position on the sky and its redshift to determine its distance along the line of sight. This is known as redshift space distortion and it allows astronomers to map out large-scale structures of the universe. A computer model may be used to help interpret the data, but the observations themselves provide the basis for the map.

Question 12

MACHOs are expected to be found only in the disk of the Milky Way, whereas WIMPs would be found only in the halo.

Accepted Answer

This statement is false. While MACHOs are believed to be primarily located in the disk of the Milky Way, they could also exist in the halo. Similarly, while WIMPs are believed to be primarily located in the halo, they could also exist in other regions such as galactic disks.

Question 13

One possible ingredient of dark matter is known as WIMPs, or weakly interacting massive particles. Our best guess is that WIMPs probably are made of protons and neutrons.

Accepted Answer

While WIMPs are weakly interacting and massive, they are not believed to be made of protons and neutrons. The exact composition of dark matter is still unknown.

Question 14

What might be causing the universe to accelerate?

Accepted Answer

Observations have shown that the expansion of the universe is accelerating, and this can be explained by the existence of a mysterious force called dark energy. Dark energy is believed to be a property of space itself, pushing everything apart and causing the acceleration of the universe's expansion. Other options, such as WIMPs or MACHOs, are potential candidates for dark matter, but they have not been observed to directly contribute to the acceleration of the universe. Gravitation is a fundamental force that can cause objects to attract each other, but it cannot explain the observed acceleration of the universe. White-dwarf supernovae explosions were used to initially detect the acceleration of the universe, but they are not causing the acceleration themselves.

Question 15

The visible parts of galaxies contribute about 10 percent of the critical density of the universe.

Accepted Answer

The answer of The visible parts of galaxies contribute about...

Question 16

If all the "dark matter" in our universe were to be instantaneously removed, which of the following would not happen?

Accepted Answer

The Solar System's dynamics are governed by the gravitational forces of its own mass, primarily the Sun, and not significantly influenced by dark matter.

Question 17

"Beauty" in Astronomy: In the absence of observational evidence, scientists often rely on other considerations to make decisions. One such consideration that we have met before (Chapter 3) is Occam's razor: the simplest explanation is to be preferred. Another consideration that is often used (and sometimes championed) by astronomers is the perceived "beauty" of the ideas themselves. Imagine you lived at a time before observational tests could be brought to bear on which of the ultimate fates (re-collapsing, critical, coasting, or accelerating) applies to our Universe. Which fate you would support based on your own personal idea of "beauty"? Give your arguments, even if you think they are not scientific. Did you choose the observationally supported theory (you may need to skip ahead to Chapter 17)? What do you think of the role such considerations play in science? Are they sometimes necessary?

Accepted Answer

The answer of "Beauty" in Astronomy: In the absence of...

Question 18

What fraction of the mass needed to halt expansion of the universe exists in the form of visible, luminous matter (such as the stars within galaxies)?

Accepted Answer

According to scientific research, the fraction of the mass needed to halt the expansion of the universe is estimated to be around 27%, of which only a small fraction (less than 1%) exists in the form of visible, luminous matter such as stars and galaxies. The remaining mass is made up of dark matter and dark energy, which have yet to be fully understood.

Question 19

MACHOs (massive compact halo objects) have been searched for using the technique of gravitational lensing, but, to date, none have been found.

Accepted Answer

MACHOs have been detected using gravitational lensing, but they are not sufficient to account for all dark matter.

Question 20

Dark matter in galaxies and clusters of galaxies is purely hypothetical because we have no way of detecting its presence.

Accepted Answer

While dark matter cannot be directly observed, its effects on the visible matter in galaxies and clusters can be detected through gravitational lensing and other observations, providing strong evidence for its existence.

Quiz 16: Dark Matter, Dark Energy, and the Fate of the Universe