Question 1

Which of the following statements about rich clusters of galaxies (those with thousands of galaxies)is not true?

Accepted Answer

The statement that galaxies in the central regions of rich clusters of galaxies are predominantly spirals is not true. In fact, galaxies in the central regions are predominantly elliptical, while spirals are found more commonly in the outskirts.

Question 2

How do we determine the amount of dark matter in elliptical galaxies?

Accepted Answer

By measuring the speeds of stars at different radii from the galactic center, we can apply Kepler's laws of motion to determine how much mass is interior to the orbit. This allows us to determine the amount of dark matter present in elliptical galaxies.

Question 3

Why wasn't the intracluster medium in galaxy clusters discovered until the 1960s?

Accepted Answer

The intracluster medium emits X-rays which are blocked by the Earth's atmosphere, requiring X-ray satellites in space to observe it. This technology was not available until the 1960s, which is why the intracluster medium was not discovered until then.

Question 4

Why do we believe 90 percent of the mass of the Milky Way is in the form of dark matter?

Accepted Answer

The high orbital speeds of stars far from the galactic center, which cannot be accounted for by the visible matter in the Milky Way, suggest that there is a significant amount of unseen matter in the form of dark matter.

Question 5

Why do we call dark matter "dark"?

Accepted Answer

Dark matter is called "dark" because it emits no or very little radiation of any wavelength, making it invisible to telescopes and difficult to detect.

Question 6

When we see that a spectral line of a galaxy is broadened,that is,spanning a range of wavelengths,we conclude that

Accepted Answer

The broadening of a spectral line in a galaxy is due to the different Doppler shifts of the individual stars within that galaxy. Each star moves at its own velocity relative to us, causing its spectral lines to shift by varying amounts. This collective effect results in the broadening of the spectral line when observed from Earth.

Question 7

What is meant by "dark energy"?

Accepted Answer

Dark energy refers to the unknown force responsible for the observed accelerating expansion of the universe. It is thought to make up about 68% of the total energy density of the universe.

Answer: A, D, E 
 None of these choices accurately describe dark energy as they refer to dark matter and the early universe. Dark matter, unlike dark energy, is thought to be composed of particles that interact only weakly with other matter, and its energy is not necessarily related to mass through E=mc². Additionally, dark matter particles are believed to be non-relativistic and cannot be highly energetic. Finally, dark energy was not present in the very early universe, but instead became dominant relatively recently.

Question 8

If there is no dark matter in the Milky Way Galaxy,what is the best alternative explanation for the observations?

Accepted Answer

If dark matter does not exist, an alternative explanation is that our understanding of gravity might be incorrect on galaxy-size scales, which could account for the observed discrepancies in orbital velocities.

Question 9

Which of the following methods used to determine the mass of a cluster does not depend on Newton's laws of gravity?

Accepted Answer

Methods A and B rely on Newton's laws of gravity to infer mass based on the motion of galaxies and the temperature of gas, which is influenced by the gravitational potential of the cluster. Method C, gravitational lensing, although it involves gravity, does not depend on Newton's laws but rather on the general theory of relativity by Einstein, which describes how mass warps space-time, causing the bending of light from distant objects.

Question 10

Compared to the central regions of spiral galaxies,we expect elliptical galaxies to have

Accepted Answer

Elliptical galaxies tend to have higher mass-to-light ratios because they are populated by older, dimmer stars compared to the younger, brighter stars found in the spiral arms of spiral galaxies. This means for a given amount of light, elliptical galaxies have more mass, leading to a higher mass-to-light ratio.

Question 11

Which of the following is not evidence for dark matter?

Accepted Answer

The expansion of the universe is not evidence for dark matter, but rather for dark energy. Dark matter is believed to affect the motion of objects within galaxies, while dark energy affects the expansion of the universe as a whole.

Question 12

The distribution of the dark matter in a spiral galaxy is

Accepted Answer

Dark matter in spiral galaxies is thought to form a roughly spherical halo that extends well beyond the visible galaxy, often about ten times the size of the galaxy's luminous halo.

Question 13

If a galaxy's overall mass-to-light ratio is 100 solar masses per solar luminosity,and its stars account for only 5 solar masses per solar luminosity,how much of the galaxy's mass must be dark matter?

Accepted Answer

We know that the galaxy's overall mass-to-light ratio is 100 solar masses per solar luminosity, which includes both visible and invisible matter. However, we also know that the visible stars account for only 5 solar masses per solar luminosity. Therefore, the remaining 95 solar masses per solar luminosity must be due to the presence of dark matter. This means that approximately 95% of the galaxy's total mass must be made up of dark matter. Hence, the correct choice is B) 95 percent.

Question 14

What evidence suggests that the Milky Way contains dark matter?

Accepted Answer

Observations of clouds of atomic hydrogen far from the galactic center show that they are orbiting the Milky Way at unexpectedly high speeds, which suggests the presence of additional mass that we cannot see. This additional mass is believed to be dark matter. The other options, such as dark material blocking starlight and observations in different wavelengths, do not directly provide evidence for the presence of dark matter.

Question 15

Gravitational lensing occurs when

Accepted Answer

Gravitational lensing occurs when a massive object, such as a galaxy or a cluster of galaxies, bends the path of light from a more distant object behind it. This bending of light can create multiple images or distort the shape of the background object.

Question 16

How do we know that there is much more mass in the halo of our galaxy than in the disk?

Accepted Answer

Stars in the outskirts of the Milky Way orbit the galaxy at much higher speeds than we would expect if all the mass were concentrated in the disk, indicating the presence of a significant amount of mass in the halo.

Question 17

What is the mass-to-light ratio for the inner region of the Milky Way Galaxy,in units of solar masses per solar luminosity?

Accepted Answer

The mass-to-light ratio for the inner region of the Milky Way Galaxy is estimated to be around 6 solar masses per solar luminosity. This means that for every unit of luminosity that we observe, there are approximately 6 solar masses of matter present. This value suggests that there is a large amount of dark matter in the center of the Milky Way, since the observed luminosity alone cannot account for the observed gravitational effects. Other choices are incorrect, as they are either too high (A, B, and C) or too low (E) compared to the estimated value.

Question 18

How are rotation curves of spiral galaxies determined beyond radii where starlight can be detected?

Accepted Answer

Rotation curves of spiral galaxies beyond radii where starlight can be detected can be determined through observations of the 21 cm line of atomic hydrogen. This technique is used to map the distribution of neutral hydrogen gas in galaxies and indirectly trace the mass distribution, allowing for the determination of rotation curves.

Question 19

A large mass-to-light ratio for a galaxy indicates that

Accepted Answer

A high mass-to-light ratio indicates that there is a large amount of matter (mass) in the galaxy relative to the amount of light being emitted. This suggests that on average, each solar mass of matter in the galaxy emits less light than our Sun, making option C the correct choice. Options A, B, and E can be eliminated because the mass-to-light ratio alone does not provide information about the overall mass of the galaxy or the size/mass of individual stars within the galaxy. Option D is also incorrect because a high mass-to-light ratio means that there is less light emitted per unit mass, not more.

Question 20

Which of the following particles are baryons?

Accepted Answer

Baryons are particles made up of three quarks. Protons are baryons as they consist of two up quarks and one down quark. Electrons, neutrinos, and photons are all leptons, while quarks are fundamental building blocks of baryons.

Quiz 23: Dark Matter, Dark Energy, and the Fate of the Universe

Which of the following statements about rich clusters of galaxies (those with thousands of galaxies) is not true?