Question 1

When we say that a cluster of galaxies is acting as a gravitational lens, what do we mean?

Accepted Answer

When we say that a cluster of galaxies is acting as a gravitational lens, we mean that it is bending or distorting the light coming from galaxies located behind it. This is due to the mass of the cluster warping the space around it, causing light to bend as it passes through.

Question 2

A mass- to- light ratio for a galaxy of much greater than one indicates that

Accepted Answer

A high mass-to-light ratio indicates that for each unit of mass, there is relatively little light being emitted. This means that, on average, each solar mass of matter in the galaxy emits much less light than our Sun, suggesting the presence of a significant amount of dark matter or non-luminous matter.

Question 3

Refer to this scenario for the following questions:
A multi- dimensional being reaches down to Earth and pulls you out of the universe. You are then thrown back into the universe at a place and time of the being's choosing, and you are permitted to leave only after you have identified your surroundings. This process is repeated several times. Through a scientifically unexplainable miracle, you are able to survive in every one of the places that you find yourself. In each scenario below, identify your surroundings (and potentially your cosmic era)from among the choices given.
-You are in a place that is extremely hot and dense. You can't see far because your surroundings are opaque to light. Around you, nuclear fusion is converting carbon into oxygen and other elements. Where are you?

Accepted Answer

The description matches the conditions of the center of a massive star near the end of its life, where nuclear fusion is converting heavier elements into lighter ones. The opacity and extreme heat are not present in the other options.

Question 4

Why can't the dark matter in galaxies be made of neutrinos?

Accepted Answer

Neutrinos travel at extremely high speeds and can escape a galaxy's gravitational pull, making them unlikely candidates for the majority of dark matter in galaxies.

Question 5

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

Accepted Answer

Measuring the amount of distortion caused by a gravitational lens does not depend on Newton's law of gravity. This method involves observing the way in which the light from background objects is bent and distorted by the gravity of the cluster of galaxies, which allows astronomers to estimate the total mass of the cluster. This method relies on the theory of general relativity rather than Newton's law of gravity. On the other hand, options B and C rely on Newton's law of gravity to determine the mass of the cluster.

Question 6

Visible, luminous matter (such as the stars within galaxies)amounts to what percentage of the critical density (the density of mass- energy needed to make the geometry of the universe flat)?

Accepted Answer

Visible, luminous matter only makes up less than 1 percent of the critical density. The majority of the critical density is made up of dark matter and dark energy.

Question 7

Measuring the amount of deuterium in the universe allows us to set a limit on

Accepted Answer

The amount of deuterium in the universe is directly related to the density of baryonic matter. By measuring the amount of deuterium, we can set limits on the density of baryonic matter in the universe. This is because deuterium is synthesized during the Big Bang and its abundance is sensitive to the number of baryons present in the early universe. Therefore, the ratio of deuterium to hydrogen can be used as a probe of the total matter density of the universe.

Question 8

Why does the temperature of the gas between galaxies in galaxy clusters tell us about the mass of the cluster?

Accepted Answer

The temperature of the gas particles in the cluster is related to their average speeds, which are held in the cluster by gravity. The gravitational force is directly related to the mass of the cluster, so by measuring the temperature of the gas, we can infer the cluster's mass.

Question 9

The quantity called the "enclosed mass" M(<r)represents ALL of the mass inside radius r. If the enclosed mass M(<r)is constant with increasing radius, where is the mass?

Accepted Answer

If the enclosed mass is constant with increasing radius, it means that the mass inside every spherical shell increases at the same rate. This implies that the mass is concentrated in the center of the object.

Question 10

A gravitational lens occurs when

Accepted Answer

A gravitational lens occurs when a massive object, such as a galaxy or cluster of galaxies, bends the path of light from a more distant object, acting as a lens and magnifying the image of the distant object. This phenomenon is a direct result of Einstein's theory of General Relativity.

Question 11

How do we know the expansion of the universe is accelerating?

Accepted Answer

Observations of white dwarf supernovae in distant galaxies show that they are fainter than they would be if the expansion of the universe were not accelerating. This indicates that the expansion is accelerating, as the light from these supernovae has to travel through more space before reaching us, causing it to be stretched and appear fainter.

Question 12

Why isn't the space within our solar system or the Milky Way expanding according to Hubble's Law?

Accepted Answer

The gravity exerted by the solar system and the Milky Way is strong enough to hold them together against the expansion of the universe. Hubble's Law applies to the space between galaxies, but not within them. Therefore, the expansion of the universe does not affect the space within our solar system or the Milky Way.

Question 13

Which of the following is not evidence for dark matter?

Accepted Answer

The expansion of the universe is not evidence for dark matter. Dark matter is a hypothetical type of matter that is thought to make up about 85% of the matter in the universe. It is invisible to telescopes and does not interact with light or other electromagnetic radiation. The expansion of the universe is a well-established fact, but it is not directly related to dark matter.

Question 14

Why is the cause of the acceleration of the universe called "dark energy"?

Accepted Answer

The term "dark energy" was coined to describe the unknown force that is causing the acceleration of the universe. It caught on with other astronomers and has become the standard term used in the field.

Question 15

What is a rotation curve?

Accepted Answer

A rotation curve is a graph showing how orbital velocity depends on distance from the center for a spiral galaxy. It is an important tool for studying the distribution and amount of mass in galaxies, and can reveal the presence of dark matter. Options A, B, and D do not accurately describe a rotation curve.

Question 16

What is the distinguishing characteristic of what we call ordinary matter (such as the matter that makes up stars, planets, and people)?

Accepted Answer

Ordinary matter is characterized by its atomic and/or ionic structure, which consists of protons and neutrons in the nucleus, surrounded by electrons. This is a defining feature of all atoms, molecules, and compounds that make up the world around us. Options A and B are not unique to ordinary matter, as other types of matter (such as dark matter) can also emit light or interact through gravity. Option D refers to a type of hypothetical particle that has yet to be observed or detected.

Question 17

If WIMPs really exist and make up most of the dark matter in galaxies, which of the following is not one of their characteristics?

Accepted Answer

WIMPs are believed to be subatomic particles that interact weakly with other matter, which is why they are difficult to detect. They are also thought to be non-luminous and do not emit or absorb light, which is why they are classified as dark matter. However, they are not expected to travel at speeds close to the speed of light. They are thought to orbit at large distances from the galactic center, but this is not their defining characteristic.

Question 18

Which of the following best sums up current scientific thinking about the nature of dark matter?

Accepted Answer

While there are many theories about the nature of dark matter, current scientific thinking suggests that most dark matter is made up of weakly interacting particles that have yet to be identified. While WIMPs are one possibility, there is not yet enough evidence to confirm that they are the main constituents of dark matter. The idea that dark matter does not exist is not widely accepted among scientists, as there is substantial evidence from multiple sources that indicates its presence. The idea that most dark matter is made up of ordinary matter in the halo of the galaxy is also not widely accepted, as the observed rotation curves of galaxies suggest the presence of vast amounts of unseen matter that is not ordinary.

Question 19

Which of the following is not one of the three main strategies used to measure the mass of a galaxy clusters?

Accepted Answer

Measuring the temperature of stars in the halos of the galaxies is not one of the three main strategies used to measure the mass of a galaxy cluster. The three main strategies are observing how the cluster bends light from galaxies located behind it, studying X- ray emission from hot gas inside the cluster, and measuring the speeds of galaxies orbiting the cluster's center.

Question 20

Refer to this scenario for the following questions:
A multi- dimensional being reaches down to Earth and pulls you out of the universe. You are then thrown back into the universe at a place and time of the being's choosing, and you are permitted to leave only after you have identified your surroundings. This process is repeated several times. Through a scientifically unexplainable miracle, you are able to survive in every one of the places that you find yourself. In each scenario below, identify your surroundings (and potentially your cosmic era)from among the choices given.
-You find yourself in a place that is unimaginably hot and dense. A rapidly changing gravitational field randomly warps space and time. Gripped by these huge fluctuations, you notice that there is but a single, unified force governing the universe. Where are you?

Accepted Answer

The description matches the conditions of the early universe before the Planck time, where the temperature and density were unimaginably high and the gravitational fields were rapidly changing. The unified force governing the universe is believed to have existed during this time.

Quiz 18: Dark Matter, Dark Energy, and the Fate of the Universe