Astronomy Ch. 23

Why do we call dark matter "dark"?

What is meant by "dark energy"?

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

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

What evidence suggests that the Milky Way contains dark matter?

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

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

The distribution of the dark matter in a spiral galaxy is

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

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

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

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

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

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?

Which of the following methods used to determine the mass of a cluster does not depend on Newton's laws of gravity? A) measuring the orbital velocities of galaxies in a cluster B) measuring the temperature of X-ray gas in the intracluster medium C) measuring the amount of distortion caused by a gravitational lens D) none of the above

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

Which of the following statements about rich clusters of galaxies (those with thousands of galaxies) is not true? A) They are sources of X-ray emission due to the presence of hot, intergalactic gas. B) There likely have been numerous collisions among the member galaxies at some time in the past. C) Galaxies in the central regions are predominantly spirals, while elliptical galaxies roam the outskirts. D) They often have a very large, central dominant galaxy near their center, perhaps formed by the merger of several individual galaxies. E) The speeds of the galaxies in the cluster indicate that most of the cluster mass is dark matter

Gravitational lensing occurs when

Which of the following is not evidence for dark matter? A) the flat rotation curves of spiral galaxies B) the broadening of absorption lines in an elliptical galaxy's spectrum C) X-ray observations of hot gas in galaxy clusters D) gravitational lensing around galaxy clusters E) the expansion of the universe

Which of the following particles are baryons? A) electrons B) neutrinos C) protons D) quarks E) photons

Which of the following is an example of baryonic matter? A) you B) the particles produced by physicists in particle accelerators C) electrons and positrons produced by pair production D) WIMPs E) neutrinos

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

Based on current evidence concerning the amount of deuterium in the universe, we can conclude that

What do we mean when we say that a particle is a weakly interacting particle?

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

Which of the following are candidates for dark matter? A) brown dwarfs B) Jupiter-size objects C) WIMPs D) faint red stars E) all of the above

Why do we expect WIMPs to be distributed throughout galactic halos, rather than settled into a disk?

Why isn't space expanding within systems such as our solar system or the Milky Way?

What are peculiar velocities?

What do peculiar velocities reveal?

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

What does the universe look like on very large scales?

What fraction of the mass needed to halt expansion is known to exist in the form of visible mass in the universe?

Based on inventoried matter in the universe, including dark matter known to exist in galaxies and clusters, the actual density of the universe is what fraction of the critical density?

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

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

What is the ultimate fate of an open universe?

Recent measurements of the expansion rate of the universe reveal that the expansion rate of the universe is doing something astronomers did not expect. What is that?

What is the evidence for an accelerating universe?

What might be causing the universe to accelerate?

What is Einstein's cosmological constant?

What is not a main source of evidence for the existence of dark matter? A) massive blue stars B) rotation curves of disk galaxies C) stellar motions in elliptical galaxies D) velocities and positions of galaxies in clusters of galaxies E) gravitational lensing by clusters of galaxies

T/F:Approximately 90 percent of the mass of the Milky Way is located in the halo of the galaxy in the form of dark matter.

T/F: Dark matter is purely hypothetical, because we have no way of detecting its presence.

T/F: If the universe is accelerating, it will expand forever.

T/F: If we learn that the universe is a recollapsing universe, it will mean that the universe is presently contracting, rather than expanding as generally believed.

T/F: By definition, our Sun has a mass-to-light ratio of 1 solar mass per solar luminosity.

T/F: One possible ingredient of dark matter is known as WIMPs, or weakly interacting massive particles. WIMPs probably are made of protons and neutrons.

T/F: Although we don't know exactly when clusters, galaxies, or stars began forming, we do know that clusters came first, with galaxies and stars forming later.

T/F: Individual galaxies generally have higher mass-to-light ratios than clusters of galaxies.

T/F: Some galaxy clusters are still growing today.

T/F: The visible parts of galaxies contribute about one-tenth of the critical density of the universe.

T/F: The only possible geometry of an accelerating universe is open.

Which of the following best summarizes what we mean by dark matter? A) matter that we have identified from its gravitational effects but that we cannot see in any wavelength of light B) matter that may inhabit dark areas of the cosmos where we see nothing at all C) matter consisting of black holes D) matter for which we have theoretical reason to think it exists, but no observational evidence for its existence

Which of the following best summarizes what we mean by dark energy? A) It is a name given to whatever is causing the expansion of the universe to accelerate with time. B) It is the energy contained in dark matter. C) It is the energy of black holes. D) It is a type of energy that is associated with the "dark side" of The Force that rules the cosmos.

The text states that luminous matter in the Milky Way seems to be much like the tip of an iceberg. This refers to the idea that

What is a rotation curve?

What is the primary way in which we determine the mass distribution of a spiral galaxy?

What do we mean when we say that the rotation curve for a spiral galaxy is "flat"?

Although we know less about dark matter in elliptical galaxies than in spiral galaxies, what does current evidence suggest?

In general, when we compare the mass of a galaxy or cluster of galaxies to the amount of light it emits (that is, when we look at it mass-to-light ratio), we expect that

Which of the following is not one of the three main strategies used to measure the mass of a galaxy clusters? A) measuring the speeds of galaxies orbiting the cluster's center B) studying X-ray emission from hot gas inside the cluster C) observing how the cluster bends light from galaxies located behind it D) measuring the temperatures of stars in the halos of the galaxies

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

Which of the following statements best summarizes current evidence concerning dark matter in individual galaxies and in clusters of galaxies? A) Dark matter is the dominant form of mass in both clusters and in individual galaxies. B) Dark matter is present between galaxies in clusters, but not within individual galaxies. C) Dark matter is present in individual galaxies, but there is no evidence that it can exist between the galaxies in a cluster. D) Within individual galaxies, dark matter is always concentrated near the galactic center, and within clusters it is always concentrated near the cluster center.

What is the distinguishing characteristic of what we call ordinary or baryonic matter?

What do we mean when we say that particles such as neutrinos or WIMPs are weakly interacting?

Which of the following best sums up current scientific thinking about the nature of dark matter? A) Most dark matter probably consists of weakly interacting particles of a type that we have not yet identified. B) Dark matter consists 90% of neutrinos and 10% of WIMPs. C) There is no longer any doubt that dark matter is made mostly of WIMPs. D) Dark matter probably does not really exist, and rather indicates a fundamental problem in our understanding of gravity.

When we speak of the large-scale structure of the universe, we mean

The critical density of the universe is the

What is the primary form of evidence that has led astronomers to conclude that the expansion of the universe is accelerating?

Which of the following best sums up current scientific thinking about the nature of dark energy? A) Dark energy most likely consists of a form of photons that we can't see or detect. B) Dark energy is most likely made up of weakly interacting particles that do not interact with light. C) Dark energy probably exists, but we have little (if any) idea what it is. D) Dark energy is the source of the mind weapon used by Sith Lords in Star Wars.

Why do we call dark matter "dark"?

Although most astronomers assume dark matter really exists, there is at least one other possible explanation for the phenomena attributed to dark matter. What is it?

Spiral galaxy rotation curves are generally fairly flat out to large distances. Suppose that spiral galaxies did not contain dark matter. How would their rotation curves be different?

The flat rotation curves of spiral galaxies tell us that they contain a lot of dark matter. Do they tell us anything about where the dark matter is located within the galaxy?

It is more difficult to determine the total amount of dark matter in an elliptical galaxy than in a spiral galaxy. Why?

How do we know that galaxy clusters contain a lot of mass in the form of hot gas that fills spaces between individual galaxies?

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

How does gravitational lensing tell us about the mass of a galaxy cluster?

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

Is space expanding within clusters of galaxies?

Which of the following statements about large-scale structure is probably not true? A) Galaxies and clusters have grown around tiny density enhancements that were present in the early universe. B) Voids between superclusters began their existence as regions in the universe with a slightly lower density than the rest of the universe. C) Many cluster and superclusters are still in the process of formation as their gravity gradually pulls in new members. D) Clusters and superclusters appear to be randomly scattered about the universe, like dots sprinkled randomly on a wall.

Based on current evidence, a supercluster is most likely to have formed in regions of space where

Based on current evidence, how does the actual average density of matter in the universe compare to the critical density?

Which of the following statements best describes the current state of understanding regarding the apparent acceleration of the expansion of the universe? A) The cause of the acceleration is well-understood, and attributed to the particles that make up dark energy. B) We have moderately strong evidence that the acceleration is real, but essentially no idea what is causing it. C) The acceleration is very important in the cosmos today, but the evidence indicates that it will eventually slow down, allowing the universe to recollapse. D) The acceleration probably is not real, and what we attribute to acceleration is probably just a misinterpretation of the data.

Some people wish that we lived in a recollapsing universe that would eventually stop expanding and start contracting. For this to be the case, which of the following would have to be true (based on current understanding)?

Hubble's constant is related to the age of the universe, but the precise relationship depends on the way in which the expansion rate changes with time. For a given value of Hubble's constant today (such as 24 km/s/Mly), the age of the universe is oldest if what is true?

Imagine that it turns out that dark matter (not dark energy) is made up of an unstable form of matter that decays into photons or other forms of energy about 50 billion years from now. Based on current understanding, how would that affect the universe at that time?