Astronomy Ch. 23
What fraction of the mass needed to halt expansion is known to exist in the form of visible mass in the universe?
1%
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?
26%
What is the mass-to-light ratio for the inner region of the Milky Way Galaxy, in units of solar masses per solar luminosity?
6
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?
95%
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.
A) Dark matter is the dominant form of mass in both clusters and in individual galaxies.
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.
A) It is a name given to whatever is causing the expansion of the universe to accelerate with time.
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.
A) Most dark matter probably consists of weakly interacting particles of a type that we have not yet identified.
Which model of the universe gives the youngest age for its present size?
A) a recollapsing universe
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
A) massive blue stars
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
A) matter that we have identified from its gravitational effects but that we cannot see in any wavelength of light
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
A) you
What is the ultimate fate of an open universe?
All matter decays to a low-density sea of photons and subatomic particles.
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.
B) We have moderately strong evidence that the acceleration is real, but essentially no idea what is causing it.
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.
C) Dark energy probably exists, but we have little (if any) idea what it is.
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
C) Galaxies in the central regions are predominantly spirals, while elliptical galaxies roam the outskirts.
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
C) measuring the amount of distortion caused by a gravitational lens
Which of the following particles are baryons? A) electrons B) neutrinos C) protons D) quarks E) photons
C) protons
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.
D) Clusters and superclusters appear to be randomly scattered about the universe, like dots sprinkled randomly on a wall.
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
D) measuring the temperatures of stars in the halos of the galaxies
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)?
Dark energy does not exist and there is much more dark matter than we are aware of to date.
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
E) all of the above
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
E) the expansion of the universe
It is more difficult to determine the total amount of dark matter in an elliptical galaxy than in a spiral galaxy. Why?
Elliptical galaxies lack the atomic hydrogen gas that we use to determine orbital speeds at great distances from the centers of spiral galaxies.
Although we know less about dark matter in elliptical galaxies than in spiral galaxies, what does current evidence suggest?
Elliptical galaxies probably contain about the same proportion of their mass in the form of dark matter as do spiral galaxies.
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.
False
T/F: Dark matter is purely hypothetical, because we have no way of detecting its presence.
False
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.
False
T/F: Individual galaxies generally have higher mass-to-light ratios than clusters of galaxies.
False
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.
False
T/F: The only possible geometry of an accelerating universe is open.
False
T/F: The visible parts of galaxies contribute about one-tenth of the critical density of the universe.
False
What does the universe look like on very large scales?
Galaxies appear to be distributed in chains and sheets that surround great voids.
What do we mean when we say that the rotation curve for a spiral galaxy is "flat"?
Gas clouds orbiting far from the galactic center have approximately the same orbital speed as gas clouds located further inward.
When we say that a cluster of galaxies is acting as a gravitational lens, what do we mean?
It bends or distorts the light coming from galaxies located behind it.
What is the distinguishing characteristic of what we call ordinary or baryonic matter?
It consists of atoms or ions with nuclei made from protons and neutrons.
Why do we call dark matter "dark"?
It emits no or very little radiation of any wavelength.
Why do we call dark matter "dark"?
It emits no radiation that we have been able to detect.
What do we mean when we say that a particle is a weakly interacting particle?
It interacts only through the weak force and the force of gravity.
Why can't the dark matter in galaxies be made of neutrinos?
Neutrinos travel at extremely high speeds and can escape a galaxy's gravitational pull.
Is space expanding within clusters of galaxies?
No, because their gravity is strong enough to hold them together even while the universe as a whole expands.
If there is no dark matter in the Milky Way Galaxy, what is the best alternative explanation for the observations?
Our understanding of gravity is not correct for galaxy-size scales.
Why do we expect WIMPs to be distributed throughout galactic halos, rather than settled into a disk?
Since they do not interact with the electromagnetic force, they do not feel friction or drag and hence do not contract with the rest of the protogalactic cloud.
How do we know that there is much more mass in the halo of our galaxy than in the disk?
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.
If all the "dark matter" in the Universe were to be, somehow, instantaneously removed, which of the following would not happen?
The Solar System would fly apart
Based on current evidence, how does the actual average density of matter in the universe compare to the critical density?
The actual density, even with dark matter included, is less than about a third of the critical density.
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?
The expansion rate has been increasing with time (an accelerating universe).
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?
The galaxies in clusters would begin to fly apart.
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?
The measurements show that the expansion is accelerating, rather than slowing under the influence of gravity.
Why wasn't the intracluster medium in galaxy clusters discovered until the 1960s?
The medium emits X rays, which are blocked by the Earth's atmosphere and require X-ray satellites in space in order to be observed.
Why do we believe 90 percent of the mass of the Milky Way is in the form of dark matter?
The orbital speeds of stars far from the galactic center are surprisingly high, suggesting that these stars are feeling gravitational effects from unseen matter in the halo.
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 orbital speeds would fall off sharply with increasing distance from the galactic center.
Why does the temperature of the gas between galaxies in galaxy clusters tell us about the mass of the cluster?
The temperature tells us the average speeds of the gas particles, which are held in the cluster by gravity, so we can use these speeds to determine the cluster mass.
Why isn't space expanding within systems such as our solar system or the Milky Way?
Their gravity is strong enough to hold them together against the expansion of the universe.
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?
There could be something wrong or incomplete with our understanding of how gravity operates on galaxy-size scales.
What do we mean when we say that particles such as neutrinos or WIMPs are weakly interacting?
They respond to the weak force but not to the electromagnetic force, which means they cannot emit light.
If WIMPs really exist and make up most of the dark matter in galaxies, which of the following is not one of their characteristics?
They travel at speeds close to the speed of light.
T/F: By definition, our Sun has a mass-to-light ratio of 1 solar mass per solar luminosity.
True
T/F: If the universe is accelerating, it will expand forever.
True
T/F: Some galaxy clusters are still growing today.
True
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.
True
How does gravitational lensing tell us about the mass of a galaxy cluster?
Using Einstein's general theory of relativity, we can calculate the cluster's mass from the precise way in which it distorts the light of galaxies behind it.
What is the primary way in which we determine the mass distribution of a spiral galaxy?
We construct its rotation curve by measuring Doppler shifts from gas clouds at different distances from the galaxy's center.
How do we know that galaxy clusters contain a lot of mass in the form of hot gas that fills spaces between individual galaxies?
We detect this gas with X-ray telescopes.
What might be causing the universe to accelerate?
We don't know!-but we call it "dark energy."
How do we determine the amount of dark matter in elliptical galaxies?
We measure the speeds of stars at different radii from the galactic center and determine how much mass is interior to the orbit.
What evidence suggests that the Milky Way contains dark matter?
We observe clouds of atomic hydrogen far from the galactic center orbiting the galaxy at unexpectedly high speeds, higher speeds than they would have if they felt only the gravitational attraction from objects that we can see.
What is the evidence for an accelerating universe?
White-dwarf supernovae are slightly dimmer than expected for a coasting universe.
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?
Yes, they tell us that dark matter is spread throughout the galaxy, with most located at large distances from the galactic center.
What is a rotation curve?
a graph showing how orbital velocity depends on distance from the center for a spiral galaxy
What is Einstein's cosmological constant?
a repulsive force that counteracts gravity and was introduced to allow for a static universe
The distribution of the dark matter in a spiral galaxy is
approximately spherical and about ten times the size of the galaxy halo.
The critical density of the universe is the
average density the universe would need for gravity to someday halt the current expansion if dark energy did not exist.
How do astronomers create three-dimensional maps of the universe?
by using the position on the sky and the redshift to determine a distance along the line of sight
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
dark matter represents much more mass and extends much further from the galactic center than the visible stars of the Milky Way.
Compared to the central regions of spiral galaxies, we expect elliptical galaxies to have
higher mass-to-light ratios because stars in elliptical galaxies are dimmer than those in spirals.
Gravitational lensing occurs when
massive objects bend light beams that are passing nearby.
What is the primary form of evidence that has led astronomers to conclude that the expansion of the universe is accelerating?
observations of white dwarf supernovae
A large mass-to-light ratio for a galaxy indicates that
on average, each solar mass of matter in the galaxy emits less light than our Sun.
What is meant by "dark energy"?
the agent causing the universal expansion to accelerate
Based on current evidence, a supercluster is most likely to have formed in regions of space where
the density of dark matter was slightly higher than average when the universe was very young.
Measuring the amount of deuterium in the universe allows us to set a limit on
the density of ordinary (baryonic) matter in the universe.
Based on current evidence concerning the amount of deuterium in the universe, we can conclude that
the density of ordinary (baryonic) matter is between 1 percent and 10 percent of the critical density.
What do peculiar velocities reveal?
the distribution of dark matter in large-scale structures
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
the higher the amount of mass relative to light (higher mass-to-light ratio), the greater the proportion of dark matter.
When we speak of the large-scale structure of the universe, we mean
the overall arrangement of galaxies, clusters of galaxies, and superclusters in the universe.
When we see that a spectral line of a galaxy is broadened, that is, spanning a range of wavelengths, we conclude that
there are different Doppler shifts among the individual stars in the galaxy.
How are rotation curves of spiral galaxies determined beyond radii where starlight can be detected?
through observations of the 21 cm line of atomic hydrogen
What are peculiar velocities?
velocities of distant objects that are not caused by the expansion of the universe
