ASTR 101 Chapter 18
Which of the models predict that galaxies should be getting farther apart now? (Keep in mind that now is located at time = 0 years on the graph.) Check all that apply.
-Accelerating -Coasting -Critical -Recollapsing
Use the words at the left to complete the sentences at right correctly. You may use the same word(s) more than once.
-Evidence suggests that two-thirds of the total mass plus energy in the universe consists of DARK ENERGY. - We say use the term DARK ENERGY as a name for whatever is causing the expansion of the universe to accelerate with time. -The bending of light by gravity is called GRAVITATIONAL LENSING. -Astronomers suspect that DARK MATTER consists of weakly interacting massive particles, or WIMPs. -Matter that emits no light of any kind is called DARK MATTER. -Large-scale structure in the universe has probably been shaped primarily by the gravity of DARK MATTER. -Stars and gas clouds are considered to be forms of LUMINOUS MATTER. -Matter that emits X-rays but not visible light is considered a form of LUMINOUS MATTER.
Which of the following provide evidence for dark matter in clusters of galaxies? Select all that apply.
-The temperature of hot, X-ray emitting gas within clusters. -Orbital speeds of galaxies within clusters. -The way clusters bend light from galaxies behind them.
What is the mass-to-light ratio of the solar system? Express your answer using two significant figures.
1 Msun/Lsun
What is the mass-to-light ratio of a 1 Msun white dwarf with luminosity 0.005 Lsun? Express your answer using one significant figure.
200 Msun/Lsun
Examine the visible light image of the Bullet Cluster. What kind of cluster is it?
A galaxy cluster
For which model would the universe begin recollapsing the soonest?
A universe with no dark energy and two times the critical mass density.
Why does the rotation curve for the solar system show speeds that become slower with increasing distance from the Sun?
Because the Sun contains most of the mass of the solar system.
Consider the portion of the video that shows two galaxy clusters that have collided, and includes a map showing the inferred distribution of dark matter among these clusters. How did astronomers infer the location of the dark matter?
By carefully observing how the light of galaxies beyond these clusters has been bent by the clusters.
At time C, is the expansion of the universe accelerating, coasting, or decelerating?
Coasting
At time D, is the expansion of the universe accelerating, coasting, or decelerating?
Decelerating
Notice that the "verdict" at end of the Extraordinary Claims essay still leaves open the possibility that dark matter does not exist and that there is instead some problem with our theory of gravity. Which of the following hypothetical observations would give scientists substantially greater confidence in the existence of dark matter?
Direct detection of WIMPs in laboratories on Earth.
Who first noticed the surprisingly high speeds of galaxies within clusters?
Fritz Zwicky
What additional data would be most valuable in helping scientists evaluate whether the accelerating model really is the best of the four models?
More observations of very distant white dwarf supernovae.
Which of the following statements best describes the pattern of the graph?
Starting from the center, speeds at first rise rapidly, and then become nearly constant with increasing distance.
According to the scientists in the article, what do the hot spots in the cosmic microwave background show?
The "seeds" for the formation of galaxies.
From parts A through C, you should understand the three major components shown in the composite Bullet Cluster image. Notice that the hot, X-ray-emitting gas (red) is not in the same place as most of the matter (blue). How do we think the hot gas came to be in a different place from the galaxies and most of the matter?
The Bullet cluster is actually two separate galaxy clusters that have collided, stripping out hot gas in the process.
Why does the rotation curve of a spiral galaxy increase with increasing distance near the galaxy's center?
The encircled mass increases so fast that gravity increases with radius.
What is the evidence there is little or no dark matter in our solar system?
The rotation curve of the planets is consistent with nearly all the mass being due to the Sun.
What is the evidence that there is dark matter beyond the disk of a spiral galaxy?
The stellar mass distribution does not account for the rotation curve of galaxies.
Which of the following best explains why a higher-mass cluster of galaxies causes light from a distant galaxy to bend more than a lower-mass cluster of galaxies?
The stronger gravity of a larger cluster curves space itself by a greater amount, and light follows the curvature of space.
What aspect of the cosmic microwave background is studied by the Planck telescope?
Tiny temperature differences in the sky.
Suppose you observe the two rotation curves shown at left. Which galaxy is more massive?
UGC 2885
Study the composite image of the Bullet Cluster, as well as the animations. What do the two large, red-colored regions in the composite image represent?
X-ray emission from hot gas.
Based on the observational evidence, is it possible that dark matter doesn't really exist?
Yes, but only if there is something wrong with our current understanding of how gravity should work on large scales.
Listed following are four models for the long-term expansion (and possible contraction) of the universe. Rank each model from left to right based on its prediction for the current age of the universe, from youngest to oldest.
Youngest -> Oldest: -Recollapsing universe -Critical universe -Coasting universe Accelerating universe
A photograph of a cluster of galaxies shows distorted images of galaxies that lie behind it at greater distances. This is an example of what astronomers call:
a gravitational lens.
If a planet (or asteroid) orbited the Sun at three times the distance of Mars, its orbital speed would be ____________.
between one-third and two-thirds the orbital speed of Mars
Strong evidence for the existence of dark matter comes from observations of:
clusters of galaxies.
If the expansion of the universe is accelerating, then the rate at which galaxies were moving apart in the past was
less than the rate at which they are moving apart today
According to the law of gravity as we understand it today, the only explanation for these this flatness is that ____________.
substantial amounts of mass must reside at great distances from the galactic center
There are also two large blue-colored regions in the composite image. These blue regions are labeled as "dark matter" based on _________.
observations of gravitational lensing by the cluster
Dark energy has been hypothesized to exist in order to explain:
observations suggesting that the expansion of the universe is accelerating.
Which of the following observations were made by Vera Rubin and Kent Ford? Select all that apply.
-Stars in the outskirts of the Andromeda galaxy orbit the galaxy's center at surprisingly high speeds. -Many spiral galaxies have hydrogen gas clouds orbiting their centers at surprisingly high speeds.
Match the words in the left-hand column to the appropriate blank in the sentences in the right-hand column. Use each word only once.
1. A massive object can distort the light of more distant objects behind it through the phenomenon that we call GRAVITATIONAL LENSING. 2. WIMPS are defined as subatomic particles that have more mass than neutrinos but do not interact with light. 3. The ROTATION CURVES of spiral galaxies provide strong evidence for the existence of dark matter. 4. Matter made from atoms, with nuclei consisting of protons and neutrons, represents what we call ORDINARY (OR BARYONIC) matter. 5. Models show that the LARGE-SCALE STRUCTURE of the universe is better-explained when we include the effects of dark matter along with the effects of luminous matter. 6. Matter consisting of particles that differ from those found in atoms is generally referred to as EXOTIC (OR NONBARYONIC) matter.
Which region of the early universe was most likely to become a galaxy?
A region whose matter density was higher than average.
At time A, is the expansion of the universe accelerating, coasting, or decelerating?
Accelerating
At time B, is the expansion of the universe accelerating, coasting, or decelerating?
Accelerating
Which model is most supported by the data on the graph?
Accelerating
Which model predicts the largest average distance between galaxies 6 billion years ago?
Accelerating
Classify the given types of matter as either ordinary (baryonic) matter that contains protons and neutrons or as exotic (nonbaryonic) matter that consists of subatomic particles different from those that build atoms. Drag the appropriate items to their respective bins.
Ordinary (Baryonic) Matter: -Matter in our bodies -Matter in stars -Matter in brown dwarfs -Dark matter consisting of Jupiter-size objects in galactic halos Exotic (Nonbaryonic) Matter: -Matter that probably makes up the majority of dark matter -Dark matter consisting of weakly interacting subatomic particles
Suppose we found out that there is no dark matter in galaxies. What is an alternative explanation of the rotation curve?
Our understanding of gravity is wrong for great distances.
If all of the dark matter were located in the visible disk of a spiral galaxy,
its rotation curve would have higher velocities than if there was no dark matter, but the velocities would decrease with distance beyond the visible disk.
If Earth's mass were half its actual value but its radius stayed the same, the escape velocity of Earth would be?
Lower than its actual value
As noted in the Introduction, some scientists have proposed that dark matter does not really exist. According to this view, all matter is ordinary (baryonic), but at large distances from matter, gravity does not precisely obey either Newton's or Einstein's theories of gravity. Is this alternative view of gravity consistent with what we observe in the Bullet Cluster? Why or why not?
No. If all matter was ordinary, then the blue region representing the location of most of the matter would line up with the red region representing the hot gas.
Which of the following describes key evidence for dark matter in individual spiral galaxies like the Milky Way?
Orbital speeds of stars stay relatively high even at great distances from the galactic center.
If gravity is the only force that plays a role in the evolution of the universe, and the mass density is less than the critical mass density, which plot correctly shows the evolution of the universe?
Plot B
If gravity is the only force that plays a role in the evolution of the universe, and the mass density is greater than the critical mass density, which plot correctly shows the evolution of the universe?
Plot C
Which model is most clearly ruled out by the data on the graph?
Recollapsing
Which of the models predict that galaxies will eventually get closer together in the future? Check all that apply.
Recollapsing
Listed following are three possible models for the long-term expansion (and possible contraction) of the universe in the absence of dark energy. Rank each model from left to right based on the ratio of its actual mass density to be the critical density, from smallest ratio (mass density much smaller than critical density) to largest ratio (mass density much greater than critical density).
Smallest -> Largest: -Coasting universe -Critical universe -Recollapsing universe
Listed following are four models for the long-term expansion (and possible contraction) of the universe. Rank the models from left to right based on their predictions for the average distance between galaxies five billion years from now, from smallest to largest.
Smallest -> Largest: -Recollapsing universe -Critical universe -Coasting universe -Accelerating universe
Based on current evidence, which of the following is considered a likely candidate for the majority of the dark matter in galaxies?
Subatomic particles that we have not yet detected.
(Universe as an Infant: Fatter Than Expected and Kind of Lumpy) What is the subject of the observations reported in the article?
The cosmic microwave background.
According to the article, what was the biggest surprise based on the analyses of the observations?
The expansion of the universe was a little slower than expected.
What is it about the observations of Zwicky, Rubin, and Ford that suggests the existence of large quantities of dark matter?
The observed orbital speeds are too high to be explained by the gravity of the visible stars and gas clouds.
The outer portion of the rotation curve of a galaxy is flat. This fact indicates that:
Where the curve is flat, the encircled mass increases with increasing distance.
Consider a distant galaxy located directly behind a cluster of galaxies, as shown in this interactive figure. As seen from Earth, the gravitationally lensed images of the distant galaxy will appear more widely separated if the intervening cluster of galaxies has ___________.
a relatively high mass and is located relatively near Earth.
(Rotation Curve for a Merry-Go-Round animation) Based on the graph, you can conclude that doubling the distance of a point from the center of the merry-go-round would _____________.
double its orbital speed
If the actual mass density of the universe is greater than the critical density and there is no dark energy, then the universe would
eventually stop expanding and begin contracting
Each data point represents a single white dwarf supernova that has been observed. The data points that appear farthest to the left on the graph represent the white dwarf supernovae that are the ___________.
farthest away from us
Consider any star on a circular orbit around the center of the galaxy. The speed of the star's orbit depends on the star's distance from the galactic center and ____________.
the mass of the galaxy that resides within the star's orbit
Consider a distant galaxy located directly behind a cluster of galaxies as shown in this interactive figure. Knowing the distance to the cluster of galaxies and the angular separation of the lensed images of the distant galaxy, astronomers can estimate:
the total amount of matter in the cluster of galaxies, including both dark matter and matter in stars.
Careful measurements show that the hot, X-ray-emitting gas in the Bullet Cluster contains about 7 times as much total mass as all the stars in the cluster's galaxies combined. Therefore, the fact that the most of the gravity is in the blue regions that surrounds the visible galaxies, rather than in the regions with the hot gas, indicates that _____________.
there is even more matter surrounding the galaxies than there is in the hot gas
Dark matter is inferred to exist because:
we can observe its gravitational influence on visible matter.
The major evidence for the idea that the expansion of the universe is accelerating comes from observations of:
white dwarf supernovae.
