21st Century Astronomy

Laura Kay, Stacy Palen, George Blumenthal

Chapter 23

Large-Scale Structure in the Universe - all with Video Answers

Educators

Chapter Questions

Problem 1

Place the following in order of size, from smallest to largest.
a. a galaxy
b. star clusters
c. the Local Group
d. a wall
e. Virgo Cluster
f. Laniakea
g. a star

Zachary Warner

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Problem 2

The dominant force in the formation of galaxies is
a. gravity.
b. angular momentum.
c. the electromagnetic force.
d. the strong nuclear force.

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Problem 3

Larger galaxies form from the merging of small protogalaxies. This process is similar to the formation of
a. stars
b. planets.
c. molecular clouds.
d. asteroids.

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Problem 4

Which is not a characteristic difference between cold and hot dark matter?
a. temperature
b. composition
c. the way they clump under the influence of gravity
d. mass

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Problem 5

Gravitational lenses can be used to find
a. dwarf galaxies near the Milky Way.
b. dust and gas in the voids.
c. the masses of galaxy clusters.
d. the structure of Laniakea.

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Problem 6

If dark energy is constant, in the far distant future the
universe
a. will be cold and dark.
b. will be bright and hot.
c. will collapse and re-form.
d. will be the same as it is now, on large scales.

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Problem 7

What is the primary difference between galaxy groups and galaxy clusters?
a. how tightly they are bound by gravity
b. the size of the largest galaxy
c. the total mass of the galaxies
d. there is no dark matter in galaxy groups

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Problem 8

Once the redshift of a galaxy has been found, its ________________is also known.
a. mass
b. velocity
c. distance
d. both b and $c$

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Problem 9

Galaxy formation is similar to star formation because both
a. are the result of gravitational instabilities.
b. are dominated by the influence of dark matter.
c. end with the release of energy through fusion.
d. result in the formation of a disk.

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Problem 10

Dark matter clumps stop collapsing because
a. angular momentum must be conserved.
b. they are not affected by normal gravity.
c. fusion begins, and radiation pressure stops the collapse.
d. the particles are moving too fast to collapse any further.

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Problem 11

Elliptical galaxies come from
a. the gravitational collapse of clouds of normal and dark
matter.
b. the collision of smaller elliptical galaxies.
c. the fragmentation of large clouds of normal and dark
matter.
d. the merging of two or more spiral galaxies.

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Problem 12

Which of the following statements about the Dark Ages of the universe is false?
a. The first stars began forming during the Dark Ages.
b. The end of the Dark Ages coincided with reionization.
c. The Dark Ages lasted from 200 million to 600 million years after the Big Bang.
d. During the Dark Ages, photons could travel freely through the universe.

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Problem 13

Astronomers have never observed a star that has no elements heavier than lithium. What does this imply about the first stars?
a. They must have died before galaxies were fully formed.
b. The first stars did not form until after galaxies formed.
c. The first stars must have had very low masses.
d. The first stars must have been enriched in heavy elements.

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Problem 14

Reionization of the neutral gas in the universe occurred because of the
a. decay of dark matter particles.
b. emission of neutrinos by the first stars that formed.
c. release of jets of charged particles from supermassive black holes.
d. radiation from the first stars, supernovae, and black holes that formed.

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Problem 15

Place the following in increasing order.
a. the fraction of the universe that is stars, planets, dust, and gas
b. the fraction of the universe that is dark energy
c. the fraction of the universe that is dark matter

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Problem 16

Suppose you were able to view the early universe at a time when galaxies were first forming. How would it be different from today's universe?

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Problem 17

Is it likely that voids are filled with dark matter? Why or why not?

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Problem 18

Imagine that there are galaxies in the universe composed mostly of dark matter, with relatively few stars or other luminous normal matter. If this were true, how might you learn of the existence of such galaxies?

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Problem 19

How are the processes of star formation and galaxy formation similar? How do they differ?

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Problem 20

What is the origin of large-scale structure?

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Problem 21

Why is dark matter essential to the galaxy formation process?

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Problem 22

Which of the following is the correct evolutionary sequence?
(a) Small star clusters formed first, which were bound together into galaxies, which were later bound together in clusters and superclusters; or (b) supercluster-sized regions collapsed to form clusters, which then later collapsed to form galaxies, which formed small clusters of stars. Justify your
answer.

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Problem 23

Why does the current model of large-scale structure require dark matter?

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Problem 24

What is the difference between a galaxy cluster and a supercluster? Is our galaxy part of either? How do we know this?

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Problem 25

How does a roughly spherical cloud of gas collapse to form a disk-like, rotating spiral galaxy?

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Problem 26

What are some of the observational signs that dark matter exists? Explain why this evidence challenged earlier theories and forced astronomers to change their minds about the existence of matter they could not see.

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Problem 27

Using the current model of galaxy formation, describe how galaxies should appear as you look further back in time. Are the features you described observed?

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Problem 28

Why do astronomers think that the dark matter in the universe must be mostly cold, rather than hot, dark matter?

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Problem 29

Describe the process of structure formation in the universe, starting at recombination and ending today.

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Problem 30

Why do scientists think that gravity, and not the other fundamental forces, is responsible for large-scale structure?

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Problem 31

Figure 23.3 shows the redshifts and velocities for a large number of galaxies. Find the average recession velocity for the galaxies in the wall indicated by the line labeled "Walls."

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Problem 32

The theory of cosmology assumes that on large scales, the structure in the universe is uniform no matter where you
look. Maps of structure, like the ones shown in Figure 23.3, support this assumption. Does the presence of large masses like Laniakea violate this principle? Explain your answer.

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Problem 33

As clumps containing cold dark matter and normal matter collapse, they heat up. When a clump collapses to about half its maximum size, the increased thermal motion of particles tends to inhibit further collapse. Whereas normal matter can overcome this effect and continue to collapse, dark matter cannot. Explain the reason for this difference.

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Problem 34

In previous chapters, we painted a fairly comprehensive picture of how and why stars form. Why, then, is it difficult to model the star formation history of a young galaxy? Is this difficulty a failure of scientific theories?

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Problem 35

If 300 million neutrinos fill each cubic meter of space, and if neutrinos account for only 5 percent of the mass density (including dark matter) of the universe, estimate the mass of a neutrino.

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Problem 36

What are the approximate masses of (a) an average group of galaxies, (b) an average cluster, and (c) an average supercluster?

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Problem 37

The lifetime of a black hole varies in direct proportion to the cube of the black hole's mass. How much longer does it take a supermassive black hole of 3 million $M_{\text {sun }}$ to decay compared to a stellar black hole of $3 M_{\text {Sun }}$ ?

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Problem 38

Knowing what elliptical galaxies are made of, estimate how old they must be. Knowing that ellipticals form via mergers of spirals, and knowing when galaxies first formed, estimate how long it took to complete the merging events that formed the elliptical galaxies seen today.

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Problem 39

The chapter-opening figure is the Hubble Ultra Deep Field 2014.
a. Explain how you might identify which galaxies might be at higher redshifts.
b. The image shows about one 10 -millionth of the entire sky. Estimate the number of galaxies in the entire sky. Explain how you used the cosmological principle in the calculation.

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Problem 40

The Bullet Cluster image in Figure 23.18 shows the collision of two galaxy clusters. Estimate the number of galaxies you can see in each cluster.

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Problem 41

It is likely that the initial fluctuations leading to large-scale structure arose from quantum fluctuations in the early universe. How would the universe look different today if those fluctuations were 10 times bigger? What about 10 times smaller?

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Problem 42

Examine Figure $23.16 .$ Is the early universe on the left or the right in this graph? (Alternatively, you could wonder if "now" was on the left or right in the graph.) Compare the star formation rate today with the star formation rate at the peak. How much more star formation occurred during the peak than occurs now?

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Problem 43

Figure 23.21 shows real data in the top panel and simulated data in the bottom panel. These two panels are not in exact agreement. Do these differences indicate a significant problem in the simulation's ability to represent reality? Why or why not?

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Problem 44

Compare galaxies of redshift $z=0.5, z=4,$ and $z=8$, respectively. About how old was the universe when the light was emitted from each of the galaxies? At what spectral wavelengths would you see the "dropout" of the spectrum? Can you observe these dropouts from the ground or do you need a telescope in space?

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Problem 45

Suppose a dwarf galaxy is orbiting a giant elliptical galaxy at the center of a cluster at a distance of $4 \mathrm{Mpc}$ and a speed of $800 \mathrm{km} / \mathrm{s} .$ Estimate the core mass of the cluster.

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Problem 46

a. Go to the website for the Sloan Digital Sky Survey III (SDSS-III; http://sdss3.org/index.php), which has made a three-dimensional map of the sky. The 2012 video flythrough can be accessed at http: $/ /$ sdss $3 .$ org / press/dr9.php, and new ones may be posted as the project acquires more data. Why did the SDSS-III scientists make this map? What were the goals of the SDSS-III project? What do astronomers learn from this fly-through?
b. Go to the website for the Dark Energy Survey (http:// ww w.darkenergysurvey.org/). What are the science goals of this project? What are the observations? Are there any results yet?

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Problem 47

Go to the website for the Bolshoi simulation (http://hipacc ucsc.edu/Bolshoi). What is this simulation? Click on "Videos" to watch the Bolshoi and some other videos that compare observed and simulated universes. What do astronomers learn from these simulations? Are there any results yet from the AGORA project?

Donald Albin

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Problem 48

Go to the website for "Galaxy Crash" (http://burro.cwru.edu/ JavaLab/GalCrashWeb), a Java applet that lets you run simple models of galaxy collisions. Read the sections under "Background" and then click on "Lab." Pick an exercise from the list (or go to the one suggested by your instructor), and work through the questions.

Sarah Mccrumb

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Problem 49

Use your favorite search engine to find the highest-redshift galaxy, quasar, and GRB observed to date. Why are astronomers interested in finding objects at higher and higher redshifts? Why is it also important for astronomers to estimate the relative frequency of such objects compared to their frequency at $z=6$ or $z=2 ?$

Declan Nell

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Problem 50

Go to the website for the new ALMA telescope (http://almaobservatory.org/en). What is unique about this telescope? How will it study the Dark Ages? Why is this telescope also going to do a "deep-field" project? How will this project be different from the deep-field observations with the Hubble Space Telescope? Look at the items under "ALMA Latest News" and "Press Releases." Are there any reports about galaxy formation?

Rodger Claar

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