Question 1

Why is the "recombination" term used to describe the epoch when the universe became transparent to photons sort of a misnomer?

Accepted Answer

It can be considered a misnomer because

Question 2

As measured from Earth, the spectrum of the galaxy NGC 7512 reveals a receding velocity of about 7,000 km/s.If an observer in that galaxy takes a spectrum of the Milky Way they would
Measure ________.

Accepted Answer

A)  the same 7,000 km/s receding speed 
B)  only 1,000 km/s, because for that observer the Hubble expansion is a lot slower 
C)  something that we cannot possibly know, because the known laws of physics apply only within the Milky Way 
D)  23,000 km/s, because the speed of light has a different value for that observer 
E)  no relative motion, because we are in fact measuring Earth's motion about the Sun. 
A)  the same 7,000 km/s receding speed 
B)  only 1,000 km/s, because for that observer the Hubble expansion is a lot slower 
C)  something that we cannot possibly know, because the known laws of physics apply only within the Milky Way 
D)  23,000 km/s, because the speed of light has a different value for that observer 
E)  no relative motion, because we are in fact measuring Earth's motion about the Sun.

Question 3

Galaxies move away from us in all directions because

Accepted Answer

A)  the force of gravity increases with distance. 
B)  the force of gravity weakens with distance. 
C)  space is expanding, and there is no center to the universe. 
D)  space is expanding, and we are near (but not at) the center of the universe. 
E)  we are at the center of the expansion of the universe. 
A)  the force of gravity increases with distance. 
B)  the force of gravity weakens with distance. 
C)  space is expanding, and there is no center to the universe. 
D)  space is expanding, and we are near (but not at) the center of the universe. 
E)  we are at the center of the expansion of the universe.

Question 4

Cosmological redshift is redshift due to

Accepted Answer

A)  the true velocity of a receding galaxy. 
B)  the cosmic microwave background radiation. 
C)  the expansion of the space that light travels through. 
D)  the higher abundance of hydrogen present in older galaxies. 
E)  light interacting with dark matter. 
A)  the true velocity of a receding galaxy. 
B)  the cosmic microwave background radiation. 
C)  the expansion of the space that light travels through. 
D)  the higher abundance of hydrogen present in older galaxies. 
E)  light interacting with dark matter.

Question 5

Which of the following would be the oldest?

Accepted Answer

A)  a 4000 K white dwarf 
B)  the Sun 
C)  the CMB 
D)  a 20-solar mass main-sequence star 
E)  a halo star with 1/250 heavy-element abundance relative to the Sun's 
A)  a 4000 K white dwarf 
B)  the Sun 
C)  the CMB 
D)  a 20-solar mass main-sequence star 
E)  a halo star with 1/250 heavy-element abundance relative to the Sun's

Question 6

What important event in the universe's early history marked the creation of the cosmic background radiation?

Accepted Answer

The CMB is a relic of the time in the un

Question 7

You observe the following three galaxies and measure their respective recession velocities: A, 2,100 km/s; B, 4,400 km/s; and C, 3,050 km/s.Which of the following must be true about these galaxies?

Accepted Answer

A)  A is smaller than B. 
B)  A has more star formation than C. 
C)  B has more stars than C. 
D)  B is farther away from us than A. 
E)  They are all members of the Local Group. 
A)  A is smaller than B. 
B)  A has more star formation than C. 
C)  B has more stars than C. 
D)  B is farther away from us than A. 
E)  They are all members of the Local Group.

Question 8

What do astronomers mean when they say that the universe is isotropic?

Accepted Answer

A)  Faraway parts of the universe look just like nearby parts. 
B)  All galaxies are spiral galaxies like our own. 
C)  Intergalactic gas has the same density everywhere in the universe. 
D)  The laws of physics are different in different places in the universe. 
E)  The universe looks the same no matter what direction you look. 
A)  Faraway parts of the universe look just like nearby parts. 
B)  All galaxies are spiral galaxies like our own. 
C)  Intergalactic gas has the same density everywhere in the universe. 
D)  The laws of physics are different in different places in the universe. 
E)  The universe looks the same no matter what direction you look.

Question 9

A galaxy is measured to have a redshift of z = 5.0.This galaxy is

Accepted Answer

A)  filled with very cold gas. 
B)  moving at 5 times the speed of light. 
C)  dominated by gravitational redshift due to its supermassive black hole. 
D)  dominated by older, redder stars. 
E)  so far away that cosmological redshift dominates our observation. 
A)  filled with very cold gas. 
B)  moving at 5 times the speed of light. 
C)  dominated by gravitational redshift due to its supermassive black hole. 
D)  dominated by older, redder stars. 
E)  so far away that cosmological redshift dominates our observation.

Question 10

Astronomers use galactic redshift as a measure of

Accepted Answer

A)  gravity. 
B)  luminosity. 
C)  rotation curves. 
D)  mass. 
E)  distance. 
A)  gravity. 
B)  luminosity. 
C)  rotation curves. 
D)  mass. 
E)  distance.

Question 11

Explain how Hubble's law can be used to infer the temperatures and densities of the universe in the past.

Accepted Answer

Hubble's law describes a universe that i

Question 12

Hubble's constant, H0, represents the

Accepted Answer

A)  rate of expansion of the universe. 
B)  speed at which galaxies are moving away from us. 
C)  time it takes a galaxy to move twice as far away from us. 
D)  size of the universe. 
E)  amount of time since the Solar System formed. 
A)  rate of expansion of the universe. 
B)  speed at which galaxies are moving away from us. 
C)  time it takes a galaxy to move twice as far away from us. 
D)  size of the universe. 
E)  amount of time since the Solar System formed.

Question 13

In the figure below, three identical photons were each emitted when their local scale factor was RU =0.5.Each is then measured after the scale factor of the universe changes to a different amount, as shown.Which photon will be measured to have the greatest energy?

Accepted Answer

A)  the one at RU =1 
B)  the one at RU = 2 
C)  the one at RU =3 
D)  The photons at RU= 2 and 3 will have identical energy. 
E)  All three will have the same energy. 
A)  the one at RU =1 
B)  the one at RU = 2 
C)  the one at RU =3 
D)  The photons at RU= 2 and 3 will have identical energy. 
E)  All three will have the same energy.

Question 14

If the temperature of the CMB changes with redshift z as T = T0(1 + z), and the recombination happened when the universe was about 1,000 times smaller than at the present time, what was the temperature at the time the universe became transparent to photons?

Accepted Answer

The currently measured CMB tem

Question 15

You measure the redshift of a distant galaxy and find, using Hubble's law and assuming a Hubble constant of 70 km/s/Mpc, that its distance is 1 billion ly (310 Mpc).Imagine that an observer in that galaxy is measuring the recession speed of the Milky Way at the same time as your measurement.What would be the speed that the remote alien observer would record?

Accepted Answer

A)  700 km/s 
B)  21,500 km/s 
C)  3,400 km/s 
D)  10,700 km/s 
E)  6,300 km/s 
A)  700 km/s 
B)  21,500 km/s 
C)  3,400 km/s 
D)  10,700 km/s 
E)  6,300 km/s

Question 16

Where did the Big Bang occur? Explain your reasoning.

Accepted Answer

The Big Bang occurred everywhere in spac

Question 17

Explain the different types of redshift you encountered in this chapter.

Accepted Answer

There are effectively 4 "types" of redsh

Question 18

Scientists indicate that the first stars probably formed at a redshift z ~ 20.At that time the universe was about

Accepted Answer

A)  5 percent its current size. 
B)  50 percent of Earth's size. 
C)  as big as an atom. 
D)  about 100 AU across. 
E)  the size of a grapefruit. 
A)  5 percent its current size. 
B)  50 percent of Earth's size. 
C)  as big as an atom. 
D)  about 100 AU across. 
E)  the size of a grapefruit.

Question 19

The CMB fluctuations of 0.001 percent seen in the WMAP sky map are due to

Accepted Answer

A)  gravitational redshifts. 
B)  technical artifacts. 
C)  faulty instrumentation. 
D)  Hubble expansion. 
E)  explosive supernovae. 
A)  gravitational redshifts. 
B)  technical artifacts. 
C)  faulty instrumentation. 
D)  Hubble expansion. 
E)  explosive supernovae.

Question 20

In an imaginary universe, astronomers find that there are thousands of galaxies within a few million light-years in all directions, but beyond those galaxies there is nothing but empty space.Such a universe would be

Accepted Answer

A)  isotropic and homogeneous. 
B)  isotropic but not homogenous. 
C)  homogeneous but not isotropic. 
D)  neither homogeneous nor isotropic. 
E)  lacking any evidence for the Big Bang. 
A)  isotropic and homogeneous. 
B)  isotropic but not homogenous. 
C)  homogeneous but not isotropic. 
D)  neither homogeneous nor isotropic. 
E)  lacking any evidence for the Big Bang.

Exam 21: The Expanding Universe

Why is the "recombination" term used to describe the epoch when the universe became transparent to photons sort of a misnomer?

As measured from Earth, the spectrum of the galaxy NGC 7512 reveals a receding velocity of about 7,000 km/s.If an observer in that galaxy takes a spectrum of the Milky Way they would Measure ________.

Galaxies move away from us in all directions because

Cosmological redshift is redshift due to

Which of the following would be the oldest?

What important event in the universe's early history marked the creation of the cosmic background radiation?

You observe the following three galaxies and measure their respective recession velocities: A, 2,100 km/s; B, 4,400 km/s; and C, 3,050 km/s.Which of the following must be true about these galaxies?

What do astronomers mean when they say that the universe is isotropic?

A galaxy is measured to have a redshift of z = 5.0.This galaxy is

Astronomers use galactic redshift as a measure of

Explain how Hubble's law can be used to infer the temperatures and densities of the universe in the past.

Hubble's constant, H0, represents the

In the figure below, three identical photons were each emitted when their local scale factor was RU =0.5.Each is then measured after the scale factor of the universe changes to a different amount, as shown.Which photon will be measured to have the greatest energy?

If the temperature of the CMB changes with redshift z as T = T0(1 + z), and the recombination happened when the universe was about 1,000 times smaller than at the present time, what was the temperature at the time the universe became transparent to photons?

Where did the Big Bang occur? Explain your reasoning.

Explain the different types of redshift you encountered in this chapter.

Scientists indicate that the first stars probably formed at a redshift z ~ 20.At that time the universe was about

The CMB fluctuations of 0.001 percent seen in the WMAP sky map are due to

In an imaginary universe, astronomers find that there are thousands of galaxies within a few million light-years in all directions, but beyond those galaxies there is nothing but empty space.Such a universe would be

Thinking Like an Astronomer

Patterns in the Skymotions of Earth and the Moon

Motion of Astronomical Bodies

Gravity and Orbits

Light

The Tools of the Astronomer

The Formation of Planetary Systems

The Terrestrial Planets and Earths Moon

Atmospheres of the Terrestrial Planets

Worlds of Gas and Liquidthe Giant Planets

Planetary Moons and Rings

Dwarf Planets and Small Solar System Bodies

Taking the Measure of Stars

Our Starthe Sun

The Interstellar Medium and Star Formation

Evolution of Low-Mass Stars

Evolution of High-Mass Stars

Relativity and Black Holes

Galaxies

The Milky Waya Normal Spiral Galaxy

Cosmology

Large-Scale Structure in the Universe

Life

Filters

Hubble's constant, H₀, represents the

In the figure below, three identical photons were each emitted when their local scale factor was R_U =0.5.Each is then measured after the scale factor of the universe changes to a different amount, as shown.Which photon will be measured to have the greatest energy?

If the temperature of the CMB changes with redshift z as T = T₀(1 + z), and the recombination happened when the universe was about 1,000 times smaller than at the present time, what was the temperature at the time the universe became transparent to photons?