Deck 21: The Expanding Universe

Question 1

Sailors could claim the distribution of islands in the ocean is both homogeneous and isotropic (as far as they can see) if

A) the only large islands are nearby, but small ones can be found near the horizon.
B) there are islands of varying sizes all around the horizon, but none near the sailors.
C) there are the same number of islands in every direction, but islands in one direction are bigger than in any other direction.
D) they can see islands in one direction, but not in other directions.
E) the average numbers, distances, and sizes of islands are the same in all directions.

Answer 1

the average numbers, distances, and sizes of islands are the same in all directions.

Question 2

If the distance of a galaxy at a redshift z = 0.05 is 220 Mpc, how many years back into the past are we looking when we observe this galaxy?

A) 5 million years
B) 220 million years
C) 720 million years
D) 6 billion years
E) 10 billion years

Answer 2

720 million years

Question 3

One implication of Hubble's law is that

A) the Milky Way is at the center of the universe.
B) galaxies can have peculiar velocities that are greater than the speed of light.
C) the density and temperature of the universe must have been higher in the past.
D) it applies only to nearby galaxies within the Local Group.
E) more distant objects appear to be older than they actually are.

Answer 3

the density and temperature of the universe must have been higher in the past.

Question 4

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?

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 5

An implication of the cosmological principle is that our location in the universe is

A) not special.
B) at the center of the universe.
C) stationary while everything else is moving.
D) near the edge of the universe.
E) something that cannot be determined.

Question 6

Where in the universe did the Big Bang take place?

A) near the Milky Way Galaxy
B) near the Virgo Cluster
C) near some unknown location on the other side of the universe
D) everywhere in the universe
E) at the center of the universe, not too far from the center of the cosmic background radiation

Question 7

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

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 8

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

A) All stars in all galaxies have planetary systems just like ours.
B) The universe looks exactly the same no matter what direction you look.
C) Galaxies are generally distributed similarly throughout the universe.
D) Generally speaking, there is little difference between conditions on Earth, in the Sun, or in outer space.
E) The universe looks the same at all times in its history.

Question 9

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

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.

Question 10

If a scientist in the distant Andromeda Galaxy studies the hydrogen gas in that galaxy, they will measure ________ what a scientist in our galaxy would measure, studying hydrogen gas
In the Milky Way.

A) a larger mass per atom than
B) a smaller mass per atom than
C) a different set of spectral lines than
D) different possible chemical bonds with other elements than
E) the same properties as

Question 11

If Hubble expansion was due to an explosion throwing galaxies into space, instead of space expanding everywhere, which of the following would be true?

A) The Hubble constant would be zero.
B) The universe would have a measurable center.
C) We would not measure a redshift for distant galaxies.
D) The rate of expansion could not change with time.
E) There would be no new star formation in the modern universe.

Question 12

If we lived in a galaxy 1 billion light-years from our own, what would we see?

A) a universe 1 billion years younger than ours
B) a universe 1 billion years older than ours
C) much the same universe as we see here
D) a universe expanding at a slower rate than we see from Earth
E) a universe expanding at a faster rate than we see from Earth

Question 13

According to Hubble's law, as the distance of a galaxy ________ its ________ increases.

A) increases; luminosity
B) increases; recessional velocity
C) decreases; luminosity
D) decreases; recessional velocity
E) decreases; peculiar velocity

Question 14

All distant galaxies appear to be moving away from us.Does this mean we are in a special place, and why?

A) Yes, it means we are at the center of the universe.
B) Yes, because we are in the newest part of the universe.
C) No, because science does not allow for special places.
D) No, because all observers will see the same phenomenon.
E) Yes, because all other possibilities have been disproven.

Question 15

Distant Galaxy A has a larger redshift than distant, but otherwise identical, Galaxy B.This means that Galaxy A

A) has a larger look-back time than Galaxy B.
B) appears to be bluer than Galaxy B.
C) appears to be brighter than Galaxy B.
D) is nearer to us than Galaxy B.
E) must be an elliptical-type galaxy if Galaxy B is also an elliptical-type galaxy.

Question 16

That "the physical laws and the properties of the universe are the same everywhere and in all directions" is a statement of

A) Hubble's law.
B) the Doppler shift.
C) Big Bang nucleosynthesis.
D) the cosmological principle.
E) Occam's razor.

Question 17

Galaxies move away from us in all directions because

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 18

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 ________.

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 19

Astronomers use galactic redshift as a measure of

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

Question 20

Hubble's constant, H₀, represents the

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 21

Galaxy peculiar velocities are typically about 300 km/s.How far away do you have to look in order to see recessional velocities that are 10 times this peculiar velocity? Note that the Hubble constant has a value of 70 km/s/Mpc.

A) 12 Mpc
B) 25 Mly
C) 37 Mpc
D) 43 Mpc
E) 52 Mpc

Question 22

Cosmological redshift is redshift due to

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 23

If the distance of a galaxy is 5 Mpc, what is its recessional velocity if the Hubble constant is 70 km/s/Mpc?

A) 70 km/s
B) 100 km/s
C) 350 km/s
D) 500 km/s
E) 700 km/s

Question 24

The inverse of the value of H₀ gives a measurement in a unit of

A) time.
B) mass.
C) density.
D) size.
E) luminosity.

Question 25

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?

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 26

The figure below uses a rubber sheet as an analogy for Hubble expansion, with coins representing galaxies.The illustration demonstrates that

A) galaxies may be sitting still, but will move farther apart as space expands.
B) galaxies stand still, and distances are unchanged by the expanding universe.
C) galaxies are repelled from each other, causing them to move away from each other.
D) space is expanding in two dimensions only.
E) galaxies are being flung outward by the Big Bang.

Question 27

Will the increasing scale factor of the universe cause the Sun to get larger over time?

A) Yes, because the scale factor will change the rate of fusion in the Sun's core.
B) Yes, because the atoms within the Sun will all expand individually.
C) No, because Earth is at center of the universe's expansion.
D) Yes, because the expansion of the universe will directly cause the Sun to expand.
E) No, because the Sun's self-gravity locally dominates over the expansion of the universe.

Question 28

The majority of the redshift that we measure from the most distant galaxies is due to

A) excess hydrogen emission.
B) most of these galaxies being old, elliptical-type galaxies.
C) the expansion of the universe "stretching" the wavelengths of photons.
D) the cosmic microwave background.
E) the actual velocity of that galaxy.

Question 29

What would you predict for the recession velocity of a galaxy whose distance (measured with the help of Type Ia supernovae) is 42 Mpc? Note that the Hubble constant has a value of
70 km/s/Mpc.

A) 2,940 km/s
B) 1.42 km/s
C) 205,800 km/s
D) 25,000 km/s
E) 386 km/s

Question 30

The apparent recessional velocities of galaxies at large distances are due mainly to

A) the actual motions of the galaxies through space.
B) the motion of our Sun around the galactic center.
C) a continuously increasing scaling factor of the universe.
D) the relativistic jets launched by supermassive black holes.
E) incorrect interpretation of spectra from galaxies.

Question 31

Two galaxies are currently separated by 55 Mpc.Roughly how long did it take for them to reach this separation?

A) 13.7 billion years
B) 250 million years
C) 4.6 billion years
D) less than a second
E) 27 billion years

Question 32

If we "reverse" Hubble expansion, we see that the universe should have been very dense in the distant past, and so it must have been ________ than it is today, and we should be able to
Observe ________ as a result.

A) much colder; cosmological redshift
B) much colder; the cosmic microwave background
C) much hotter; gravitational redshift
D) much hotter; the cosmic microwave background
E) much hotter; black holes

Question 33

If a galaxy has an apparent velocity of 700 km/s, what is its distance if the Hubble constant is 70 km/s/Mpc?

A) 1 Mpc
B) 7 Mpc
C) 10 Mpc
D) 70 Mpc
E) 100 Mpc

Question 34

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?

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

Question 35

If the slope of the graph v vs.D is H_o, the slope of the D vs.v graph is the

A) age of the universe.
B) cosmological constant.
C) scaling factor of the universe.
D) temperature of the CMB.
E) amount of dark energy in the universe.

Question 36

Hubble originally estimated the "Hubble constant" to be about 500 km/s/Mpc.What age of the universe would such a number imply?

A) 2 billion years
B) 13.8 billion years
C) 16 billion years
D) 250 million years
E) 500 million years

Question 37

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 cosmological redshift?

A) the one at R_U =1
B) the one at R_U = 2
C) the one at R_U = 3
D) The photons at R_U = 2 and 3 will be identical, and have the greatest redshift.
E) All three will have the same cosmological redshift.

Question 38

The high temperature and density of the early universe are directly responsible for

A) the expansion of the universe.
B) the cosmic microwave background.
C) star formation.
D) cosmological redshift.
E) nucleosynthesis of the heaviest elements.

Question 39

If you measured the distances and recessional velocities for a sample of galaxies and plotted the data to get the figure shown below, what value would you derive for the Hubble constant?

A) 10 km/s/Mpc
B) 50 km/s/Mpc
C) 70 km/s/Mpc
D) 100 km/s/Mpc
E) 500 km/s/Mpc

Question 40

If the Hubble constant had a value that was half of its current measured value of 70 km/s/Mpc, the age of the universe would be about

A) 7 billion years.
B) 14 billion years.
C) 22 billion years.
D) 28 billion years.
E) 33 billion years.

Question 41

The CMB can be regarded as a "wall" beyond which we cannot see light because

A) photons couldn't leave the hot plasma before the recombination period.
B) we haven't discovered the technology to detect light from earlier epochs.
C) galaxies absorb all the photons from earliest times, closer to the Big Bang.
D) there existed only massive particles within the universe in the first 380,000 years of existence; there were no photons.
E) behind the CMB "wall" there is a huge black hole that doesn't allow light to escape.

Question 42

If the wavelength of the background radiation peaked at 1 µm at the time of recombination, how old was the universe then compared with its age today?

A) one second old
B) 1/100th of a second old
C) 1/1,000th of a year old
D) 3,000 years old
E) several hundred thousand years old

Question 43

In the early 1960s, physicists Penzias and Wilson detected a persistent noise at a wavelength of about 1 mm in their radio telescope that came from all directions in the sky due to

A) synchrotron emission from the Crab Nebula.
B) emission from newly formed stars in the Orion Nebula.
C) cellphone usage.
D) photons left over from the Big Bang.
E) television and radio broadcasting.

Question 44

What is the correct interpretation of a redshift larger than 1?

A) The object is moving faster than the speed of light.
B) The universe's scale factor universe has more than doubled in size since the light from that object was emitted.
C) The object has an extremely large peculiar velocity.
D) The light was shifted to longer wavelengths from gravitational radiation.
E) The rate of expansion of the universe is increasing.

Question 45

The CMB fluctuations seen in the WMAP temperature distribution measures are of crucial importance in that

A) they prove that the cosmological principle is incorrect; the universe is in fact anisotropic.
B) they reveal the signatures of the large mass concentrations where subsequent galaxies and clusters of galaxies formed.
C) they reveal the technical limitation of the detectors on board the WMAP satellite.
D) they give us clues about the black-hole evaporation happening in the early universe.
E) they emphasize the limited value of general relativity.

Question 46

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

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 47

The COBE microwave map shown in the figure below seems to indicate that one part of the sky is 0.003 K warmer than the other.What is the cause of such an apparent anisotropy?

A) Earth and the Sun are moving at 368 km/s relative to the CMB reference frame.
B) The map was obtained when the Sun was at its peak of activity within a given 11-year cycle.
C) It is due to interference from the many microwave ovens people use in homes.
D) The boundaries between the warmer and cooler areas are sites of strong supernova activity.
E) It indicates that galaxies could form in only half of the universe.

Question 48

The cosmic microwave background radiation was emitted when the universe had a size about 1/1,000 of today's value.What was the temperature of the microwave background radiation at the epoch of recombination?

A) 30 K
B) 300 K
C) 3000 K
D) 30,000 K
E) 300,000 K

Question 49

Redshift z = v_r/c, where v_r is the apparent recessional velocity, and c is the speed of light.Is it possible to observe a redshift greater than 1.0, or lower than=1.0?

A) Neither is possible.
B) Yes, it is possible to measure both.
C) It is possible to measure z $\lt$ -1.0, but not z $\gt$ 1.0.
D) It is possible to measure z $\gt$ 1.0, but not z $\lt$ -1.0.
E) Redshifts greater than 1.0 and less than -1.0 are the only possible redshifts.

Question 50

Which of the following would be the oldest?

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 51

The existence of the cosmic background radiation tells us that the early universe was

A) much hotter than it is today.
B) much colder than it is today.
C) composed entirely of radiation at early times.
D) composed entirely of stars at early times.
E) about the same temperature as today but much more dense.

Question 52

The oldest photons we detect in the universe come from

A) distant quasars.
B) the first generations of stars.
C) the most distant galaxies.
D) the epoch of recombination.
E) the first supernovae.

Question 53

Why is it not possible to look all the way back to the Big Bang itself?

A) Photons are not produced until the stars begin to shine at a redshift of z 20.
B) From redshifts of z=0 to 100, photons are gravitationally lensed by the dark matter in the universe.
C) At redshifts of z $\gt$ 1,000, most of the photons are blocked by large amounts of cold gas and dust.
D) For redshifts of z $\gt$ 1,000, photons cannot travel freely because they easily interact with individual protons and electrons in the universe.
E) Photons from the Big Bang would be so strongly redshifted that we could never detect them.

Question 54

The temperature of the CMB is hotter on one side of the sky than on the other by approximately 3 milliKelvin.What does this tell us?

A) The true center of the universe is in the direction of the hotter side of the CMB.
B) Earth is moving with respect to the CMB.
C) Earth is near the center of the universe's expansion but not exactly at the middle.
D) The universe is expanding faster on one side of us than on the other.
E) The universe is homogeneous but not isotropic.

Question 55

The current temperature of the cosmic background radiation of 2.73 K means that the peak of its spectrum occurs at a wavelength of

A) 0.1 µm.
B) 1 µm.
C) 10 µm.
D) 100 µm.
E) 1,000 µm.

Question 56

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

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

Question 57

It is possible for a galaxy to have an apparent velocity greater than the speed of light.This means that the galaxy

A) has an active nucleus capable of producing relativistic jets.
B) is approaching us and the measured speed is a mere artifact of projecting its velocity along the line of sight.
C) has been ejected at tremendous speed by the Big Bang.
D) is being temporarily accelerated by large numbers of simultaneous supernovae blasts.
E) is being carried by the expansion of the universe.

Question 58

After the Big Bang, as the universe cooled and protons and electrons combined, what important consequence happened?

A) Protons and neutrons combined to form nuclei such as deuterium and helium.
B) Neutrinos ceased to interact with normal matter.
C) Dark matter ceased to interact with normal matter.
D) Photons began to travel freely through the universe.
E) Lithium and other light elements were formed by the fusion of hydrogen and helium.

Question 59

The CMB spectrum shown in the figure below is

A) an absorption line spectrum.
B) an emission line spectrum.
C) a thermal spectrum.
D) identical to the spectrum of our Sun.
E) blueshifted into the microwaves domain.

Question 60

The apparently "grainy" CMB distribution shown in the WMAP in the figure below represents temperature fluctuations of about

A) 0.001 percent.
B) 0.1 percent.
C) 10 percent.
D) 50 percent.
E) 27.4 percent.

Question 61

A recent record for the most distant galaxy yet discovered was held by a galaxy that has a redshift of z =8.68.Compared with the current scale factor of the universe, what was the scale factor when the light from that galaxy started its long journey to us?

A) about 10 times smaller
B) about 50 percent smaller
C) about 0.001 percent smaller
D) about 99 percent smaller
E) about 250 percent bigger

Question 62

The natural abundances of various forms of the elements hydrogen, helium, and lithium are predicted by

A) Einstein's special theory of relativity.
B) Einstein's general theory of relativity.
C) the cosmological principle.
D) Hubble expansion.
E) the Big Bang theory.

Question 63

Describe the two assumptions regarding the universe that are made by the cosmological principle.

Question 64

The CMB is a snapshot of the radiation in the universe at a redshift of z 1,000 when the universe was about

A) 1,000 times smaller than it is today.
B) 10 times smaller than it is today.
C) 2 times smaller than it is today.
D) the same size as it is today.
E) 10 times larger than it is today.

Question 65

Provide an example of something that is homogenous but not isotropic.You may use an imaginary (but clear) situation, if desired.

Question 66

Employing Hubble's law, calculate the distance to the Andromeda Galaxy, which is approaching at
110 km/s.The directly measured distance to Andromeda is 2.5 million light-years.How do the two numbers agree or disagree? Explain.

Question 67

Where did the Big Bang occur? Explain your reasoning.

Question 68

We see the universe around us expanding at 70 km/s/Mpc, a rate given by the Hubble constant.If you were an astronomer living today in a galaxy that was located 1 billion light-years from the Milky Way, what would you measure for your Hubble constant?

Question 69

Imagine a balloon that you can inflate to visualize the expansion of the universe.Space is symbolized by the surface of the balloon.You use a marker to draw galaxies on its outside surface, and you start inflating the balloon.Name at least one aspect of this model that is analogous to the real expansion of the universe, and at least one aspect that is not analogous.

Question 70

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

Question 71

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

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 72

Does the expansion of the universe make the Sun bigger? What about the Milky Way? Why or why not?

Question 73

Explain the difference between the universe and the observable universe.

Question 74

Does Hubble's law imply that our galaxy is sitting at the center of the universe? Explain.

Question 75

Why did Big Bang nucleosynthesis only produce elements less massive than beryllium?

A) The laws of physics do not allow the formation of massive nuclei outside stars.
B) The universe was never hot enough for nucleosynthesis by fusion before stars formed.
C) There was no black hole to provide sufficient energy for advanced nucleosynthesis.
D) The universe expanded and cooled rapidly.
E) More massive elements did form, but they decayed very rapidly into lighter ones.

Question 76

Distant galaxies we can see today with a redshift of z 6 emitted their light when the universe was

A) 5 times smaller than it is today.
B) 6 times smaller than it is today.
C) 7 times smaller than it is today.
D) 8 times smaller than it is today.
E) the same size as it is today.

Question 77

When we look at galaxies in the universe and measure their star formation rates, we find that galaxies at redshifts z 1 have higher star formation rates than they do now.At that time, the universe was ________ times the size it is today.

A) 0.1
B) 0.2
C) 0.5
D) 0.7
E) 0.9

Question 78

Movies and television shows often portray the Big Bang as a tiny bright spot that loudly explodes to fill the screen.Why is this depiction inaccurate?

Question 79

Prior to detailed studies of the cosmic microwave background, the age of the universe was poorly known.The age of the universe was thought to be around 16 billion years.If the universe were 16 billion years old, what would be the value of the Hubble constant, assuming constant expansion? Note that 1 Mpc = 3.086 * 10¹⁹ km and 1 year= 3.154 *10⁷ s.

Question 80

Galaxies with redshifts greater than 1 ________ moving greater than the speed of light due to ________.

A) appear to be; cosmological redshift
B) appear to be; hydrogen absorption
C) appear to be; gravitational redshift
D) are actually; leftover inertia from the Big Bang
E) are actually; a poorly understood repulsive force