Exam 3 Key

Multiple Choice

Identify the choice that best completes the statement or answers the question.

1. What recent observation prompted astronomers to resurrect the idea of the cosmological constant?

a.	The Universe was static until about 10 billion years ago.
b.	The rate of expansion of the Universe seems to be constant, i.e., independent of time.
c.	The rate of expansion of the Universe seems to decrease with time.
d.	The rate of expansion of the Universe seems to increase with time.
e.	The size of the Universe pulsates around some average value.

2. The resolution of Olber's paradox is that:

a.	We cannot see those stars that are farther away from us than the distance that light has traveled since the beginning of the universe.
b.	Matter cannot have traveled farther than light has traveled during the age of the universe, so there are no stars beyond a certain distance from us.
c.	The light from very distant stars is bent out of our line of sight by the gravitational fields of nearby galaxies.
d.	The light from stars beyond a certain, very large distance is completely absorbed by matter between us and the star.
e.	The light from stars beyond a certain, very large distance comes to a complete stop.

3. Pair production can only occur if:

a.	The energy of a photon is at least equal or greater than the combined mass-energy of a particle-anti-particle pair.
b.	The wavelength of a photon is at least equal or greater than the combined diameters of a particle-anti-particle pair.
c.	The photon becomes stationary.
d.	Two photons collide head on.
e.	A photon having a positive electric charge collides with a photon having a negative electric charge.

4. If the Universe were empty, its age would be equal to:

a.	H0
b.	1/H0
c.	2H0
d.	H0^2
e.	1/H0^2

5. What do cosmologists study?

a.	The origin, structure, and evolution of the solar system
b.	The formation, structure, and evolution of galaxies
c.	The formation, structure, and evolution of stars
d.	The origin, structure, and evolution of the universe
e.	The origin, structure, and evolution of the Earth

6. What is the “cosmic light horizon” or “cosmological horizon”?

a.	It is the maximum distance to which our own radio and television signals will have traveled through the universe since radio was invented.
b.	It is the distance from which light can travel to us over the finite age of the universe, representing a viewing distance limit for us upon Earth.
c.	It is the distance beyond which we cannot see because of absorbing matter in the universe.
d.	It is the distance at which (because we see back in time as we look out into space) galaxies are just being formed.
e.	It is the distance to the nearest supermassive black hole.

7. What causes cosmological redshift of photons that reach us from distant galaxies?

a.	The photons have moved from high gravitational field regions toward lower fields, thus becoming reddened.
b.	The photons were emitted from the galaxies much earlier in time when the overall temperature of matter was much lower. So, the redder the observed photons, the farther away from Earth they were produced.
c.	The photons have traveled across space that has been expanding and their wavelengths have expanded with it, becoming redder.
d.	The photons were emitted by objects that were moving rapidly away from us, and so have been reddened by the Doppler effect.
e.	The photons are redshifted by intergalactic medium (mostly gas and dust).

8. Where are we?

a.	Near the edge of an expanding universe, as shown by the Great Wall of Galaxies
b.	Somewhere in an expanding universe, but not in any special part of it
c.	At the exact center of an expanding universe, as shown by the universal expansion away from us in all directions
d.	Off-center in an expanding universe, as shown by the fact that the microwave background radiation is at a different temperature in one direction than in the opposite direction
e.	Exactly on the edge of an expanding universe

9. What is meant by the statement that “a telescope is a time machine”?

a.	A telescope collects photons, which are ageless.
b.	A telescope allows an astronomer to observe the Earth as it will be in the future.
c.	A telescope allows an astronomer to observe the Earth as it was in the past.
d.	As we observe objects in space that are farther away, we see them as they were farther back in time.
e.	As we observe objects in space that are farther away, we see them as they will be farther into the future.

10. The question asked in Olber's paradox of cosmology is:

a.	“Why is the sky dark at night?”
b.	“How old is the universe?”
c.	“What is beyond the edge of the universe?”
d.	“Where did the universe come from?”
e.	“What is the origin of gold?”

11. The specific characteristics that identify most quasars are:

a.	Diffuse circular image, no redshift of the spectrum, often a very bright radio source
b.	That they look like elliptical galaxies, but with high spectral redshifts
c.	Spiral-galaxy appearance, and very high spectral blueshift, indicating that they are coming toward the Sun at high speed
d.	Starlike appearance, and very high spectral blueshift, indicating that they are approaching the Sun very fast
e.	Starlike appearance, very high redshifts, and hence very large distances, indicating very energetic sources

12. In cosmology, what is the “inflationary epoch”?

a.	The period of universal expansion from the Big Bang to the present
b.	The first 300,000 years of the life of the universe, when matter and radiation interacted vigorously
c.	A short period of extremely rapid expansion when the universe was very young
d.	A period of about 15 minutes soon after Big Bang when most of helium-4 was produced
e.	The first 10^-43 seconds, also known as Planck’s time

13. Because of the general expansion of space, all distant galaxies appear to be moving away from us, with speeds that increase with distance from our galaxy. An observer in one of these distant galaxies would apparently see:

a.	All galaxies on one side of the observer moving toward her and all galaxies on the other side moving away from her; the more distant the galaxy, the faster its motion
b.	All galaxies moving away from her, the more distant galaxies moving faster
c.	All galaxies moving away from her, with closer galaxies moving faster
d.	All galaxies moving toward her, with more distant galaxies moving faster
e.	All galaxies moving toward her, with more distant galaxies moving slower

14. How many fundamental forces are there now in nature?

a.	6
b.	5
c.	4
d.	3
e.	2

15. What is the effective temperature of the microwave background radiation?

a.	5,800 K
b.	Room temperature
c.	2.7 K
d.	Temperature at which water boils
e.	32 F

16. The “central engine” of an active galaxy is thought to be:

a.	A massive accumulation of radioactive uranium and plutonium at the center of the galaxy
b.	A supermassive black hole at the center of an accretion disk, with jets of material being ejected perpendicular to the disk
c.	The violent merger of two galaxies, in which the collision throws out jets of matter along the rotation axis of the larger galaxy
d.	Supernova explosions in an extremely dense star cluster at the center of the galaxy
e.	Annihilation of matter and anti-matter at the center of the galaxy

17. The light curve of a quasar shows an increase in brightness over a 1-month period. This implies that:

a.	The quasar moves away from us at a rate of 1 light-month per year.
b.	The quasar is 1 light-month away from us.
c.	The accretion disk has a period of rotation of about 1 month.
d.	The quasar has a black hole with a period of rotation of about 1 month.
e.	The quasar is about 1 light-month in diameter.

18. In cosmology, the phrase “critical density” refers to:

a.	The smallest density that will produce inflation of the universe
b.	The density below which stars will never form
c.	The density above which the universe is opaque to radiation
d.	The density needed to produce precisely flat space on average throughout the universe
e.	The density needed to produce a static (i.e., non-expanding) universe

19. The energy output of a bright quasar is equivalent to:

a.	1000 times that of the Sun
b.	10^6 solar-type stars
c.	Supernova Type I
d.	1000 bright galaxies
e.	That of the Milky Way galaxy

20. The cosmic microwave background radiation is not uniform over the sky—it is slightly hotter toward the constellation Leo and slightly cooler in the opposite direction, toward Aquarius. Why?

a.	The background radiation really is uniform; the observed difference is due to Earth's motion through the universe.
b.	That is the way the universe began—hotter in one direction and cooler in the other.
c.	The difference is probably a statistical fluctuation, and therefore not real.
d.	Earth is slightly off-center in the universe, so one side of the universe is a bit closer and the other side is a bit farther away.
e.	The difference is due to unequal distribution of mass in the universe: more in the direction of Leo and less in the direction of Aquarius.