Physics Homework #4

If the Universe is open

• Material will get further and further apart, and eventually no new stars will form

• Its density exceeds the critical density

• Its expansion will eventually stop but it will never recollapse

• It will eventually start to recollapse and it will get hotter and denser

How old is the Universe, and how do we know?

• Infinitely old, we observe that old galaxies exist at the largest distances we can probe from Eart;

• 8 billion years old; the oldest globular clusters and white dwarfs are all about this age

• 500 billion years old; the microwave background radiation appears to be 500 billion light years away from us

• 14 billion years old; the expansion of the Universe appears to have started about this long ago

The Universe is said to be closed if

• the Universe is infinite

• the density of the Universe is less than the critical density

• gravity is strong enough to stop the expansion in a finite time

• all of the above

Based on current evidence, how does the actual average density of matter in the Universe compare to the critical density?

• If we include dark matter, the actual density of matter equals the critical density

• The actual density of matter, even with dark matter included, is less than about a third of the critical density

• The actual density of matter is many times higher than the critical density

• The actual density of dark matter and luminous matter combined is no more than about 1% of the critical density

Based on current evidence, a supercluster is more likely to have formed in regions of space where

• the acceleration of the expansion was proceeding faster than elsewhere

• the density of dark matter was slightly higher than average when the Universe was very young

• there was an excess concentration of hydrogen gas when the Universe was very young

• supermassive black holes were present in the very early Universe

Is space expanding within clusters of galaxies?

• No, because the expansion of the Universe affects only empty space, not space in which matter is present

• No, because their gravity is strong enough to hold them together even while the Universe as a whole expands

• Yes, and that is why clusters tend to grown in size with time

• No, because the Universe is not old enough yet for these objects to have begin their expansion

If WIMPs really exist and make up most of the dark matter in galaxies, which of the following is not one of their characteristics?

• They travel at speeds close to the speed of light

• They tend to orbit at large distances from the galactic center

• They can higher emit nor absorb light

• They are subatomic particles

How does gravitational lensing tell us about the mass of a galaxy cluster?

• Newton's universal law of gravitation predicts how mass can distort light, so we can apply Newton's law to determine the mass of the cluster

• Using Einstein's general theory of relativity, we can calculate the cluster's mass from the precise way it distorts the light of galaxies behind it

• The lensing broadens spectral lines, and we can use the broadening to "weigh" the cluster

• The lensing allows us to determine the orbital speeds of galaxies in the cluster, so that we can determine the mass of the cluster from the orbital velocity law

How are rotation curves of spiral galaxies determined beyond radii where starlight can be detected?

• By extrapolation

• Through observation of the 21 cm line of atomic hydrogen

• Through observations of spectral lines of dark amtter

• By watching the galaxies rotate over a period of years

What is meant by "dark energy"?

• The agent causing the universal expansion to accelerate

• Highly energetic particles that are believed to constitute dark matter

• Any unknown force that opposes gravity