CP 23 Dark Matter, Dark Energy, and the Fate of the Universe

Question 1

Dark matter is inferred to exist because:

Answer 1

we can observe its gravitational influence on visible matter.

Question 2

Why do we call dark matter “dark”?

Answer 2

It emits no radiation that we have been able to detect.

Question 3

Dark energy has been hypothesized to exist in order to explain:

Answer 3

observations suggesting that the expansion of the universe is accelerating.

Question 4

When we say that a cluster of galaxies is acting as a gravitational lens, what do we mean?

Answer 4

It bends or distorts the light coming from galaxies located behind it.

Question 5

What is the distinguishing characteristic of what we call ordinary (or baryonic) matter?

Answer 5

It consists of atoms or ions with nuclei made from protons and neutrons.

Question 6

Strong evidence for the existence of dark matter comes from observations of:

Answer 6

clusters of galaxies.

Question 7

Based on current evidence, which of the following is considered a likely candidate for the majority of the dark matter in galaxies?

Answer 7

subatomic particles that we have not yet detected in particle physics experiments

Question 8

What is the primary way in which we determine the mass distribution of a spiral galaxy?

Answer 8

We construct its rotation curve by measuring Doppler shifts from gas clouds at different distances from the galaxy’s center.

Question 9

Which of the following best summarizes what we mean by dark energy?

Answer 9

It is a name given to whatever is causing the expansion of the universe to accelerate with time.

Question 10

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

Answer 10

Using Einstein’s theory of relativity, we can calculate the cluster’s mass from the way in which it distorts the light of galaxies behind it.

Question 11

What do we mean when we say that the rotation curve for the Milky Way Galaxy is “flat”?

Answer 11

Gas clouds orbiting far from the galactic center have approximately the same orbital speed as gas clouds located further inward.

Question 12

Measuring the amount of deuterium in the universe allows us to set a limit on __________.

Answer 12

the density of ordinary (baryonic) matter the universe

Question 13

When we speak of the large-scale structure of the universe, we mean __________.

Answer 13

the overall arrangement of galaxies, clusters of galaxies, and superclusters in the universe

Question 14

Based on evidence from measurements of the acceleration of the expansion rate and from careful study of the cosmic microwave background, about what percentage of the universe’s total mass and energy takes the form of ordinary atomic matter (protons, neutrons, and electrons)?

Answer 14

5%

Question 15

The primary observation that has led astronomers to conclude that the expansion of the universe is accelerating comes from __________.

Answer 15

observations of white dwarf supernovae

Question 16

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

Answer 16

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

Question 17

Is space expanding within clusters of galaxies? Why or why not?

Answer 17

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

Question 18

Which model presumes the existence of some type of dark energy in the universe

Answer 18

accelerating universe

Question 19

Which of the following possible types of universe would not expand forever?

Answer 19

a recollapsing universe

Question 20

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

Answer 20

The actual average density of matter, even with dark matter included, is only about a quarter of the critical density.

Question 21

Which statement best describes current understanding of the eventual fate of the universe?

Answer 21

Although the accelerating expansion suggests that the universe will continue to expand forever, it is possible that future discoveries will indicate a different fate.

Question 22

Hubble’s constant is related to the age of the universe, but the precise relationship depends on the way in which the expansion rate changes with time. For a given value of Hubble’s constant (such as 22 km/s/Mly), the age of the universe is oldest if in which universe

critical
recollapsing
accellerating
coasting

Answer 22

Accellerating