Astronomy

Based on inventoried matter in the universe, including dark matter thought to exist in galaxies and clusters, the actual density of the universe is what fraction of the critical density?

25 percent

Why do we call dark matter "dark"?

It emits no or very little radiation of any wavelength.

What do we mean when we say that a particle is a weakly interacting particle?

It interacts only through the weak force and the force of gravity.

Why can't the dark matter in galaxies be made of neutrinos?

Neutrinos travel at extremely high speeds and can escape a galaxy's gravitational pull.

If there is no dark matter in the Milky Way Galaxy, what is the best alternative explanation for the observations?

Our understanding of gravity is not correct for galaxy-size scales.

Why do we expect WIMPs to be distributed throughout galactic halos, rather than settled into a disk?

Since they do not interact with the electromagnetic force, they do not feel friction or drag and hence do not contract with the rest of the protogalactic cloud.

If all the "dark matter" in the Universe were to be, somehow, instantaneously removed, which of the following would not happen?

The Solar System would fly apart.

Why isn't space expanding within systems such as our solar system or the Milky Way?

Their gravity is strong enough to hold them together against the expansion of the universe.

Some MACHOs emit radiation. Why are they considered dark matter?

They are not as bright as a normal star and are not visible across great distances of space.

How do we determine the amount of dark matter in elliptical galaxies?

We measure the speeds of stars at different radii from the galactic center and determine how much mass is interior to the orbit.

What evidence suggests that the Milky Way contains dark matter?

We observe clouds of atomic hydrogen far from the galactic center orbiting the galaxy at unexpectedly high speeds, higher speeds than they would have if they felt only the gravitational attraction from objects that we can see.

Which model of the universe gives the youngest age for its present size?

a recollapsing universe

The distribution of the dark matter in a spiral galaxy is

approximately spherical and about ten times the size of the galaxy halo.

How do astronomers create three-dimensional maps of the universe?

by using the position on the sky and the redshift to determine a distance along the line of sight

Which of the following are candidates for dark matter?

faint red stars WIMPs brown dwarfs Jupiter-size objects ALL OF THE ABOVE

What does the universe look like on the billion lightyear scale?

galaxies appear to be distributed in chains and sheets that surround great voids

What fraction of the mass needed to halt expansion is known to exist in the form of visible mass in the universe?

less than 1 percent

Gravitational lensing occurs when

massive objects bend light beams that are passing nearby.

Which of the following methods used to determine the mass of a cluster does not depend on Newton's laws of gravity?

measuring the amount of distortion caused by a gravitational lens

A large mass-to-light ratio for a galaxy indicates that

on average, each solar mass of matter in the galaxy emits less light than our Sun.

Which of the following particles are baryons?

protons

Which of the following is not evidence for dark matter?

the expansion of the universe

When we see that a spectral line of a galaxy is broadened, that is, spanning a range of wavelengths, we conclude that

there are different Doppler shifts among the individual stars in the galaxy.

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

through observations of the 21 cm line of atomic hydrogen

What are peculiar velocities?

velocities of distant objects that are not caused by the expansion of the universe

What is causing the expansion of the universe to accelerate?

we don't know! - it's a subject of continuing research