Chapter 14

Dark matter is purely hypothetical, because we have no way of detecting its presence.

False

If they exist, MACHOs are expected to be found only in the disk of the Milky Way, whereas WIMPs would be found only in the halo.

False

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.

How do we know that there is much more mass in the halo of our galaxy than in the disk?

Stars in the outskirts of the Milky Way orbit the galaxy at much higher speeds than we would expect if all the mass were concentrated in the disk.

Why do we believe 90 percent of the mass of the Milky Way is in the form of dark matter

The orbital speeds of stars far from the galactic center are surprisingly high, suggesting that these stars are feeling gravitational effects from unseen matter in the halo.

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.

Approximately 90 percent of the mass of the Milky Way is located in the halo of the galaxy in the form of dark matter.

True

Currently we think the matter accounts for only 27% of the cosmos, with dark energy making up the rest.

True

It is now believed the majority of mass for most galaxies lies in their dark halos.

True

The distribution of the dark matter in a spiral galaxy is

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

Which of the following are candidates for dark matter?

faint red stars brown dwarfs WIMPs Jupiter-size objects all of the above

The luminous matter in the universe accounts for what percent of the total mass of the universe?

less than 4%

Which of the following particles are baryons?

protons

What is meant by "dark energy"?

the agent causing the universal expansion to accelerate