Module 23
What does the universe look like on very large scales?
Galaxies appear to be distributed in chains and sheets that surround great voids.
Which of the following statements about rich clusters of galaxies (those with thousands of galaxies) is not true?
Galaxies in the central regions are predominantly spirals, while elliptical galaxies roam the outskirts.
Why do we call dark matter "dark"?
It emits no or very little radiation of any wavelength.
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.
Recent measurements of the expansion rate of the universe reveal that the expansion rate of the universe is doing something astronomers did not expect. What is that?
The measurements show that the expansion is accelerating, rather than slowing under the influence of gravity.
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.
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.
What might be causing the expansion of the universe to accelerate?
We don't know!-but we call it "dark energy."
The distribution of the dark matter in a spiral galaxy is
approximately spherical and much bigger than the size of the galaxy halo.
Gravitational lensing occurs when
massive objects bend light beams that are passing nearby.
What is meant by "dark energy"?
the agent causing the universal expansion to accelerate
