Astronomy Test 23

Why do we call dark matter "dark"?
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Why do we call dark matter "dark"?
It emits no or very little radiation of any wavelength.
What is meant by "dark energy"?
the agent causing the universal expansion to accelerate
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.
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
The distribution of the dark matter in a spiral galaxy is
approximately spherical and about ten times the size of the galaxy halo.
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.
Compared to the central regions of spiral galaxies, we expect elliptical galaxies to have
higher mass-to-light ratios because stars in elliptical galaxies are dimmer than those in spirals.
Gravitational lensing occurs when`
massive objects bend light beams that are passing nearby.
Which of the following particles are baryons?
protons
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.
What is the ultimate fate of an open universe?All matter decays to a low-density sea of photons and subatomic particles.What is the evidence for an accelerating universe?White-dwarf supernovae are slightly dimmer than expected for a coasting universe.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.If a galaxy's overall mass-to-light ratio is 100 solar masses per solar luminosity, and its stars account for only 5 solar masses per solar luminosity, how much of the galaxy's mass must be dark matter?95%Which of the following is an example of baryonic matteryouMeasuring the amount of deuterium in the universe allows us to set a limit onthe density of ordinary (baryonic) matter in the universe.What does the universe look like on very large scales?Galaxies appear to be distributed in chains and sheets that surround great voids.What might be causing the universe to accelerate?We don't know!-but we call it "dark energy."