Jessica Horn: Chapter 16 Quiz

1. What do we mean by dark matter and dark energy? Dark matter and dark energy have never been directly observed, but each has been proposed to exist because it seems the simplest way to explain a set of observed motions in the universe. Dark matter is the name given to the unseen mass whose gravity governs the observed motions of stars and gas clouds. Dark energy is the name given to whatever may be causing the expansion of the universe to accelerate.
2. What is the evidence for dark matter in galaxies? The orbital velocities of stars and gas clouds in galaxies do not change much with the distance from the center of the galaxy. Applying Newton's laws of gravitation and motion to these orbits leads to the conclusion that the total mass of a galaxy is far larger than the mass of its stars. Because no detectable visible light is coming from this matter, we call it dark matter.
3. What is the evidence for dark matter in clusters of galaxies? We have three different ways of measuring the amount of dark matter in clusters of galaxies: from galaxy orbits, from the temperature of the hot gas in clusters, and from the gravitational lensing predicted by Einstein. All of these methods agree, indicating that the total mass of a cluster is about 50 times the mass of its stars, implying huge amounts of dark matter.
4. Does dark matter really exist? We infer that dark matter exists from its gravitational influence on the matter we can see, leaving two possibilities: either dark matter exists or there is something wrong with our understanding of gravity. We cannot rule out the latter possibility, but we have good reason to be confident that our current understanding of gravity is correct and dark matter is real.
5. What might dark matter be made of? There does not appear to be enough ordinary matter in the form of stars, planets, and gas clouds to account for all the dark matter. Therefore, scientists suspect that most of it is made of exotic particles known as WIMPs that have yet to be directly detected.
6. What is the role of dark matter in galaxy formation? Because most of a galaxy's mass is in the form of dark matter, the gravity of the dark matter is probably what formed protogalactic clouds and galaxies from slight density enhancements in the early universe.
7. What are the largest structures in the universe? Galaxies appear to be distributed in gigantic chains and sheets that surround great voids. These giant structures trace their origin directly back to regions of slightly enhanced density early in time.
8. Will the universe continue expanding forever? Even before we consider the possibility of dark energy, the evidence points to external expansion. The critical density is the average matter density the universe would need for the strength of gravity to eventually halt the expansion. The overall matter density of the universe appears to be only about 25% of the critical density.
9. Is the expansion of the universe accelerating? Observations of distant supernovae indicate that the expansion of the universe is speeding up. No on knows the nature of the mysterious force due to dark energy that could be causing this acceleration.
10. Who discovered this acceleration? The 2006 Shaw Prize in Astronomy and the 2011 Nobel Prize in Physics were both awarded to Saul Perlmutter, Brian P. Schmidt, and Adam G. Riess for the 1998 discovery of the accelerating expansion of the Universe through observations of distant supernovae.