Chapter 16 Quiz: Olivia Ward

1. What do we mean by dark matter and dark energy?
   
   Dark matter is an undetected form of mass that emits little or no light but whose existence we infer from its gravitational influence. Dark energy is an unknown form of energy that seems to be the source of a repulsive force 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. The total mass of a galaxy is much larger than the mass of its stars, which only account for 2% of the mass, and hot gas, which only accounts for 13%. Dark matter accounts for 85% of mass.
   
3. What is the evidence for dark matter in clusters of galaxies?
   
   We can measure the velocities of galaxies in a cluster from their Doppler shifts. The mass we find from galaxy motions in a cluster is about 50 times larger than the mass in the stars. This is true even when taking the cluster's hot gas into consideration.
   
   
4. Does dark matter really exist?
   
   We have two options when it comes to the existence of dark matter, one being that dark matter really exists and we are observing the effects of its gravitational attraction, and two being that something is wrong with our understanding of gravity, causing us to mistakenly infer the existence of dark matter. Most astronomers prefer the first option.
   
   
5. What might dark matter be made of?
   
   Dark matter is thought to be made out of two basic options. Ordinary Matter (MACHOs) is made from dead or failed stars in halos of galaxies. Exotic Particles (WIMPs) are made from mysterious neutrino-like particles, which are considered to be the best bet.
   
   
6. What is the role of dark matter in galaxy formation?
   
   Gravity of dark matter is what caused protogalactic clouds to contract early in time, but WIMPs can't contract to the center because they don't radiate away from their orbital energy. Dark matter is still pulling things together.
   
7. What are the largest structures in the universe?
   
   Maps of galaxy positions reveal extremely large structures: superclusters and voids. Models show that the galaxy of dark matter pulls mass into denser regions.
   
8. Will the universe continue expanding forever?
   
   The expansion of the universe is accelerating. There is not enough dark energy to stop the acceleration.
   
9. Is the expansion of the universe accelerating?
   
   The brightness of distance white dwarf supernovae tells us about how much the universe has expanded since they exploded. An accelerating universe is the best fit to supernova data.
   
10. Who discovered this acceleration?
    
    Alex Filippenko made this discovery in 1998, but his let his students be awarded. The 2006 Shaw Prize in Astronomy and the 2011 Nobel Prize in Physics were both awarded to Sual Perlmutter, Brian P. Schmidt, and Adam G. Riess.