Chapter 18 (Page 399) Review 3, 9 Chapter 19 (Page 416) Review 1, 10

Chapter 18, Review 3. Briefly describe how we can construct rotation curves for spiral galaxies and how these curves lead us to conclude that spiral galaxies contain dark matter.

From the book:

“Even beyond the point in the disk at which starlight fades, we can still see radio waves from atomic hydrogen gas. We can therefore use Doppler shifts of the 21-cm emission line of atomic hydrogen to determine how quickly this gas moves toward us or away from us. On one side of a spiral galaxy the gas is rotating away from us, so its 21-cm line is redshifted a little more than the redshift of the galaxy as a whole. On the other wise, the 21-cm line is blueshifted relative to the redshift of the galaxy as a whole. From the Doppler shifts of these clouds, we can construct a rotation curve- a plot showing the orbital velocities of gas clouds and stars.”

The rotation curves observed for spiral galaxies are flat.

“Because the orbital speeds of gas clouds tell us the amount of mass contained within their orbital paths, the flat rotation curves imply that a great deal of matter lies far from the galactic center.” But since we can’t see any visible matter, there must be dark matter instead. (pg 387).

- solid bodies (like records) rotate slow at the center, fast at the outside
- Keplerian orbits (like our solar system) rotate fast at the center (Mercury), slow at the outside (Pluto)

- "center" means that there is mass in the system outside of the orbit

- "outside" means that there is no mass in the system outside of the orbit

- we observe the outer edges of galaxies to not slow down like they should at the outside of the system
--> must not be the outside then!  There must be more mass in the system outside the orbit
- but we can't see any mass outside the orbit --> dark matter!

Chapter 18, Review 9. Briefly explain what we mean by baryonic and nonbaryonic matter. Which type are you made of?

“Protons and neutrons belong to a category of particles called baryons, so ordinary matter is sometimes called baryonic matter. By extension, extraordinary matter is called non-baryonic matter.” (392)

Nonbaryonic matter refers to exotic matter that is not part of the normal composition of atoms, such as neutrinos or the hypothetical WIMPS”

Baryonic matter refers to ordinary matter made of atoms (because the nuclei of atoms contain protons and neutrons, which are both baryons” (index)

Humans are baryonic matter.

Chapter 19, Review 1. Briefly explain why the early universe must have been much hotter and denser than the universe is today.

Observations show that the universe is cooling and becoming less dense as it ages. Therefore, working backwards scientists can infer that at a time before now, the universe was hotter and denser. When we look back in time (also meaning when we look as far away in distance as we can) we are looking back at the universe at a time shortly after it was born. We see a relatively uniform background of radiation that is redshifted a huge amount. To achieve the redshift that it has over such a long amount of time, the radiation must have been very hot.

Chapter 19, Review 10. How long did the era of nucleosynthesis last? Why is this era so important in determining the chemical composition of the universe forever after?

The era of nucleosynthesis lasted for almost exactly 3 minutes (3 minutes minus 0.001 second). During this era, conditions in the universe were like the inside of a star- heat and density were high enough so that protons could fuse into helium. When the era was over, about 25% of the mass of the universe had fused to form helium, and the other 75% stayed hydrogen. After this period, the universe had cooled down so that fusion could no longer take place, and the amount of hydrogen and helium (and deuterium and lithium) was fixed in the universe. (The amount of hydrogen that a single star eventually fuses to helium is miniscule compared to the amount of hydrogen in the whole universe, so a single star doesn’t do much to change the chemical composition of the universe).