Physics 209: 2nd Midterm

Due: Friday, May 26, 1995

Answer 4 out of the 5 questions

Question 1:

The Horizon Problem arises because of consideration of the notion of causality.
• Explain in more everyday language what this means and what the horizon problem is.
• A chain reaction collision on a highway is an example of a situation where causal connection has significant implications. Explain the connection.

Question 2:

• What is meant by the term dark matter?
• How is the existence of dark matter deduced?
• Is the dark matter likely to be 100 % baryonic? Why or why not.

Question 3:

Suppose that Omega = 1 today. How different would the Universe be if
• there was 1 family of light neutrinos
• there was 5 families of light neutrinos
Be sure to justify your answers by explaining the effects different numbers of light neutrino families have on the evolution of the Universe.

Question 4:

Today, the Universe is matter-dominated.

• What does this mean?
• Suppose that the CMBR had a temperature of 10 Kelvin and that Omega = 1, for what z was this hypothetical universe radiation-dominated?

Question 5:

Suppose that the Universe is flat and that the cosmological constant is zero.
• Sketch how the scale factor, R(t), evolves in a matter-dominated universe
• Sketch how the scale factor, R(t), evolves in a radiation-dominated universe
• Roughly, how does R(t) depend upon the time, t, in both cases?