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- Exam 3 is next time: Thursday April 11. Format will be 7 MC’s and 4 SA’s (out of 7). Covers chapters 7 – 11. Sample questions are posted. Additional sample essays for Chapter 11 have also been posted.
- The stuff covered today will not be on the exam

Why is the night sky dark?

First detected by Arno Penzias and Robert Wilson

The “noise” came from all directions so it had to be receiver based. The even cleaned out the horn of pigeon droppings and other debris but it wouldn’t go away

Robert Dicke, who originally built the receiver Penzias and Wilson were using, was working with Peeples and others

Peebles had predicted a CBR based on a calculations of a cyclic universe

Most of the CBR is in the infrared region which is difficult to measure from the Earth’s surface

There were many other problems with the Steady State model but the CBR was the final nail in the coffin.

Early calculations by George Gamow, Ralph Alpher and Robert Herman started with neutrons which decayed into protons then fused to make everything else. They predicted a cosmic background at 5 Kelvin.

Note the extremely low levels of Beryllium, Lithium and Boron around the “stability gap”.

Big Bang nucleosynthesis makes most of the helium. Stars make everything beyond helium. Only in stellar cores is the density of Beryllium high enough to get past the stability gap.

What was the universe like before the CBR? How about before nucleosynthesis? How far back can we go if we can’t see anything farther than this?

This term is due to the decrease in energy from the expansion of space

Energy decreases as 1/R4 so

But energy density is proportional to T4 so

During the energy dominated era, the universe expands as the square root of time

Virtual particles of all types of elementary particles were popping in and out of existence everywhere

There must have been particle production processes that did not conserve baryon number

The photon is the exchange particle for the electromagnetic field

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