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Announcements

- 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

Olber’s Paradox

Why is the night sky dark?

The Cosmic Background Radiation

First detected by Arno Penzias and Robert Wilson

Arno Penzias and Robert Wilson had planned on mapping the Milky Way in the microwave

They discovered a persistent “noise”

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

James Peebles, at Princeton, was trying to measure the CBR

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

Penzias and Wilson only measured the CBR at one wavelength: 7.35cm

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

From the early 1960’s through the late 1980’s may tried measuring the CBR on balloons

Launched in 1989, the COBE mission finally got an accurate spectrum for the CBR

A closer look reveals fluctuations in the background radiation at the one part in 100,000 level

The discovery of the CBR killed the Steady State Model by proving the existence of a hot early universe

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

Theories that said all elements were made in stars couldn’t make enough Helium

Nucleosynthesis in the early universe

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.

The “stability Gap” was a problem for theories that made everything in the early universe

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

The answer was that both theories were correct

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.

The Early Universe

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?

Matter density decreases differently than energy density

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

Solving the Friedmann equation for an energy dominated universe

Energy decreases as 1/R4 so

But energy density is proportional to T4 so

At the extremely high density of the early universe, the density term dominates the Friedmann equation

Putting it all together gives a simple relationship between time and size

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

So the farther back in time we go, the higher the density gets and the hotter it gets

What was a universe dominated by energy like?

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

If only particle-antiparticle pairs are produced there would not be any matter in the universe

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

So to understand the early universe we must understand high-energy particle physics

Some fields are easy to picture

The photon is the exchange particle for the electromagnetic field

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