Announcements

1. 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

2. Olber’s Paradox Why is the night sky dark?

3. The Cosmic Background Radiation First detected by Arno Penzias and Robert Wilson

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

5. 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

6. 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

7. 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

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

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

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

11. 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.

12. Where did all this come from?

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

14. 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.

15. 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”.

16. 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.

17. 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?

18. Matter density decreases differently than energy density This term is due to the decrease in energy from the expansion of space

19. Thus, there must have been a time when energy dominated over matter

20. Solving the Friedmann equation for an energy dominated universe Energy decreases as 1/R4 so But energy density is proportional to T4 so

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

22. 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

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

24. What was a universe dominated by energy like? Virtual particles of all types of elementary particles were popping in and out of existence everywhere

25. 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

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

27. At the elementary particle level we talk about particle fields

28. Some fields are easy to picture The photon is the exchange particle for the electromagnetic field

29. One of the important things in particle physics is symmetry

30. Spontaneous Symmetry Breaking is like a phase transition

31. Before the Planck time (10-43 seconds) we know nothing ?