VERY Early Universe

1. VERY Early Universe Tuesday, January 29(planetarium show tonight: 7 pm, 5th floor Smith Lab)

2. It’s about time! Different calendars have different starting times (birth of Christ, hijra to Medina, etc.) The Big Bang (start of expansion) provides an absolute zero for time.

3. Universe started expanding at a time t = 0. What is the current time t = t0? (That is, how much time has elapsed since the Big Bang?) We’ve already answered that question (approximately).

4. Flashback slide! At a finite time in the past (t ≈ 1/H0), the universe began in a very dense state. 1/H0, called the “Hubbletime”, is the approximate age of the universe in the Big Bang Model.

5. The Hubble time, 1/H0, is approximately equal to t0 (time elapsed since Big Bang). If expansion has been slowing down, the universe is younger than 1/H0. If expansion has been speeding up, the universe is older.

6. Redshift (z) of a distant object: measure of how much the universe has expanded since light was emitted. Since universe has been expanding continuously, each z corresponds to a unique time t.

7. Looking at the Cosmic Microwave Background: z ≈ 1000, t ≈ 350,000 years

8. As time (t) increases, density and temperature decrease.

9. What the *&@% do I mean by “the temperature of the early universe”? Today, the universe is full of things with many different temperatures. The early universe was dense: particles frequently collided, and came to the same equilibrium temperature.

10. The very early universe was a nearly homogeneous “soup” of elementary particles.

11. Particle Physics for Dummies Electron: low mass, negative charge Proton: higher mass, positive charge Neutron: ≈ proton mass, no charge Neutrino: VERY low mass, no charge

12. What’s a photon? Cosmic Gall (John Updike) Neutrinos, they are very small. They have no charge and have no mass And do not interact at all. The earth is just a silly ball To them, through which they simply pass, Like dustmaids down a drafty hall Or photons through a pane of glass.

13. A photonis a particle of light. On very small scales, the laws of quantum mechanics apply. One of these laws states that a particle can have the properties of a wave, and vice versa.

14. This concept of “wave-particle duality” is mind-bending but useful. Light of a given color can be treated as: 1) waves of a given wavelength 2) photons of a given energy

15. Energy can be measured in BTUs, kilowatt-hours, calories, ergs, etc… The energy of individual particles is usually measured in electron-volts. 1 electron-volt (eV) is the energy gained by an electron when its electrical potential increases by 1 volt.

16. An electron-volt is a tiny amount of energy, appropriate for dealing with single particles and atoms. photon of red light: energy = 1.8 eV photon of violet light: energy = 3.1 eV

17. The temperature Tof the early universe determines the average particle energy E.

18. t T E 30,000 yr 10,000 K 3 eV 12 days 10 million K 3 keV 1 second 10 billion K 3 MeV 10-6 sec 10 trillion K 3 GeV 1 GeV = 1 billion electron-volts = energy of a gamma ray photon

19. How far back in time dare we go?

20. Looking again at the CMB: z ≈ 1000, t ≈ 350,000 years, T ≈ 3000 K Universe became transparent because hydrogen went from ionized to neutral.

21. Hydrogen atom: a proton and electron held together by electrostatic attraction. It takes 13.6 eV of energy to ionize a hydrogen atom. Any photon with E > 13.6 eV (ultraviolet, X-ray, gamma-ray) can ionize hydrogen.

22. At T = 3000 K, some photons are energetic enough to ionize hydrogen. At T < 3000 K, hydrogen forms neutral atoms: too few ionizing photons! 13.6 eV photons

23. Deuterium nucleus: a proton and neutron held together by strong nuclear force. nucleus It takes 2,200,000 eV of energy to dissociate a deuterium nucleus. Any photon with E > 2.2 MeV (gamma-ray) can dissociate deuterium.

24. If hydrogen atoms are safe from ionization when T < 3000 K, then deuterium nuclei will be safe from dissociation when T < ???

25. The temperature of the universe fell below 480 million K when its age was t ≈ 7 minutes. Photons were no longer energetic enough to blast apart deuterium nuclei. Deuterium nuclei could form and be safe from destruction.

26. Primordial nucleosynthesis: gamma ray (energetic photon) deuterium nucleus proton neutron p + n → D + γ The very early universe was a nuclear fusion reactor.

27. tritium nucleus There’s not a lot of deuterium in the universe today. Why not? Because fusion continued: D + n → T + γ

28. There’s not a lot of tritium in the universe today. Why not? helium nucleus For one thing, tritium is unstable. For another, fusion continued: T + p → He + γ

29. Before primordial nucleosynthesis, there were 2 neutrons for every 14 protons. (Neutrons tend to decay into protons.)

30. 2 neutrons combine with 2 protons to form 1 stable helium nucleus, with 12 lonely protons (hydrogen nuclei) left over.

31. 25% of the initial protons & neutrons (and hence 25% of their mass) should be in helium: the rest will be hydrogen.

32. When we look at the spectra of the first stars that formed, they consist of 25% helium by mass, and 75% hydrogen. TRIUMPH FOR PRIMORDIAL NUCLEOSYNTHESIS! There’s just the amount of H & He that was predicted.

33. 350,000 yr 7 min 1 billion yr nucleo-synthesis trans- parency galaxy formation

34. Gosh! We understand what the universe was like when it was a few minutes old! 1) At t < 1 minute, things get more speculative. 2) Cosmologists love to speculate.

35. Thursday’s Lecture: Gravity and the Expanding Universe Reading: Chapter 5