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

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

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

4. Hubbletime 1/H 0, called the “Hubble time”, is the approximate age of the universe in the Big Bang Model. At a finite time in the past (t ≈ 1/H 0 ), the universe began in a very dense state. Flashback slide!

5. The Hubble time, 1/H 0, is approximately equal to t 0 (time elapsed since Big Bang). slowing down younger If expansion has been slowing down, the universe is younger than 1/H 0. speeding up older 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. z t 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. decrease As time (t) increases, density and temperature decrease.

9. the What the *&@% do I mean by “the temperature of the early universe”? Today, the universe is full of things with many different temperatures. early same 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. 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. Cosmic Gall (John Updike) What’s a photon?

13. photon particle A photon is a particle of light. quantum mechanics 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. wavelength energy Light of a given color can be treated as: 1) waves of a given wavelength 2) photons of a given energy

15. Energy Energy can be measured in BTUs, kilowatt-hours, calories, ergs, etc… electron-volts 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. tiny 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 violet photon of violet light: energy = 3.1 eV

17. T E The temperature T of 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. again 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. 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. Hydrogen atom: Hydrogen atom: a proton and electron held together by electrostatic attraction.

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. 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. Deuterium nucleus: neutron strong nuclear force Deuterium nucleus: a proton and neutron held together by strong nuclear force. nucleus

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

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

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

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! TRIUMPH FOR PRIMORDIAL NUCLEOSYNTHESIS! There’s just the amount of H & He that was predicted.

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

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: Reading: Chapter 5 Gravity and the Expanding Universe