Lecture 24 Early Universe -Testing Models

1. Lecture 24Early Universe -Testing Models ASTR 340 Fall 2006 Dennis Papadopoulos Chapters 12- 13

2. The Big Bang-Brief Introduction • What were conditions like in the early universe? • What is the history of the universe according to the Big Bang theory?

3. BACKGROUND: THE STRUCTURE OF MATTER • Atom is made up of… • Nucleus (very tiny but contains most off mass) • Electrons (orbit around the nucleus) • Atom held together by attraction between positively-charged nucleus and negatively-charged electrons. • Binding Energy eV~ 3000-8000 K • Electron mass .5 MeV ~ 5x109 K

4. Atomic nuclei • The nucleus is itself made up of: • Protons,p(positively charged) • Neutrons,n (neutral; no charge) • Collectively, these particles are known as baryons • p is slightly less massive than n(0.1% difference) • Protons and neutrons bound together by the strong nuclear force (exchange of “gluons”) • Binding Energy per nucleon few Mev~1010 K

5. Elements & isotopes • Number of protons determines element: • Hydrogen – 1 proton • Helium – 2 protons • Lithium – 3 protons • Beryllium – 4 protons • Boron – 5 protons • Carbon – 6 proton • … • Number of neutrons determines the isotope • e.g., for hydrogen (1 proton), there are three isotopes • Normal Hydrogen (H or p) – no neutrons • Deuterium (d) – 1 neutron • Tritium (t) – 2 neutrons

6. Quarks • There’s one more level below this, consisting of quarks… • Protons & Neutrons are made up of trios of quarks • Up quarks & Down quarks • Proton = 2 up quarks + 1 down quark • Neutron = 1 up quark + 2 down quarks • There are other kinds of quarks (strange, charm, top, bottom quarks) that make up more exotic types of particles…

7. Quarks • In particle physics, quarks are one of the two basic constituents of matter (the other Standard Modelfermions are the leptons). • Antiparticles of quarks are called antiquarks. Quarks are the only fundamental particles that interact through all four of the fundamental forces. The word was borrowed by Murray Gell-Mann from the book Finnegans Wake by James Joyce, where seabirds give "three quarks", akin to three cheers (probably onomatopoetically imitating a seabird call, like "quack" for ducks). • The names of quark flavours (up, down, strange, charm, bottom, and top) were also chosen arbitrarily based on the need to name them something that could be easily remembered and used. • An important property of quarks is called confinement, which states that individual quarks are not seen because they are always confined inside subatomic particles called hadrons (e.g., protons and neutrons); an exception is the top quark, which decays so quickly that it does not hadronize, and can therefore be observed more directly via its decay products. Confinement began as an experimental observation, and is expected to follow from the modern theory of strong interactions, called quantum chromodynamics (QCD). Although there is no mathematical derivation of confinement in QCD, it is easy to show using lattice gauge theory.

8. Nuclear fusion • Heavier nuclei can be built up from lighter nuclei (or free n, p) by fusion • Need conditions of very high temperature and density to overcome repulsion of protons • These conditions are present only in cores of stars and… in the early Universe! • The original motivation of Gamow, Alpher, & Herman in advocating big bang was that it could provide conditions conducive to nuclear reactions

9. The early universe must have been extremely hot and dense 3 minutes T about 109 K

10. History of Universe according to BIG BANG Theory

11. Planck Era Before Planck time (~10-43 sec) No theory of quantum gravity

12. Photons converted into particle-antiparticle pairs and vice-versa E = mc2 Early universe was full of particles and radiation because of its high temperature

13. GUT Era Lasts from Planck time (~10-43 sec) to end of GUT force (~10-38 sec)

14. Electroweak Era Lasts from end of GUT force (~10-38 sec) to end of electroweak force (~10-10 sec)

15. Particle Era Amounts of matter and antimatter nearly equal (Roughly 1 extra proton for every 109 proton-antiproton pairs!)

16. Era of Nucleo-synthesis Begins when matter annihilates remaining antimatter at ~ 0.001 sec Nuclei begin to fuse

17. Era of Nuclei Helium nuclei form at age ~ 3 minutes Universe has become too cool to blast helium apart

18. Era of Atoms Atoms form at age ~ 380,000 years Background radiation released

19. Era of Galaxies Galaxies form at age ~ 1 billion years

20. Primary Evidence We have detected the leftover radiation from the Big Bang. The Big Bang theory correctly predicts the abundance of helium and other light elements.

21. What have we learned? • What were conditions like in the early universe? • The early universe was so hot and so dense that radiation was constantly producing particle-antiparticle pairs and vice versa • What is the history of the universe according to the Big Bang theory? • As the universe cooled, particle production stopped, leaving matter instead of antimatter • Fusion turned remaining neutrons into helium • Radiation traveled freely after formation of atoms

22. Evidence for the Big Bang • How do we observe the radiation left over from the Big Bang? • How do the abundances of elements support the Big Bang theory?

23. How do we observe the radiation left over from the Big Bang?

24. The cosmic microwave background – the radiation left over from the Big Bang – was detected by Penzias & Wilson in 1965

25. Background radiation from Big Bang has been freely streaming across universe since atoms formed at temperature ~ 3,000 K: visible/IR

26. Background has perfect thermal radiation spectrum at temperature 2.73 K Expansion of universe has redshifted thermal radiation from that time to ~1000 times longer wavelength: microwaves

27. WMAP gives us detailed baby pictures of structure in the universe

28. In early Universe… • At t=1s, neutrinos began free-streaming • At t=14s, e stopped being created and destroyed • Temperature continued to drop until protons and neutrons, if they combined, were not necessarily broken apart

29. NUCLEOSYNTHESIS IN THE EARLY UNIVERSE • Nucleosynthesis: the production of different elements via nuclear reactions • Consider universe at t=180s • i.e. 3 minutes after big bang • Universe has cooled down to 1 billion (109) K • Filled with • Photons (i.e. parcels of electromagnetic radiation) • Protons (p) • Neutrons (n) • Electrons (e) • [also Neutrinos, but these were freely streaming]

30. The first three minutes… • Protons and Neutrons can fuse together to form deuterium (d) • But, deuterium is quite fragile… • Before t=180s, Universe is hotter than 1 billion degrees. • High-T means that photons carry a lot of energy • Deuterium is destroyed by energetic photons as soon as it forms

31. After the first 3 minutes… • But, after t=180s, Universe has cooled to the point where deuterium can survive • Deuterium formation is the first step in a whole sequence of nuclear reactions: • e.g. Helium-4 (4He) formation:

32. Protons and neutrons combined to make long-lasting helium nuclei when universe was ~ 3 minutes old

33. Big Bang theory prediction: 75% H, 25% He (by mass) Matches observations of nearly primordial gases

34. Further reactions can give Lithium (Li) • Reactions cannot easily proceed beyond Lithium due to the “stability gap”… more about that later

35. If this were all there was to it, then the final mixture of hydrogen & helium would be determined by initial number of p and n. • If equal number of p and n, everything would basically turn to 4He… Pairs of protons and pairs of neutrons would team up into stable Helium nuclei. • Would have small traces of other species • But we know that most of the universe is hydrogen… why are there fewer n than p? What else is going on?

36. Neutron decay • Free neutrons (i.e., neutrons that are not bound to anything else) are unstable! • Neutrons spontaneously and randomly decay into protons, emitting electron and neutrino • Half life for this occurrence is 15 mins (i.e., take a bunch of free neutrons… half of them will have decayed after 15 mins).

37. While the nuclear reactions are proceeding, supply of “free” neutrons is decaying away. • So, speed at which nuclear reactions occur is crucial to final mix of elements • What factors determine the speed of nuclear reactions? • Density (affects chance of p/n hitting each other) • Temperature (affects how hard they hit) • Expansion rate of early universe (affects how quickly everything is cooling off and spreading apart).

38. Full calculations are complex. Need to: • Work through all relevant nuclear reactions • Take account of decreasing density and decreasing temperature as Universe expands • Take account of neutron decay • Feed this into a computer… • Turns out that relative elemental abundances depend upon the quantity BH2 • Here, Bis the density of the baryons (everything made of protons+neutrons) relative to the critical density.

39. Full calculations are complex. Need to: • Work through all relevant nuclear reactions • Take account of decreasing density and decreasing temperature as Universe expands • Take account of neutron decay • Feed this into a computer… • Turns out that relative elemental abundances depend upon the quantity BH2 • Here, Bis the density of the baryons (everything made of protons+neutrons) relative to the critical density.

40. We can use the spectra of stars and nebulae to measure abundances of elements • These need to be corrected for reactions in stars • By measuring the abundance of H, D, 3He, 4He, and 7Li, we can • Test the consistency of the big bang model -- are relative abundances all consistent? • Use the results to measure the quantity Bh2

41. How do the abundances of elements support the Big Bang theory?

42. Dependence of abundances on BH2 From M.White’s webpage, UC Berkeley Bh2

43. Results • All things considered, we have Bh20.019. • If H0=72km/s/Mpc, • h=0.72 • B0.04 • This is far below =1! • Baryons alone would give open universe Bh2

44. What have we learned? • How do we observe the radiation left over from the Big Bang? • Radiation left over from the Big Bang is now in the form of microwaves—the cosmic microwave background—which we can observe with a radio telescope. • How do the abundances of elements support the Big Bang theory? • Observations of helium and other light elements agree with the predictions for fusion in the Big Bang theory

46. RECAP • The density parameter for matter is defined as • Value of M very important for determining the geometry and dynamics (and ultimate fate) of the Universe • Constraints from nucleosynthesis • To get observed mixture of light elements, we need the baryon density parameter to be B0.037 • If there were only baryonic matter (“normal” stuff made of protons, neutrons, & electrons) in the Universe, then this would imply that M0.037. • In that case, and if  were =0, the Universe would be open (hyperbolic) and would expand forever

47. Standard model evolution diagrams

48. Preview… • But life is more complicated than that… • Much evidence shows that Mmay be 5 or 10 times larger than B , yet stillM <1 • Additional evidence suggests that nevertheless, the Universe is flat, withk=0so k =0 (i.e. neither hyperbolic nor spherical geometrically) • This implies the cosmological constant  must be nonzero…and in fact, there is observational evidence for accelerating expansion! • We’ll start with the accounting of all forms of mass in the Universe…

49. THE MASS OF STARS IN THE UNIVERSE • Stars are the easiest things to see and study in our Universe… • Can study nearby stars in detail • Can see the light from stars using “normal” optical telescopes in even distant galaxies. • But…what we see is the light, and what we’re interested in is the mass… • Need to convert between the two using the mass-to-light ratioM/L.

50. The Sun • Msun=21030 kg • Lsun=41026 W • Actual numbers not very instructive… • From now on, we will reference mass-to-light ratios to the Sun (Msun/Lsun).