Today: “Nucleosynthesis… another phase change in early universe… and why is the Universe so Flat?” HW for next time: Onion, “the nucleus and forces of nature” chapters 3 & 4 reminder: use the Glossary Session 7: 2/6/07 ? min

geometry of universe: determined by total density of matter & energy Density = Critical Density > Critical Density < Critical why is the density ~ critical…i.e. why is our universe so“flat?”

Inflation flattens overall geometry like blowing up a balloon, overall density of matter plus energy becomes almost critical

…during the first 3 minutes, Big Bang nucleosynthesis created deuterium (d), 3 He, 4 He, traces of Li…but nothing else at t ~1 minute, universe cooled to temperature T ~ 1MeV, nuclear energies E = kT, 1 MeV ~ °K 1/40 eV ~10,000 °K too cool for n↔p equilibrium since m n > m p by 1.3 MeV, m n ~ m p = 1 GeV = 1000 MeV so neutrons weakly decay n → p e beta decay between 1 and 3 minutes, temperature high enough to fuse n + p → d, then d + t → He t=tritium= 3 H not too high to dissociate weakly-bound d 2.2 MeV binding energy nor to destroy very stable 4 He (=α) 24 MeV binding energy at t = 1 min, Big Bang predicts protons:neutrons = 7:1 after 3 minutes, universe too cool to fuse a neutron and proton to deuterium, but all neutrons already bound up in helium! 4 He 2 = α= 2 protons + 2 neutrons (read Steve Weinberg’s “The First Three Minutes”)

In hot primordial plasma, protons and neutrons combine making ultra-stable, long-lasting helium but universe only hot enough up to ~ 3 minutes of age one of the reaction chains… JET, next ITER

Big Bang theory prediction: 75% H, 25% He (12/16 vs 4/16 by mass) Matches observations of primordial gases in star forming regions

Big Bang theory: correctly predicts trace abundances of elements 2 H = d, 3 He, Li? (will discuss horizontal axis when we discuss at dark matter)

But, can you observing the Big Bang for yourself? the snow on the TV… darkness of the night sky…evidence for Big Bang

first Kepler, then Olbers’ Paradox If universe were 1) infinite 2) unchanging 3) everywhere the same stars would cover the night sky

Night sky is dark because the universe changes with time As we look out in space, we can look back to a time when there were no stars

Night sky is dark because the universe changes with time As we look out in space, we can look back to a time when there were no stars

…now, in summary a brief history of the universe as motivation for our study of particle physics

Four known forces in universe: Strong Force Electromagnetism Weak Force Gravity Do forces unify at high temperatures? Yes! Fermilab & Cern Proton Decay (GUT?) Who knows? (String Theory)

Planck Era Before Planck time (~ sec) No theory of quantum gravity

GUT Era Lasts from Planck time (~ sec) to end of GUT force (~ sec)

Electroweak Era Lasts from end of GUT force (~ sec) to end of electroweak force (~ sec)

Quark Era Amounts of matter and antimatter nearly equal (Roughly 1 extra proton for every 10 9 proton- antiproton pairs!)

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

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

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

Era of Galaxies Galaxies form at age ~ 1 billion years

RESERVE

PHOTONS DECOUPLING from H & He atoms once formed interactive

History of Universe with black body spectra red shifted for each era interactive

Light from disconnected regions coupled after inflation interactive

The early universe must have been extremely hot and dense