Lecture 24
May 6, 2008 Evidence for the Big Bang

Key Concepts:

Exam 3 and Final Exam

The Hubble expansion implies that the universe was ever smaller in the earlier times.
Smaller volume means higher density of matter and energy.
The universe was extremely hot at one point, and there must be observable effects! (a smoking gun?)
The question is "what should one look for?"

Around 1950, G. Gamow speculated that heavy elements were synthesized by thermonuclear reactions when the universe was very young.
Arno Penzias and Robert Wilson at Bell Labs in 1963 (The Astrophysical Journal article reporting the discovery titled "A Measurement of Excess Antenna Temperature at 4080 Mc/c")
a theoretical prediction by the Princeton group ("Cosmic Black-Body Radiation" by Dicke, Peebles, Roll, & Wilkinson 1965)
Nobel Prize awarded to Penzias and Wilson in 1978

mass density: ρ_m = M/V (ρ_c ~ 10^-29 g/cm³)
radiation density: ρ_r = aT⁴/c² (ρ_r = 6.5 x 10^-34 g/cm³ for T = 2.7K)
looking back in time, mass density increases as ρ_m ∝ (R₀/R)³
looking back in time, radiation density increases as ρ_r ∝ (R₀/R)⁴ - an extra factor of R from Hubble expansion on radiation temperature T
ρ_r/ρ_m ∝ R₀/R
ρ_r/ρ_m ~ 10^-3 now, but ρ_r/ρ_m ~ 1 when R₀/R ~ 1000 (or 1+z ~ 1000)

==> Universe was radiation-dominated at z > 1000

Marks an important transition from fully ionized universe to formation of atoms about 400,000 years ago (transition from opaque to transparent to radiation -- e.g., watch the movie of the water freezing rapidly!)
A departure from perfect uniformity is present, seeding the future growth of structures like galaxies. (Why?)

t < 10^-43 sec (the "Planck Era"): current understanding of physics does not work, and we need a better theory of gravity.
t < 10^-38 sec (T > 10²⁹ K; the "GUT Era"): all forces were unified to a single GUT force
t < 10^-10 sec (T > 10¹⁵ K; the "Electroweak Era"): electromagnetic and weak forces still combined
t < 0.001 sec (T > 10¹² K; the "Particle Era")

at T > 10¹²K, the creation and destruction of baryon-antibaryon pairs were in thermal equilibrium the ambient radiation field.
quark-antiquark annihilation at t ~ 10^-6 sec (T ~ 10¹³ K) -- asymmetry in weak force produced slightly more quark than anitquark (one in a billion)

at T < 10¹⁰K (a few seconds after the BB), electron-positron annihilation begins. By a few 10s of seconds later, all positrons are gone, and electron neutrinos free-stream ("cosmic neutrino background" at ~2K)
Cosmic Neutrino Background
neutron ("beta") decay becomes important as electron and antineutrinos with enough energy are disappearing (half-life of 10 minutes)
more on nucleosynthesis below.....

Quiz 24C: why is a proton-neutron collision easier/faster than a proton-proton collision?