Week 3: Thermal History of the Universe

• T ∼ 10−4 eV, t ∼ 1010 yr: today; baryons and the CMB are entirely decoupled, stars and galaxies have been around a long time, and clusters of galaxies (gravitationally bound systems of ∼ 1000s of galaxies) are becoming common. • T ∼ 10−3 eV, t ∼ 109 yr; the first stars and (small) galaxies begin to form. • T ∼ 10−1−2 eV, t ∼ millions of years: baryon drag ends; at earlier times, the baryons are still coupled to the CMB photons, and so perturbations in the baryon cannot grow; before this time, the gas distribution in the Universe remains smooth.