Cosmic Microwave Background
The thermal relic of the early universe — and what its tiny ripples tell us.
About 380 000 years after the Big Bang ($z \approx 1100$), the universe cooled enough for electrons + protons to combine into neutral hydrogen — recombination. Photons decoupled and have been free-streaming ever since.
Today the CMB is observed as a nearly perfect blackbody at $T = 2.7255 \pm 0.0006$ K (FIRAS), with anisotropies $\Delta T/T \sim 10^{-5}$ (COBE 1992, WMAP, Planck).
The angular power spectrum $C_\ell$ encodes pre-recombination physics:
- Acoustic peaks: baryon-photon plasma oscillations frozen at recombination. First peak at $\ell \approx 200$ measures the sound horizon → spatial flatness ($\Omega_k \approx 0$).
- Peak ratios: ratio of even/odd peaks measures baryon density $\Omega_b$.
- Damping tail ($\ell \gtrsim 1000$): Silk diffusion damping → photon mean free path at recombination.
- Polarization: E-modes from scalar perturbations (detected); B-modes from primordial GWs (not detected; bound $r < 0.036$).
Combined CMB analyses give the most precise cosmological parameters: $\Omega_m \approx 0.315$, $\Omega_\Lambda \approx 0.685$, $H_0 \approx 67$ km/s/Mpc (or $73$ via local distance ladder — the "Hubble tension"). 2006 Nobel for Smoot & Mather.