Cosmic Microwave Background

Microwave radiation filling all space — the afterglow of the big-bang, released when the universe first became transparent at ~380,000 years old. The key experimental evidence for Big Bang cosmology. Contains more photons than all the stars in the universe have produced combined.

What It Is

In the early universe, everything was an opaque, hot plasma of sub-atomic particles — photons couldn't travel freely because they were constantly scattered by free electrons (Thomson scattering). As the universe expanded and cooled, protons and electrons combined into neutral hydrogen atoms (recombination) — and suddenly the universe became transparent. The photons released at that moment have been travelling freely ever since, now cooled by the expansion of the universe to 2.7255 K — just barely above absolute zero, in the microwave range.

The CMB contains the vast majority of photons in the universe — one billion times more photons per unit volume than there is matter. Without the universe's expansion continuously cooling it, the night sky would shine as brightly as the Sun.

Discovery

1964: Arno Penzias and Robert Wilson at Bell Labs discovered an unexplained uniform microwave noise from all directions — accidentally while testing a radio antenna. This was the CMB. They won the 1978 Nobel Prize in Physics.

The detection confirmed predictions made by Alpher, Herman, and Gamow in the 1940s and decisively tipped the scientific community toward Big Bang over Steady State models.

Temperature and Uniformity

The CMB temperature is 2.7255 K — an almost perfect blackbody spectrum. It is isotropic to roughly one part in 25,000 — the most uniform thing in the observable universe. The tiny remaining variations (anisotropies) are the key data.

Temperature variations are ~100 μK — one part in 100,000 — after subtracting a dipole caused by our motion through the universe.

The Anisotropies — A Map of the Early Universe

The small temperature variations encode the density fluctuations present in the early universe — the seeds of all large-scale structure (galaxies, clusters, filaments).

First acoustic peak: determined by the overall curvature of the universe → the universe is spatially flat Second acoustic peak: density of normal baryonic matter Third acoustic peak: density of dark matter

Together these peaks provide precise measurements of cosmological parameters.

Key Missions

COBE (1989): first precise CMB measurements; confirmed perfect blackbody spectrum; discovered anisotropies at 1 part in 10⁵ → 2006 Nobel Prize (Mather, Smoot)

WMAP (2001–2010): precise anisotropy maps; confirmed flat universe; placed tight constraints on dark matter and dark energy

Planck (2009–2013): most precise CMB measurements to date; mapped temperature and polarisation with unprecedented accuracy

BOOMERanG (2000): balloon experiment that first confirmed spatial flatness

Connection to the Vault

The CMB is the third of the four pillars of Big Bang evidence (with Hubble's law, Big Bang nucleosynthesis, and large-scale structure) documented in big-bang. It constrains dark-matter (second and third peaks), dark-energy (geometrical effects), and inflation (B-mode polarisation — not yet detected to required precision). It sets the baseline temperature of outer-space: 2.7 K.