Cosmology and astrophysics

Cosmic microwave background (CMB)

The CMB is relic light released when the universe became transparent about 380,000 years after the Big Bang. Its temperature map contains tiny fluctuations that seeded cosmic structure.

The CMB is relic light released when the universe became transparent about 380,000 years after the Big Bang. Its temperature map contains tiny fluctuations that seeded cosmic structure.

Model and observations

Map colors represent tiny departures from the mean temperature, after removing our motion dipole and foreground emission. The colors are not a photograph of glowing matter.

As the primordial plasma cooled, electrons combined with protons to form neutral hydrogen and photons began to travel freely. We observe the last-scattering surface at roughly $z\sim1100$, not through the earlier opaque era using light. Expansion stretches photon wavelengths in proportion to the scale factor $a$, cooling the original Planck spectrum from thousands of kelvin to today’s temperature near $2.725\,\mathrm K$. The near-perfect thermal spectrum is a strong test of a hot early history.

The tiny anisotropies are not merely noise: the angular power spectrum $C_\ell$ has acoustic peaks that encode pre-recombination photon–baryon oscillations and the sound horizon. The first peak constrains geometry, while relative peak heights respond to baryon and dark-matter densities. Interpretation requires separating Galactic dust, radio sources, and the kinematic dipole; polarization adds information about reionization history. A CMB map therefore constrains several parameters jointly rather than measuring one cosmic component in isolation. Penzias and Wilson discovered the microwave background signal in 1965, a landmark observational confirmation.

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