The cosmic microwave background

Relic light from the early universe

The cosmic microwave background, or CMB, is radiation left over from the early hot universe. It fills space almost uniformly and has a nearly perfect blackbody spectrum with temperature about

$Tapprox2.725 K$

The CMB is one of the strongest pieces of evidence for the Big Bang model.

Early opaque universe

In the early universe, matter was hot and ionized. Free electrons scattered photons efficiently, so the universe was opaque. Photons could not travel far without scattering.

As the universe expanded and cooled, electrons combined with nuclei to form neutral atoms. This process is called recombination, though it was actually the first long-lasting combination of electrons and nuclei.

Last scattering

After recombination, photons traveled freely through space. The surface from which CMB photons last scattered is called the surface of last scattering. This occurred when the universe was about 380,000 years old.

The redshift of last scattering is approximately

$zapprox1100$

Since

1+z=rac{a_0}{a_{emit}}

this means the universe was about 1100 times smaller in scale factor than today.

Blackbody spectrum

The CMB has a blackbody spectrum, meaning its intensity as a function of frequency matches thermal radiation extremely well. Expansion stretches photon wavelengths and lowers temperature, but preserves the blackbody form.

The present microwave temperature is the cooled remnant of a much hotter early radiation bath.

Anisotropies

The CMB is nearly uniform but not perfectly. Tiny temperature variations exist at the level of roughly

rac{Delta T}{T}sim10^{-5}

These anisotropies are seeds of later structure. Slightly denser regions gravitationally attracted more matter and eventually helped form galaxies and clusters.

Acoustic peaks

Before recombination, ordinary matter and radiation formed a coupled plasma. Gravity pulled matter into overdense regions, while radiation pressure pushed back. This produced acoustic oscillations.

The pattern of CMB anisotropies, especially the acoustic peaks in the angular power spectrum, reveals the universe's geometry, matter density, baryon density, dark matter, and other cosmological parameters.

Polarization

The CMB is also weakly polarized. Polarization patterns provide additional information about early-universe conditions, reionization, gravitational lensing, and possibly primordial gravitational waves.

Detecting certain polarization patterns is technically difficult but scientifically powerful.

The big idea

The cosmic microwave background is the cooled relic radiation from the early hot universe. Its blackbody spectrum confirms a thermal early phase, while its tiny anisotropies reveal the initial conditions for cosmic structure. The CMB is a precision map of the young universe.