Cosmic Microwave Background

In the early universe, matter and light interacted in a hot, dense plasma. About 380,000 years after the Big Bang, cooling allowed electrons to combine with atomic nuclei, chiefly forming neutral hydrogen. With far fewer free electrons to scatter photons, light could travel over great distances and the universe became transparent. The radiation released at that stage is the cosmic microwave background. It is the oldest light that can be observed directly; it does not come from the Big Bang instant itself.

The universe has expanded since the CMB photons began travelling freely. Expansion stretched their wavelengths, shifting the light into the microwave part of the electromagnetic spectrum and lowering its temperature. Measurements place the CMB’s mean temperature at about 2.7 kelvin, only a few degrees above absolute zero. It fills the sky in every direction.

The CMB is remarkably uniform, but sensitive instruments detect small temperature differences across the sky. These anisotropies correspond to slight variations in the early universe’s density. Over cosmic time, gravity amplified such differences, helping structure such as galaxies and galaxy clusters form. Mapping the variations allows cosmologists to test models and infer properties of the universe, in combination with other observations; the CMB is not by itself a complete account of cosmic history.

The ground-breaking Cosmic Background Explorer (COBE) mission measured the CMB spectrum and detected large-scale temperature anisotropy. NASA’s Wilkinson Microwave Anisotropy Probe (WMAP) mapped the full sky with greater detail, and the European Space Agency’s Planck mission made higher-sensitivity observations across a wider range of microwave and submillimetre frequencies. Separating the CMB from foreground emission, including radiation from the Milky Way, is an important part of interpreting these maps.