The Oldest Light in the Universe
The cosmic microwave background is the thermal afterglow of the Big Bang — photons released when the universe was just 380,000 years old. At that moment, the expanding universe had cooled to about 3,000 K, allowing electrons and protons to combine into neutral hydrogen atoms for the first time. Suddenly, the universe became transparent to light, and those photons have been traveling freely ever since, now cooled to a frigid 2.725 K by 13.8 billion years of cosmic expansion.
A Perfect Blackbody
The CMB spectrum is the most perfect blackbody ever measured in nature. The COBE satellite's FIRAS instrument found deviations of less than 50 parts per million from a theoretical Planck curve. This extraordinary thermal equilibrium was established in the first few hundred thousand years when photons and matter constantly exchanged energy. The peak emission at 1.06 mm wavelength falls squarely in the microwave band — hence the name cosmic microwave background.
Temperature Fluctuations and Cosmic Seeds
While the CMB is remarkably uniform, it contains tiny temperature variations of about ±200 microkelvin — one part in 100,000. These fluctuations are the imprint of quantum density variations from the inflationary epoch, stretched to cosmic scales. Regions slightly denser than average appear slightly warmer, while underdense regions appear cooler. These seeds of structure grew through gravitational instability over billions of years into the galaxies, clusters, and cosmic web we observe today.
Precision Cosmology from the CMB
The angular power spectrum of CMB fluctuations encodes a wealth of cosmological information. The positions and heights of acoustic peaks tell us the universe is geometrically flat, composed of 5% ordinary matter, 27% dark matter, and 68% dark energy. The CMB has transformed cosmology into a precision science, with parameters measured to percent-level accuracy. Ongoing experiments like the Simons Observatory aim to detect primordial gravitational waves imprinted on the CMB polarization.