Cosmic Background Radiation

Cosmic background radiation is thermal radiation that fills the entire observable universe nearly uniformly. It is the electromagnetic afterglow of the Big Bang — specifically, the light released approximately 380,000 years after the universe began, when it had cooled enough for electrons and protons to combine into neutral hydrogen atoms for the first time. Before that moment the universe was an opaque plasma; afterwards, it became transparent, and those released photons have been travelling through space ever since. Today we detect them in the microwave band at a temperature of just 2.725 Kelvin.

Cosmic background radiation comes from the surface of last scattering — a spherical shell around us representing the state of the universe at recombination, approximately 380,000 years after the Big Bang. At that moment, the universe cooled to around 3,000 Kelvin, electrons and protons combined into neutral hydrogen, and the plasma that had previously scattered all light became transparent. The photons released at that instant decoupled from matter and have been travelling freely through space in all directions ever since. Because the universe is expanding, we receive these photons from every direction in the sky, giving the CMB its characteristic uniformity.

Cosmic background radiation was caused by the cooling of the early universe following the Big Bang. In the first hundreds of thousands of years, the universe was a hot, dense plasma so energetic that photons could not travel more than a tiny distance before being scattered by free electrons. As the universe expanded, it cooled. At around 380,000 years old — a transition called recombination — the temperature dropped to approximately 3,000 Kelvin. At this point electrons and protons combined into neutral hydrogen atoms for the first time, removing the particles that had been scattering light. The photons were suddenly free to travel unimpeded, and the radiation we detect today as the CMB is those same photons, cooled and stretched by cosmic expansion to 2.725 Kelvin.

Cosmic background radiation was accidentally discovered in 1964 by Arno Penzias and Robert Wilson, two radio astronomers working at Bell Labs in New Jersey. While calibrating a large horn antenna intended for satellite communications, they detected a persistent, uniform microwave hiss that could not be eliminated regardless of the direction they pointed the antenna or the time of year. After ruling out all terrestrial sources — including pigeon droppings inside the horn — they connected with a team at Princeton led by Robert Dicke, who was independently searching for exactly this signal. The two groups immediately recognised that Penzias and Wilson had found the thermal afterglow of the Big Bang. Penzias and Wilson were awarded the Nobel Prize in Physics in 1978 for the discovery.

Cosmic background radiation is evidence for the Big Bang because its existence, temperature, spectrum, and structure are all precisely predicted by Big Bang cosmology and cannot be explained by competing models. A universe that began in a hot, dense state and has been expanding and cooling for 13.8 billion years would inevitably produce a uniform thermal glow in the microwave band — exactly what we observe. The CMB’s blackbody spectrum matches Planck’s law to better than one part in 10,000, consistent only with a universe that was once in near-perfect thermal equilibrium. The competing Steady State model, which held that the universe has always existed in roughly its current form, predicted no such radiation and was effectively ruled out by the CMB’s discovery in 1965.

Cosmic background radiation indicates that the universe had a hot, dense beginning approximately 13.8 billion years ago and has been expanding and cooling ever since. More specifically, it indicates that the universe passed through a stage called recombination — when it was about 380,000 years old — at which point matter and radiation decoupled and the cosmos became transparent for the first time. The small temperature variations imprinted on the CMB indicate that the early universe was not perfectly smooth: it contained tiny density fluctuations, seeded by quantum processes during inflation, that gravity would later amplify into every galaxy, star, and planet we observe today. The overall pattern of the CMB also indicates that the geometry of the universe is spatially flat, and that its energy content is approximately 5% ordinary matter, 27% dark matter, and 68% dark energy.

Cosmic background radiation cannot be seen with the human eye because it falls in the microwave band of the electromagnetic spectrum, far below the frequencies our eyes can detect. It is observed using microwave-sensitive radio receivers and antennas. On Earth, dedicated radio telescopes can detect it, though ground-based observations are hampered by absorption and emission from the atmosphere. The most detailed maps have been produced by satellites placed above the atmosphere: NASA’s COBE (1989), WMAP (2001), and ESA’s Planck (2009) satellites each produced increasingly precise full-sky maps of the CMB. Interestingly, a small fraction of the static seen on analogue television sets tuned to an empty channel — typically around 1% — comes from CMB photons striking the antenna, making it one of the few ways a person can indirectly “see” this ancient light with everyday technology.