Cosmic Microwave Background

Cosmic microwave background radiation (CMB) is the leftover radiation from the Big Bang and serves as a source of evidence for it actually happening. It is extremely cold at just 2.725 Kelvin, so emits blackbody wavelengths in the microwave part of the electromagnetic spectrum. It was discovered in 1964 by Arno Penzias and Robert Wilson, who won the 1978 Nobel Prize in physics alongside Pyotr Kapitsa, a pioneer in low temperature physics. Let’s look at where it comes from and how it was detected.

A map of the cosmic microwave background taken with the Planck satellite in 2013. Credit: ESA.

When the universe first formed, it was incredibly hot at around 273 million Kelvin. This was so hot that any potential atoms that could have formed were broken into protons and electrons, a sort of hydrogen plasma. The photons of light hit the electrons and scattered in random directions. After 380,000 years, the universe cooled and hydrogen atoms formed. These could no longer scatter photons through Thomson radiation, so the universe became “transparent” and light travelled in straight lines.

The cosmic microwave background allows us to see the universe as it was only 380,000 years after its formation. This radiation is mostly uniform, but there are fluctuations based on the early stages of galaxies and stars, which are visible with the color changes on the map. Furthermore, these fluctuations tell us more about how the Big Bang happened.

Scientists are also looking for evidence of the faster than light inflation of the universe right after the Big Bang. The model is theoretically sound and could be confirmed by examining the polarization of the CMB. Moreover, it can be used to study gravitational waves.

The story of its detection is one of luck and inspiring collaboration. The CMB was first theoretically predicted in 1948, by Gamow, Alpher and Herman. In 1965, researchers at the Bell Telephone Laboratories were constructing a new radio receiver. They were constantly bothered by a certain noise, which was coming from the sky. At the same time, a team at Princeton led by Robert Dicke was trying to detect the cosmic microwave background. The two groups heard about each other’s work, and Dicke realized that this unaccounted for noise was due to the CMB. The group swiftly published their results in the Astrophysical journal and the Big Bang theory became the new paradigm.

The Holmdel Horn Antenna that was used by the teams to detect the CMB. Credit: Bell Labs.

The cosmic microwave background is an ancient artefact from the Big Bang. Its an inspiring story about an accidental discovery. The resolution of our maps is getting better and better with each decade, so we can reasonably expect that it will be the key for future research about the Big Bang. If you want to learn more about the CMB and its discovery, check out this video from Khan Academy.


Decoding the cosmic microwave background. (2018, July 27). Astronomy.Com.

Howell, E. (2018, August 24). Cosmic Microwave Background: Remnant of the Big Bang. Space.Com.

Unfortunate pigeons and the search for a theory of everything. (2013, August 8). Early Universe @UCL.

Williams, M. (2018, September 8). What is the Cosmic Microwave Background? Universe Today.

Published by Mateusz Ratman

High school student from Warsaw, Poland. JHU Class of 2026.

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