FactSentinel

The universe is permeated by a faint glow of microwave radiation known as the Cosmic Microwave Background (CMB), which has a temperature of approximately 2.7 degrees Kelvin, or about -270.4 degrees Celsius. This is only a few degrees above absolute zero, the theoretical point at which all molecular motion ceases. This radiation is a direct relic of the Big Bang, dating back to roughly 380,000 years after the universe's formation, when the cosmos cooled enough for atoms to form and light to travel freely for the first time.

The evidence supporting this claim is extensive and precise. The CMB was first detected in 1965 by Arno Penzias and Robert Wilson, who initially mistook it for background noise in their radio antenna. Since then, satellite missions including COBE, WMAP, and Planck have mapped this radiation with extraordinary accuracy, confirming its temperature and near-uniform distribution across the sky. The radiation's blackbody spectrum matches theoretical predictions almost perfectly, providing one of the strongest pieces of evidence for the Big Bang model. As the universe has expanded over roughly 13.8 billion years, this primordial radiation has stretched and cooled from extremely high temperatures down to its current near-absolute-zero state.

A common misconception is that space is a perfect vacuum with zero temperature. In reality, even the emptiest regions of space contain this faint thermal radiation, meaning the universe has a measurable, non-zero baseline temperature everywhere. This background temperature is not uniform to zero variation, either; tiny fluctuations of about one part in 100,000 reveal the seeds of galaxy formation, making the CMB not just evidence of the Big Bang but also a map of cosmic structure's origins.