Cosmic microwave background is a sea of radiation that provides us with evidence for the big bang.
When around 1916 Einstein first used general relativity to build a cosmic model, he followed the orthodoxy of the day and assumed that the universe was static: that is, neither expanding nor contracting. He even added in an extra term in his equations, a sort of anti-gravity called the cosmological constant, to make sure it was so. Einstein later called it his “greatest blunder” – although the constant has since been revived to describe the mysterious dark energy that is currently speeding up the universe’s acceleration.
It seemed to be a blunder because observations by Edwin Hubble and others in the 1920s showed that distant galaxies are “redshifted” as if they are moving away from us. Others then used his theory to build models of an expanding universe. This was the beginning of today’s standard cosmological model, which describes a universe that began in the hot, dense, infinitesimal pinprick of the big bang some 13.8 billion years ago.
The big bang is now orthodoxy, although without a theory than unifies general relativity with quantum theory, we’re stuck in explaining exactly what it was or why it happened. But the clinching evidence for it came by accident in 1964. Arno Penzias and Robert Wilson were telecoms engineers working on an early version of today’s mobile-phone technology when they discovered an unexplainable, continuous noise in a gigantic microwave receiver.
This is the oldest light in the universe, sent on its way some 380,000 years after the big bang, when the cosmos had cooled sufficiently for the first atoms to form, allowing photons to travel freely. Probes collecting this light, most recently ESA’s Planck mission, have mapped it in fine detail, providing information on the universe’s earliest years and its make-up today – confirming the picture of the standard model of cosmology, with its enigmatic elements of dark matter and dark energy. Richard Webb