Astronomy
Cosmic Microwave Background: Evidence for the Big Bang
Quick fact
The cosmic microwave background was discovered accidentally in 1965 by Arno Penzias and Robert Wilson, who won the Nobel Prize—all because they couldn't get rid of a persistent hiss from their antenna.
Why this is interesting
When you tune an old analog TV to a channel with no signal, about 1% of the static you see is the oldest light in the universe—the cosmic microwave background. But how can a tiny hiss of radiation confirm a massive explosion billions of years ago?
Read the full explanation
Understanding Cosmic Microwave Background: Evidence for the Big Bang
The Big Bang theory predicts that the universe began as an incredibly hot, dense point and has been expanding ever since. About 380,000 years after the Big Bang, the universe cooled enough for protons and electrons to combine into neutral hydrogen, making it transparent for the first time. The light released at that moment has been traveling through space ever since, but because the universe has expanded, that light has been stretched from visible to microwave wavelengths. This radiation, called the cosmic microwave background (CMB), now fills the entire universe with a near-uniform temperature of about 2.725 Kelvin. Imagine the entire sky glowing faintly in microwaves—this is the CMB, a snapshot of the universe when it was just 380,000 years old.
A deeper explanation
The CMB is compelling evidence for the Big Bang because it matches predictions made before its discovery. Its spectrum is nearly a perfect blackbody, consistent with the hot, dense early universe. More importantly, the CMB is not perfectly uniform; tiny variations (anisotropies) correspond to density fluctuations that eventually grew into galaxies and galaxy clusters. The pattern and size of these fluctuations match predictions of the Big Bang model, specifically the inflationary period. By studying the CMB, cosmologists can determine the universe's age (13.8 billion years), its composition (about 5% ordinary matter, 27% dark matter, 68% dark energy), and its geometry (flat). Without the CMB, the Big Bang would remain a speculative idea; with it, we have direct observational proof that the universe began in a hot, dense state and has evolved as predicted.