Britannica Sources used in this answer

How does cosmic inflation help explain the cosmic microwave background’s uniform temperature?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

Oval map of the cosmic microwave background radiation from NASA's WMAP mission, showing a mottled pattern of blue, green, yellow, and red representing temperature variations. — Cosmic microwave background A full-sky map produced by the Wilkinson Microwave Anisotropy Probe (WMAP) showing cosmic background radiation, a very uniform glow of microwaves emitted by the infant universe more than 13 billion years ago. Color differences indicate tiny fluctuations in the intensity of the radiation, a result of tiny variations in the density of matter in the early universe. According to inflation theory, these irregularities were the “seeds” that became the galaxies. WMAP's data support the big bang and inflation models, and cosmic microwave background is at the farthest limits of the observable universe.

The high degree of isotropy in the cosmic microwave background presents a challenge known as the "light-horizon" problem. Because opposite sides of the observable universe are beyond each other's light horizons, they should not have had time to exchange information or reach a uniform temperature since the beginning of the universe.1

Cosmic inflation provides a solution to this dilemma. The theory posits that before this rapid expansion occurred, the matter and radiation that now make up the entire observable universe were in close thermal contact within a small cosmic event horizon. During this period, these regions were able to interact and acquire the same thermodynamic characteristics and temperature.12

When the universe underwent exponential inflation, these regions were stretched far apart, moving outside their individual event horizons. As the universe later resumed normal expansion, these distant portions reappeared on our horizon. Because they were in thermal equilibrium before the inflationary period, they are observed today to maintain the same uniform temperature.12