Why does the universe look the same in every direction?

Look at the cosmic microwave background in one direction, then look at the exact opposite direction. The two patches of sky are so far apart that, given the universe's age and the speed-of-light limit on any signal, they could never have exchanged so much as a single photon. And yet they look almost identical in temperature. That puzzle is called the horizon problem, and for decades it had no good answer.

There's a second, related puzzle. The density of the universe sits remarkably close to one precise 'critical' value, the dividing line between a universe that eventually recollapses and one that expands forever. Any tiny deviation from that value should have snowballed enormously over billions of years, so for the universe to look this balanced today, its density right after the Big Bang must have been tuned to within one part in 10^62 of that critical value. That's the flatness problem. Such precise tuning by pure chance seemed absurd.

Alan Guth, a young physicist at Cornell, came at these puzzles almost sideways. In 1978 he heard Robert Dicke lecture on the flatness problem. In 1979, while working on a different puzzle about particles, he found that the early universe could have gone through a brief burst of exponential expansion. It became known as cosmic inflation. Inflation flattens any curvature, the way the Earth looks flat when you stand on a tiny patch of it. Two weeks later, Guth heard colleagues discussing the horizon problem, and saw that inflation solved it too. Before inflation, the region that became our observable universe was tiny. All its parts could exchange signals and even out. Inflation then blew up that uniform patch, carrying its evenness along with it.

Guth submitted his paper in 1980. Later, satellites like COBE and WMAP measured patterns in the cosmic microwave background that match what inflation predicts. Still, nobody knows what physical field drove the expansion, and some scientists dissent. Inflation remains the leading, but unproven, explanation for why the universe looks so strikingly uniform.