Although nobody can be sure how the Universe began, most scientists believe that it was horn from an enormous explosion 13 billion years ago. This explosion, called the “Big Bang”, was the point where space and time came into existence and all of the matter in the cosmos started to expand. Before the Big Bang, everything in the Universe was compressed into a minuscule area no bigger than the nucleus of an atom. The Big Bang was an unimaginably violent explosion that sent particles flying in every direction. A process called cosmic inflation caused the Universe to expand into an area bigger than the entire Milky Way in less than a second. Moments later, the temperature began to decrease, and the Universe began to settle down. Stars and galaxies began to form roughly one billion years after the Big Bang.
Initially, the universe was permeated only by energy. Some of this energy congealed into particles, which assembled into light atoms like hydrogen and helium. These atoms clumped first into galaxies, then stars, inside whose fiery furnaces all the other elements were forged.
This is the generally agreed-upon picture of our universe’s origins as depicted by scientists. It is a powerful model that explains many of the things scientists see when they look up in the sky, such as the remarkable smoothness of space-time on large scales and the even distribution of galaxies on opposite sides of the universe.
But there are things about this story that make some scientists uneasy. For starters, the idea that the universe underwent a period of rapid inflation early in its history cannot be directly tested, and it relies on the existence of a mysterious form of energy in the universe’s beginning that has long since disappeared.
“Inflation is an extremely powerful theory, and yet we still have no idea what caused inflation or whether it is even the correct theory, although it works extremely well,” said Eric Agol, an astrophysicist at the University of Washington.
For some scientists, inflation is a clunky addition to the Big Bang model, a necessary complexity appended to make it fit with observations. This wouldn’t be the last addition.
“We’ve also learned there has to be dark matter in the universe, and now dark energy,” said Paul Steinhardt, a theoretical physicist at Princeton University. “So the way the model works today is you say, ‘OK, you take some Big Bang, you take some inflation, you tune that to have the following properties, then you add a certain amount of dark matter and dark energy.’ These things aren’t connected in a coherent theory.”