c. 13.7 Billion BCE

Big Bang

Edwin Hubble (1889–1953)

There’s no better place to start considering the broad sweep of astronomical history than the beginning—that is, the actual beginning of both space and time. Twentieth-century astronomers such as Edwin Hubble discovered that the universe is expanding by observing that large-scale structures like galaxies are all moving away from each other, in any direction that we look. This means that, in the past, the universe was smaller and that, at some point in the far distant past, everything started out as a single point of space and time: a singularity. Years of careful observations by the Hubble Space Telescope and other facilities have revealed that the universe was born in a violent explosion of this singularity about 13.7 billion years ago.

The details of big bang theory—as it was initially dubbed by astronomers in the 1930s—have been rigorously tested with decades of astronomical observations, laboratory experiments, and mathematical modeling by cosmologists and astronomers who specifically focus their research on the origin and evolution of the universe. What we have learned about the early history of our universe from these studies is impressive: within the first second of the universe’s existence, the temperature dropped from a million billion degrees to “only” 10 billion degrees, and all of the universe’s present supply of protons (hydrogen atoms) and neutrons formed out of this primordial plasma. By the time the universe was only three minutes old, helium and other light elements had been formed from hydrogen in the same kind of nuclear fusion process that still occurs today deep inside of stars.

It’s mind-blowing to think about both space and time being created at a single instant, 13.7 billion years ago. What caused the explosion? What was there before the big bang? Cosmologists tell us that we can’t really ask that question because time itself was created in the big bang. It’s also humbling to realize that the most abundant element within each of our bodies—hydrogen—was created in the very first second that ever was. We are ancient!

Graphically depicting the beginning of the universe is just as challenging as trying to understand it! Here, an artist has fancifully captured the idea that the big bang was triggered by a collision with another three-dimensional universe that had been hidden in higher dimensions.