A Universe that expands and cools today, like ours does, must have been hotter and denser in the past. Initially, the Big Bang was regarded as the singularity from which this ultimate, hot, dense state emerged. But we know better today. Image credit: NASA / GSFC.

The Big Bang Wasn’t The Beginning, After All

Why you can’t extrapolate back to a singularity.

“Despite its name, the big bang theory is not really a theory of a bang at all. It is really only a theory of the aftermath of a bang.” -Alan Guth

The Universe began not with a whimper, but with a bang! At least, that’s what you’re commonly told: the Universe and everything in it came into existence at the moment of the Big Bang. Space, time, and all the matter and energy within began from a singular point, and then expanded and cooled, giving rise over billions of years to the atoms, stars, galaxies, and clusters of galaxies spread out across the billions of light years that make up our observable Universe. It’s a compelling, beautiful picture that explains so much of what we see, from the present large-scale structure of the Universe’s two trillion galaxies to the leftover glow of radiation permeating all of existence. Unfortunately, it’s also wrong, and scientists have known this for almost 40 years.

First noted by Vesto Slipher, the more distant a galaxy is, on average, the faster it’s observed to recede away from us. For years, this defied explanation, until Hubble’s observations allowed us to put the pieces together: the Universe was expanding. Image credit: Vesto Slipher, (1917): Proc. Amer. Phil. Soc., 56, 403.

The idea of the Big Bang first came about back in the 1920s and 1930s. When we looked out at distant galaxies, we discovered something peculiar: the farther away from us…