There is a large suite of scientific evidence that supports the picture of the expanding Universe and the Big Bang. The entire mass-energy of the Universe was released in an event lasting less than 10^-30 seconds in duration; the most energetic thing ever to occur in our Universe’s history. (NASA / GSFC)

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What Was It Like When The Big Bang First Began?

13.8 billion years ago, our Universe as-we-know-it came into existence. Here’s what it was like.

Ethan Siegel
6 min readJul 11, 2018

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Looking out at our Universe today, we not only see a huge variety of stars and galaxies both nearby and far away, we also see a curious relationship: the farther away a distant galaxy is, the faster it appears to move away from us. In cosmic terms, the Universe is expanding, with all the galaxies and clusters of galaxies getting more distant from one another over time. In the past, therefore, the Universe was hotter, denser, and everything in it was closer together.

If we extrapolate back as far as possible, we’d come to a time before the first galaxies formed; before the first stars ignited; before neutral atoms or atomic nuclei or even stable matter could exist. The earliest moment at which we can describe our Universe at hot, dense, and uniformly full-of-stuff is known as the Big Bang. Here’s how it first began.

If you look farther and farther away, you also look farther and farther into the past. The earlier you go, the hotter and denser, as well as less-evolved, the Universe turns out to be. The earliest signals can even, potentially, tell us about what happened prior to the moments of the hot Big Bang. (NASA / STScI / A. Feild (STScI))

Some of you are going to read that last sentence and be confused. You might ask, “isn’t the Big Bang the birth of time and space?” Sure; that’s how it was originally…

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Starts With A Bang!
Starts With A Bang!

Published in Starts With A Bang!

The Universe is out there, waiting for you to discover it.

Ethan Siegel
Ethan Siegel

Written by Ethan Siegel

The Universe is: Expanding, cooling, and dark. It starts with a bang! #Cosmology Science writer, astrophysicist, science communicator & NASA columnist.

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