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The Big Bang Wasn’t The Beginning, After All

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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.

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.

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 they were, the faster they appeared to be receding from us.

According to the predictions of Einstein’s General Relativity, a static Universe would be gravitationally unstable; everything needed to either be moving away from one another or collapsing towards one another if the fabric of space obeyed his laws.

The observation of this apparent recession taught us that the Universe was expanding today, and if things are getting farther apart as time goes on, it means they were closer together in the distant past.

An expanding Universe doesn’t just mean that things get farther apart as time goes on, it also means that the light existing in the Universe stretches in wavelength as we travel forward in time.

Since wavelength determines energy (shorter is more energetic), that means the Universe cools as we age, and hence things were hotter in the past.

It’s tempting, therefore, to keep extrapolating backwards in time, to when the Universe was even hotter, denser, and more compact.

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.

Theorists thinking about these problems started thinking of alternatives to a “singularity” to the Big Bang, and rather of what could recreate that hot, dense, expanding, cooling state while avoiding these problems.

The conclusion was inescapable: the hot Big Bang definitely happened, but doesn’t extend to go all the way back to an arbitrarily hot and dense state.

Instead, the very early Universe underwent a period of time where all of the energy that would go into the matter and radiation present today was instead bound up in the fabric of space itself.

That period, known as cosmic inflation, came to an end and gave rise to the hot Big Bang, but never created an arbitrarily hot, dense state, nor did it create a singularity.

What happened prior to inflation — or whether inflation was eternal to the past — is still an open question, but one thing is for certain: the Big Bang is not the beginning of the Universe!

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Pass it on: New Scientist

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