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Why the Big Bang Wasn't the Beginning of the UNIVERSE

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.

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: National Aeronautics and Space Administration (NASA) / GSFC.

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

Did the Universe begin with the Big Bang? When we discovered the cosmic microwave background, and its properties matched exactly the prediction of the Big Bang theory, it was a watershed moment for cosmology. For the first time, we had uncovered the origins to the entire Universe, having learned where all of this came from at long last. Emerging from a hot, dense, expanding, and cooling state, the matter-and-radiation-filled early Universe gave rise to everything we see today.


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. Image credit: NASA / STScI / A. Felid.

Except there were a few pesky problems that the Big Bang couldn't explain. If the Universe truly emerged from an arbitrarily hot, dense state, and if space and time themselves were born at that exact moment, the Universe would have signatures that we simply don’t see. Instead, theorists came up with an alternative beginning: cosmic inflation. Inflation made a bold prediction about the scale and magnitude of the fluctuations that should arise from this early state, and when our technology finally caught up to our imaginations, we measured them.

 

The quantum fluctuations inherent to space, stretched across the Universe during cosmic inflation, gave rise to the density fluctuations imprinted in the cosmic microwave background, which in turn gave rise to the stars, galaxies, and other large-scale structure in the Universe today. Image credit: E. Siegel, with images derived from ESA/Planck and the DoE/NASA/ NSF interagency task force on CMB research.

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