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.
“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 GuthThe
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. Extrapolate this back far enough, and you’ll come to a time where everything was so hot that not even neutral atoms could form. If this picture were correct, we should see a leftover glow of radiation today, in all directions, that had cooled to just a few degrees above absolute zero. The discovery of this Cosmic Microwave Background in 1964 by Arno Penzias and Bob Wilson was a breathtaking confirmation of the Big Bang. READ MORE