This Is How We Know The Cosmic Microwave Background Comes From The Big Bang
“The outer layers are extremely tenuous and rarified, and the radiation we receive here on Earth doesn’t all originate from the very edge of that plasma. Instead, much of what we see originates from about the first 500 kilometers, where the interior layers are significantly hotter than the outermost ones. The light coming from our Sun — or any star, for that matter — is not a blackbody, but the sum of many blackbodies that vary in temperature by many hundreds of degrees.
It’s only when you add all these blackbodies together that you can reproduce the light we see coming from our parent star. The cosmic microwave background, when we look at its spectrum in detail, is a far more perfect blackbody than any star could ever hope to be.”
If you get your science from the internet, you might hear about all sorts of alternatives to the Big Bang. Grandiose claims are often made, decrying the Big Bang as a religion that can never be falsified, while simultaneously touting ideas that most scientists discarded decades or even centuries ago.
But there is no ideology at play; science is a game that we play with predictive power and evidence. The Big Bang makes explicit predictions, and so do alternative ideas that rely on atomic emissions, reflected starlight, photonic energy loss, or heated-up dust.
We can look at every idea we can conceive of, but in the end, only one matches what we observe. Here’s how the Cosmic Microwave Background points to the Big Bang, and away from every other alternative.