Cosmic Background Radiation


I’ve never been able to understand this phenomenon. How is it proof of the Big Bang?

In: 7

So imagine that everything is milky. Air is milky, glass is milky, everything. It’d be really hard to see any light because it always gets blocked by something.

That’s basically what the universe was for the first 300,000 years or so. It was so hot and so dense that light couldnt get very far.

But at that 300,000 year point, the universe expanded enough that light started to be able to actually go places. The light we see from when that happened is called the CMB, Cosmic Microwave Background.

We have been able to look at this radiation because every moment of every day, more of it, coming from even further away, hits the Earth, having traveled for about 13.7 billion years before hitting anything.

As for proof of the Big Bang, it’s not really proof, it’s evidence that supports the Big Bang, because the Big Bang/inflation offers reasonable explanations for why we have this blanket of low energy radiation hitting us all the time.


In the next town over, they’re blasting away rock with dynamite, to make a new Amazon warehouse.


You’re a town away, but you still hear the boom. In fact, it’s so loud, that it has this echo. You hear the boom, but then the boom takes a few seconds to “fade out”.

Now imagine the boom is even bigger! It’s so big, it takes AN HOUR to fade out. You sit there after the big boom, and the sound gets gradually less and less until you have to strain to hear it. But you can still hear it up to an hour later. That was a mighty big boom!

Imagine the creation of the universe as the biggest boom ever.

The “sound” from it is STILL fading away, today, billions of years later.

That “sound” is very faint now, and you can’t even really hear it, you need special equipment just to detect it.

Scientists have “proven” that the echo of a big boom that we can barely detect today, is evidence of something really really big happening many billions of years ago.

(**THIS WAS ELI5** — So it’s not really sound, it’s radiation, but the concept still holds).

When you look out into space you’re also looking back in time.

Light from something a billion light years away took a billion years to get here, so we see it as it was a billion years ago.

This leads to an interesting phenomenon as you look further away: galaxies get increasingly crude, clustered closer together, and filled with increasingly young stars.

Eventually you can’t see anything at all. The ability to “zoom” further back in time stops at the “cosmic microwave background”. This haze of ancient heat and radiation from primordial hot gas appears to be the oldest thing we can see.

In a much older universe you would just expect to see fully formed galaxies forever in every direction, but we don’t see that – we see a baby universe ~14 billion years ago and then nothing beyond that.

It’s “proof” of the Big Bang because it is in every direction we look and theory says that the Big Bang happened everywhere in the universe. This means that no matter in which direction you look you will see it and as time passes more light from it will continue to reach us (since it happened everywhere in the universe as more time passes the light from the Big Bang will eventually reach us from further and further away). It’s also considered “proof” because it was predicted before it was discovered. They were looking for it. That’s one of the big things about science is that theory predicts its own evidence.

Oh… So many wrong explanations…

First, what is CMBR?
After the big bang, the universe was a lot denser than today. If there’s more energy at less space, than the temperature at that space is higher. In fact, it was so hot, that atoms didn’t exist (actually, as you go back, even baryons didn’t exist, because there was so much energy at one place, that quarks couldn’t combine).
So, at some point we have a bunch of free electrons and protons, and the radiation scatters like crazy when it hits an electron. Think of it as a lamp behind a really thick fog. In other words: The Universe wasn’t transparent to those remnants of the big bang.

But as the universe expanded, it also cooled down, allowing those electrons and protons to combine into hydrogen atoms. And Hydrogen atoms don’t really interact with that radiation, so, the universe became transparent to the radiation, so the radiation follows a straight path now, like a ray of light. That’s what CMBR is: Radiation that we can see after the universe was cold enough to allow the formation of atoms. (One analogy is comparing to a cloudy sky: The Sun is the heat from the big bang, the clouds are the “soup” of particles before the formation of atoms, and the light that pass those clouds is the CMBR).

Why is it evidence for big bang?
1. Redshift indicates expansion of the universe.
2. In theory, you would expect a certain amount of radiation, at a certain temperature and energy distribution. Guess what? The measurements made of the CMBR fits those predictions (and to a scary level of accuracy).