Question: How do you know the big bang occured?

Cesar Lopez-Monsalvo answered on 16 Jun 2011:

In most of my previous answers I have made use of relativity to explain black holes. Now I will use it to explain part of the big bang theory.

First, let me put things in context. Einstein finished his theory of gravity (relativity) in 1917. The interaction between gravity and matter is described by the Einstein’s equations (the modern version of Newton’s law of gravitation). Soon after that, solutions to these equations started to appear. One of them was a black hole solution, which I talk about here http://ias.im/47.833. Some other type of solutions appeared as well which made Einstein feel a bit uncomfortable. It seemed that if you applied the theory to the universe as a whole, you wouldn’t be able to keep it “steady”, back then it was not very popular the idea of a dynamical universe. Later, Hubble made some astronomical observations that indicated that farther away objects seemed to be moving away from us faster than nearby ones. You can picture this if you draw some dots on a balloon and then see how the move away from each other as you blow some air into it. That basically indicated that the universe is expanding. So, if everything is expanding now, the universe must have been smaller in the past. If you take that idea seriously, then at some point in the very distant past everything should have been very condensed. But here is a very cool thing about it, that it is not just the different chunks of matter which were closer to one another, spacetime (see here http://ias.im/47.305) was also “smaller”. If we do our physics correctly, then the very distant past should have been way hotter and smaller than it is now.

People didn’t take the idea too seriously at the beginning. It is the general case that you need to provide some evidence that your idea is correct, even if that evidence is is just a prediction of something which has never been observed before, that is what makes physical theories strong. So, if the idea that a the universe started in a “kind of” massive explosion and has been expanding ever since is right, then some trace of such an event must be still out there for us to discover. In fact there is and it was worth a Nobel prize! A couple of guys Penzias and Wilson experimenting with a super-antenna to detect signals from satellites, discovered that there was some “noise” coming from every direction they looked at. Eventually they convinced themselves that their apparatus was working properly and that the noise was coming from space. That happened to be an “echo” from the big bang, a very faint temperature that is everywhere in outer space (right now is about 2.7 degrees above absolute zero, so it is very cold really). That was an epic discovery and, without really looking for one, they were awarded a Nobel prize for it.

Recent satellite missions have been observing that relic temperature and now we have a very accurate map of it. It is literally a picture of the universe when it was about 300000 years old (that is very very young). We cannot see further with light cause before that the universe was not transparent because it was very hot (try to see through a flame in a match).

Right now, we are expecting the new data from a satellite called Plank which was launched a couple of years ago to measure this temperature even more precisely. If we manage to understand this really well, we will have an even better idea of what happened really close to the very beginning. We do not know yet what happen…say at the very first instant of time.

If you want to see a picture and a video of how does it work, look at this

http://www.esa.int/export/SPECIALS/Planck/SEMF2FRZ5BG_1.html