Question: What sort of evidance is there leading toward proof is there that the Big Bang existed?

Laurence Perreault Levasseur answered on 18 Jun 2014:

Great, and very important(!!), question!

I already gave a long explanation here (it explains things like what does it mean for the universe to be expanding, and what was the universe like just after the Big Bang, which helps understanding what the proofs are):
/energyj14-zone/2014/06/14/what-is-the-evidence-for-the-big-bang-what-happened-during-the-big-bang/

But here are the evidences:
1) Galaxies are moving away from us today, and the further away form us they are, the faster they are moving away. This tells us that a) the Universe is in expansion, and b) if we go backward in time, there is a point where all of space and all of matter was contracted in a single point, the Big Bang!

2) If we were able to look in the past, we would see a point where galaxies were so close together that all the matter they are made of was mixed together in a big ‘soup’.
The trick is that we actually CAN look in the past. We can do that because light travel at a finite speed. For example, at that speed, it takes 3 seconds for the light to travel from the moon to the Earth. So when we see the moon, we see it as it was 3 seconds ago, when the light left the moon! Same with the sun, it’s further away, so it takes about 8 minutes for the light to travel from the sun to us, so we always see the sun as it was 8 minutes ago. So, the further something is, the further in the past we see it from the Earth! If a galaxy is a million light-years away, we see it as it was 1 million years ago.
If we look far enough, we see the ‘soup’ I was talking about, when the Universe was just 400 000 years old (now it’ s about 13.8 billions years old)! The light coming from that soup is called the Cosmic Microwave Background (or CMB).

3) If we look at how much light gases there are in the Universe (these are the first few in the periodic table – don’t worry if you don’t know what it is!), like Hydrogen, Helium, Deuterium, and Lithium, they are the same to high accuracy everywhere in the Universe around us, in all directions. This tells us that those elements were not formed in stars, like all the other ones, but shortly after the Big Bang, when the universe was still a uniform ‘soup’ (the same soup as in 2).

4) Galaxy evolution and galaxies distribution in the sky also support the Big Bang scenario. Since we can look at galaxies at different ages (depending on how far from us they are, we see them when they were younger – further away from us- or older – closer to us- ) and this is consistent with them all starting to form about at the same time, and points at the same age of the Universe as other evidences. This one is less important, but it mostly tell us that the Universe is evolving with time and it’s not always been the same.

5) a very cool one: the sky is dark at night, as opposed to very bright. This is called Olbers’ Paradox. check it out here:
http://en.wikipedia.org/wiki/Olbers'_paradox
This also tells us that the Universe is not static and eternal. If it were, the sky at night would be very bright, instead of dark. This is because even if the further a star is, the dimmer it is, the further away you look, the more stars there are in a sphere of fixed radius around us, in the precise way that those two things cancel, and everywhere in the sky should be as bright as the surface of the sun!!! But this doesn’t happen because 1) the Universe isn’t eternal, it likely began in a Big Bang and 2) the Universe is expanding, so all further stars really contribute less light