Question: What is the evidence for the Big Bang? What happened during the Big Bang?

Laurence Perreault Levasseur answered on 14 Jun 2014:

By looking at the galaxies in the sky, we know that our Universe is in expansion.

To understand what this means, you can think about a raisin bread baking in the oven. As the bread raises, the dough inflates and the raisins get further and further away from each other. The important point is that they move away from each other NOT because they are moving through the dough, but because the dough ITSELF between all the raisins is inflating.

Something very similar is happening in our Universe right now. You can think of the raisins as galaxies in the Universe, and the inflating dough as space between galaxies. So this means that, just like the bread, the Universe is expanding, which means galaxies are constantly moving further and further away from each other, not because they are moving through space, but because space itself is constantly expanding. One consequence of this is that the further a galaxy is from ours, the faster it seems like it’s moving away from us, because there is more space that is expanding.

Ok. Now, if we start running the clock of the Universe backwards, it means that the further back in time I go, the closer galaxies were to each other. And if I go far enough in the past, all the galaxies (or, more precisely, the stuff – mostly a gas called hydrogen – they are made of) were all clumped together in a very hot and dense soup. If I go even further back in time, all that soup was contracted at the same point, with basically no space at all. That is the beginning of the Universe, and that’s what we call the Big Bang.

So the Big Bang lasted an infinitely short amount of time, and all of the Universe was infinitely small (it had a zero volume). It’s really hard (I mean, kind of impossible) to imagine, but you shouldn’t think of the Big Bang as an ‘explosion’ that happened at some place. It’s more like the moment when all of the infinite space itself (and even time!!) appeared simultaneously, out of literally nothing.

Currently, scientists don’t really understand what happens there. The Big Bang model doesn’t say why this happened, or how. Physicists (including myself) think that we need a better theory of fundamental physics to understand it (a theory that works when things are insanely HOT and insanely DENSE, something called quantum gravity, but we don’t know what it is yet), and that once we understand this theory, we will be able to explain what happened, that space wasn’t created out of zero volume (or if it was, why).

Ok, now for the evidence. You can read some of them from what I said above:

1) Galaxies are moving away from us today, and the further away form us they are, the faster they are moving away.

2) If we were able to look in the past, we would see this ‘soup’ from when the universe was so dense that all the gas galaxies are made of was touching each other. 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 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.