Did the universe come into existence through the big bang, and was the big bang an explosion?
The unfortunate thing about the Big Bang is its name. It was invented by Sir Fred Hoyle in a radio interview on BBC in 1949 to ridicule the Big Bang theory.
At that time, the only evidence that existed for it, was cosmic redshift and the fact that Einstein’s General Relativity – the core of the Big Bang theory – worked really well. From General Relativity, physicist Alexander Friedmann concluded that the universe could not be stable, and physicist and astronomer Georges Lemaître figured out that if the universe was not stable, it must have a beginning where space and time was zero – an Ur-Atom or a Cosmic Egg. There’s lots of maths behind their conclusion, based on the field equations in General Relativity, so it’s not just an idea pulled out someone’s arse.
The problem is that the paradigm at the time was that the universe was eternal and stable, without a beginning. And since Lemaître’s hypothesis contradicted the paradigm, he had an uphill battle to fight to prove his idea. Cosmic redshift helped, but it did not convince everyone, like for instance Fred Hoyle.
Another thing that was against Lemaître’s idea is that he was also a Catholic priest, and his hypothesis sounded a lot like “let there be light”.
So, atheist Sir Fred Hoyle did not like Lemaître’s hypothesis, and thus called it “The Big Bang”.
Sadly, the name stuck.
The problem with the name is that it implies an explosion, which the Big Bang is not. It is an expansion of space – space simply got bigger. “Inflation” is a better term, since it implies that the universe kind of behaved like a balloon being inflated. “Everywhere stretch” is an even better name, since it tells you exactly what happened. But they don’t have the “oomph” of “Big Bang”.
So, the universe didn’t explode. What caused it to expand or stretch then?
Well, we don’t know.
The problem is that we are looking at the Big Bang from the inside and looking back towards the beginning. We don’t have a top-down outside view of the universe. And that means that there are some horizons in the way. There is the cosmic microwave background – in itself important evidence of the Bang, but it also blocks light from before the background formed as the opaque plasma of the early universe cooled down to ordinary gas, some 380,000 years after the Bang.
We can infer earlier stuff from the surface (i.e. the light of the background that reaches us now) of the cosmic microwave background, and we can simulate the conditions of the early universe in particle colliders. We have well-tested theories on quantum mechanics and general relativity, which allow us to make predictions of what we will see beyond the testable conditions.
But we can’t reach all the way back, not even hypothetically. At the first fraction of a second, the conditions are so extreme that the physics we have is not enough to predict what happened. So everything beyond that is beyond our reach. Until we find a better physics – specifically one that combines general relativity and quantum physics – we cannot go beyond that first fraction of a second and explained what made the universe bang.
So right now, we are limited to explaining everything that happened after the bang up to now and predicting what will happen in the future.