Almost everybody has heard about the big bang. That marked the beginning of our universe some 13.8 billion years ago, which has been expanding ever since.
But it is very difficult for most of us, me certainly included, to get a solid understanding of what the big bang really was. That's because our common sense intuitions of reality, which are founded in everyday experience, aren't of much help in domains of science such as quantum mechanics and big bang cosmology.
Religions appeal to those intuitions by making the creation of the universe into something our minds can easily grasp.
Like, that God stands outside the universe in His own heavenly realm and used his divine power to bring the universe into being in much the same way as a human artist would stand at an easel and create a painting on a canvas.
We can picture this in our minds. However, the big bang is very difficult to picture, since it is like nothing we're familiar with. So to grasp the big bang we have to embrace scientific knowledge, which is much better than religion or intuition/common sense when it comes to understanding reality.
In the December 31, 2022 issue of New Scientist the question was posed, "In which direction in the sky did the big bang take place 13.8 billion years ago?" The answers from scientifically-minded readers are illuminating.
The universe doesn't have a centre we can point towards. This is because the big bang wasn't a conventional explosion. If it were, you would be able to track the debris back to its origin and to a point in the sky. Instead, the big bang was the instant that both time and space were created.
The abstract ideas that cosmology raises are often very difficult to intuit... Many years ago I discovered a lovely way to think about this inside a church in the Dee valley in Aberdeenshire, UK. The message said: "The Universe is a Circle whose Circumference is Nowhere and whose Centre is Everywhere." So the answer to the question is that the big bang happened in whichever direction you care to look. And you will never find its boundary.
The big bang took place everywhere in the universe -- it was the universe. Hence, regardless of where you point, you always point in the direction of the big bang... Imagine you are an ant living on the surface of a huge balloon. For the ant, the surface is its entire universe. The inside or outside of the balloon can't be observed.
Now, imagine the balloon expanding. For the ant, there is no direction from which the expansion started, it is happening everywhere, in all directions. It is a bit like standing at the South Pole and trying to point in the direction of north. At the South Pole, all directions are north.
Regarding what the universe has been expanding into following the big bang, the answer is similar: nothing. Or rather, even less than nothing, if "nothing" is viewed as the absence of something, which isn't really correct when it comes to the big bang.
For the expansion of the universe is of space and time itself. So it isn't correct to view that expansion as akin to a balloon being inflated, for a balloon grows larger within already existing space. Instead, our universe is expanding. Period. It isn't expanding into anything. It is simply expanding.
I can't visualize this, because it is impossible to picture in any sort of familiar fashion. I suspect that only those comfortable with advanced mathematics can fathom how the big bang brings spacetime into being, yet didn't occur within spacetime.