Nope – we can, in fact, trace the universe back to where it all started. Unfortunately, it rapidly gets complicated, because the answer is that it began where you are sitting. And also where I am sitting. And also where everyone else on the planet is sitting. And also at the center of our galaxy, and at the center of every other galaxy.
The idea here is that our current universe is expanding, so the universe must have been smaller in the past. So if you take every point in space that exists now, and trace it backwards, all those points get closer and closer together until they reach a mathematical and physical stopping point – a singularity. A singularity is an infinitesimally small point, which can contain quite a large amount of matter or energy – the centers of black holes are also singularities.
The singularity we reach if we trace back the whole universe must have contained all the energy that now exists in our universe, as either mass or light, or dark matter, or dark energy. But it also contained all the space – so all the points of space that we now see as very widely separated were present within that singularity. So the “where” of the Big Bang is, quite literally, everywhere.
We have quite a lot of evidence pointing us to this idea of a very tiny universe at the very beginning of our universe; one of the more important being the detection of the cosmic microwave background (or CMB for short). This background radiation is called “background” because our universe has a fundamental glow in the microwave that you can’t escape – any other observations you’re making at this wavelength will be in addition to the CMB.
Critically, the CMB is very precisely almost the exact same in every direction that we look, and even though this glow is the oldest light in the universe, and the universe was much, much smaller than it is now, you would still not expect it to be the exact same everywhere — unless the universe had been even smaller previously. The theory of the Big Bang produces this naturally, because in between all space being compressed into the singularity, and the production of the light we see as the CMB, there is predicted to be a period of super fast expansion — inflation. Or, if you’re tracing the universe backwards in time, the universe shrinks dramatically down.
The thing to keep in mind with the Big Bang and the expansion of the universe is that it wasn’t an “explosion” like a detonation here on earth, with a definite center, and the universe spooling outwards into a pre-existing space. The closest you can get while thinking of conventional explosions would be if you managed to really effectively explode a tiny object, and then asked “Where was this piece when the explosion happened?” It was at the center, with all the other scattered pieces. For our universe’s expansion, each of those pieces would have to be markers in space itself. Where did the universe’s big bang happen? It happened where the universe was small, and each fragment of our current universe was there to witness it.
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