What does the has a center comment have to do with the rest of the comment? The big bang had a center too. That doesn't mean anything in the context though.
The other two sibling comments expressed some doubt, but I can tell you with absolute certainty that in the standard model, and to the best of our knowledge in reality, the big bang did not have a center.
This makes no sense to me. Are we saying the universe existed before the big bang? The big bang went boom and everything started expanding from it. How is it not the center?
Start with a tiny sphere. Expand it. The sphere gets bigger, but no point on the 2D-like surface is the centre.
It's like that, but the surface is a 4D spacetime.
You can't use your imagination to visualise this. But you can use math to describe it.
Whether there's some kind of ultradimensional hypercentre is a different question. Even if there was, it wouldn't be accessible from this universe.
And don't forget we don't have a clue what spacetime is. Relativity has some nice descriptions of what it does, but there's no fundamental explanation of how the universe generates the phenomena we call position, time, and distance.
That is the best explanation I've ever heard about that "where is the center question". Thank you.
Perhaps some day I'll be able to wrap my head around your closing caveat. I suspect any question I ask on that would be another "where is the center" type question.
We observe that the matter density in the universe is decreasing with time. As far as we know, this looks the same from every point in the universe: galaxies move away from each other. The speed at which they move away from each other doesn’t depend on where in the universe you are. (It only depends on the time, as the speed is slowly increasing.) There is no center from which they move away from. Or alternatively, every point in the universe is such a center. It’s like when zooming into a grid. It doesn’t matter at which point of the grid you zoom in, the grid expands in the same way regardless. The grid has no center, and also no boundary.
Due to the laws of general relativity, extrapolating that observation into the past, the implication is that the matter in the universe was ever denser the further you go into past. Furthermore, there is a point in time where it becomes infinitely dense. This is what we call the Big Bang. Because our equations break down at that point, we don’t know what happened at that point in time, or before, or whether there was a before. However, observational evidence like the cosmic microwave background strongly support the theory: https://en.wikipedia.org/wiki/Big_Bang#Observational_evidenc...
Note that “infinitely dense” does not imply that the universe shrinks to a single point at that time. Rather, it was infinitely dense at every point. (Again, this is like zooming out from an infinite grid infinitely far. The grid always remains infinitely large.) This means that the Big Bang happened everywhere at once. It’s a point in time, not a location.
The Big Bang is not a physical explosion and the universe is not a balloon. All space was condensed together, but space is (to our current ability to measure it) topologically flat and infinite. It was still infinitely big in every dimension, it was just unfathomably dense.
The expansion that's occurring is that the space between things is growing. But it's every space between everything all at once, everywhere.
No, we are not saying that. We are saying that the entirety of space - and not just its contents - have been compressed into a point (ignoring quantum gravity for now). Any two points in space have been arbitrarily close if you play the movie backwards sufficiently close to the first moment.
No, it’s not space itself. Relative to what would it expand? It’s galaxies moving away from one another, which necessarily increases the space between them.
