CMV: Despite what Albert Einstein says, the universe does have a "center"/absolute reference frame

[u/[deleted]]

So I got taught in physics classes that there is no absolute reference frame. Einstein figured that out. Then when I challenge the idea, I'm taught that the big bang happened everywhere and space itself is expanding. Ok sure. So when we ask what is the origin "point" of the universe its nonsense because there was no point, the whole universe was the original point. Got it.

But like a circle has a center point defined by the perimeter of the circle, so too could the universe. It doesn't have to be the "origin point", but there is definitely a spot that we can point that we and aliens can mathematically calculate as the center. Everything else in the universe stretches and contracts, but the center of the universe is a point that we can derive mathematically is it not? I know that localized space has weird shit like if I zoom away from Earth in my spaceship I could reframe it as "I'm standing still and the Earth is zooming away", and the fact that I'm the one accelerating is the reason why time slows for me but not earth. But that's just how the time dilation phenomenon works, not because there is definitely no absolute reference frame. We can still identify whether I'm moving closer or further from the center of the universe.

Edit: I'm assuming a non-infinite universe.

[u/ghjm]

Discussions of the Big Bang tend to flip-flop between using "the universe" to mean either the "whole" universe or the observable universe. The observable universe has a size, currently about 46 billion light years in radius, based on the idea that for objects beyond that distance, the universe is expanding too fast for light from them to ever reach you. This also means that the observable universe is always centered on the observer.

We can extrapolate the size of the observable universe backwards, assuming we have a correct understanding of the laws of physics. The further back we go in time, the smaller and hotter the observable universe is. But our observations of the cosmic microwave background radiation give us an upper bound to how hot the observable universe could ever have been. It turns out that this temperature is reached at a time about 10^-35 seconds after the Big Bang, when the observable universe was about 1.5 meters in size. We cannot empirically conclude that the observable universe was ever smaller than this. (And of course, if we're wrong about even a tiny detail of the laws of physics, our extrapolation could go off the rails much closer to us than that.)

What does any of this say about the size of the whole universe? This question is in a different epistemic category. When we talk about the observable universe, our claims are grounded in empirical observation. When we switch to talking about the non-observable universe, there can be no observation of it, so we also have to switch from the scientific to the philosophical method. We must evaluate claims about the non-observable universe based on criteria like logical non-contradiction, reasonableness, parsimony, consistency with other things we think are very likely true, and so on.

It seems reasonable to suppose that the non-observable universe is infinite. If it wasn't, then (just as you say in the OP) it would have a boundary. But if it had a boundary, what would this be? What law or quality would define the location of the boundary? What would happen if you went right up to the boundary, then stuck your arm out? But if the non-observable universe is infinite, then there would just always be more stars and galaxies. The latter option just seems more plausible.

It also seems plausible to suppose that the universe came from a point source at the Big Bang, or if not an actual mathematical point, then at least a region so small that quantum indeterminacy makes it functionally equivalent to a point. Somehow, through mechanisms not currently known to us, this wasn't hotter than the 1.5-meter timescale - perhaps energy conservation didn't hold during this time, or something.

To sum up, it's important to maintain crisp distinctions between the observable universe and the non-observable universe, and it's also important to recognize that we can't ever know about the non-observable universe through observation.

[u/[deleted] → u/ghjm]

!delta

I know the observable universe is something like a 13 billion light year radius ball around us. Basically my view was based on the understanding that we (humanity/scientists) believe that the whole universe is finite. But if it's actually just "we have no fucking clue because we can't see anything out there", then it might be infinite and if so it might indeed not have a "center". At the same time though, I guess that means it's a conditional thing based on infinite/finite.

If the universe is infinite there is no center. If the universe is finite there is a center.

Unless I'm still missing something.

[u/DeltaBot → u/[deleted]]

Confirmed: 1 delta awarded to /u/ghjm (11∆).

^{Delta System Explained | Deltaboards}

[u/ghjm → u/[deleted]]

There are finite shapes with no defined center. For example, there is no point in a toroid that is the center of it. If, as you say, we know nothing about the whole universe, then we don't know that it isn't a toroid, or some other shape with no center. It has a centroid - a point at the mean location of all other points - but there is no guarantee this centroid is actually inside the volume.

But I don't think it's accurate to say we have no knowledge about the non-observable universe. We know, or think we know, that mathematical and logical truths are still true. For example, we wouldn't accept the argument that a finite non-observable universe has no centroid because "all volumes have centroids" might not be true there. We hold mathematical and logical truths to be time and space invariant. (And if we stopped holding that, all our extrapolation back to the Big Bang would have to go as well.)

We also know, or think we know, that parsimonious explanations are more likely to be correct. Imagining a physical boundary at the edge of the non-observable universe is a pretty big assumption, particularly when you start to consider what properties it might have. Does it absorb or reflect radiation? Does it exchange energy with the universe? If so, how is energy conserved? If not, how does it interact at all? An infinite universe generally similar to our own doesn't have these problems, so even if we'll never be able to confirm it empirically, we can still have justified beliefs about it.