I Does a beginning to time imply absolute speed?

[Hiero]

TL;DR Summary

If someone understands the theory behind the Big Bang please explain the conventional wisdom.

I’m just imagining spacetime that is spatially infinite but has a beginning in time. Naively, there would be only one (class of) reference frame(s with the same velocity) in which time starts simultaneously everywhere. (This is a naive application of “relativity of simultaneity” from SR.) But this would give a universal rest frame to measure absolute speeds.

What’s the generally accepted view on how time began? Is space thought of as infinite or growing? (Or are “infinite” and “growing” not exclusive?)

One other thought; is there a concept of a universal center of (all) mass? That would also give a nature rest frame.

 

[PeterDonis]

I’m just imagining spacetime that is spatially infinite but has a beginning in time.

The idealized spatially flat FRW spacetime has this property, yes.

Naively, there would be only one (class of) reference frame(s with the same velocity) in which time starts simultaneously everywhere.

But not naively, it is easily shown by looking at the math that this is not in fact the case, because the very concept of "reference frame" that you are implicitly using is not valid in the spacetime in question. It's only valid in the Minkowski spacetime of SR.

What’s the generally accepted view on how time began?

"How time began" is ambiguous. A more precise question would be whether models like the idealized FRW spacetime with a "beginning" in time (an initial singularity, more precisely) describe (in a suitable approximation) our actual universe. The answer to that is that we don't know for sure; it's possible that our universe had an initial singularity in the past, but it's also possible that it didn't and existed for an infinite time in the past.

Is space thought of as infinite or growing?

Infinite, yes, according to our best current model. But it is still possible that space in our universe is finite, but much, much larger than our observable universe.

"Growing" is not really meaningful. Our universe is expanding, and pop science books and articles often describe that as "space expanding", but that description is not really a good one and can be misleading, so it's best not to use it.

is there a concept of a universal center of (all) mass?

Not as far as our universe is concerned, no.

 

[Hiero]

Hi, thank you for your thorough reply.

"How time began" is ambiguous. A more precise question would be whether models like the idealized FRW spacetime with a "beginning" in time (an initial singularity, more precisely) describe (in a suitable approximation) our actual universe. The answer to that is that we don't know for sure; it's possible that our universe had an initial singularity in the past, but it's also possible that it didn't and existed for an infinite time in the past..

(Sorry I don’t know GR yet but) What I really want to know is, in models which do have a beginning (like “FRW”?) how does time begin? In particular, is space itself initially infinite with all matter localized? Or is space itself taken to be expanding from a point too? Is space really any different than the set of distance relations between matter? (But if all matter were hypothetically in a finite volume, would quantum fluctuations still occur beyond that volume?)

As a side question, are there any well accepted eternal models?

Sorry for any vague questions; the concepts are mystifying.

 

[Bandersnatch]

But not naively, it is easily shown by looking at the math that this is not in fact the case, because the very concept of "reference frame" that you are implicitly using is not valid in the spacetime in question. It's only valid in the Minkowski spacetime of SR.

I don't know if it's that naive - it sounds just like comoving coordinates.
@Hiero comoving coordinates can be thought as a particular set of worldlines, observers on which see the universe as homogeneous and isotropic, and of the same age. This gives you a >convenient< frame to measure speeds against, or describe the expansion of the universe in general - hence its use in cosmology - but it's not a >preferred< frame in the relativistic sense.

 

[Bandersnatch]

In particular, is space itself initially infinite with all matter localized?

No, if space is infinite, then there is also infinite amount of matter, spread throughout that infinite space throughout the history of the universe. There's never a point in time when matter coalesces into a point in space, leaving empty space behind - on a contrary, the earlier in time, the more uniformly spread it is.
That uniformity is there with finite space too.

Or is space itself taken to be expanding from a point too?

There is no point to expand from. The only thing that qualifies as a point in that infinite space-time you described in the OP is the temporal point - the beginning. The spatial part always was infinite if it is infinite now.

Is space really any different than the set of distance relations between matter?

No, there's no practical difference. Not in the context of expansion of the universe, at least. Whenever you hear about space expanding, it's equivalent to saying the distances between objects are increasing.

What I really want to know is, in models which do have a beginning (like “FRW”?) how does time begin?

Now, this question seems to be talking about something different than your follow-up questions. In the follow-up you were asking about how the universe looked like at the beginning. This appears to be asking how did the time start, as in what happens that the time starts flowing or has a beginning at all.
The answer would be that in the standard model there is no answer. There is a singularity at t=0, which 1) means that the model doesn't ever reach t=0, so it can't tell you what happens there (the temporal boundary is not a part of the space-time) 2) suggests that the model stops being valid sometime before that, and one would need some additional physics to either extend the temporal scale or describe the singularity region.
From what I read, in the second category there is e.g. the Hartle-Hawking theorem, which attempts to eliminate the singularity (time stops being distinguishable from spatial dimensions at some point in the past - but it's beyond my ability to understand in more detail). In the former category, there is e.g. inflation or bounce cosmology, both of which are in some sense eternal.

 

[PeterDonis]

it sounds just like comoving coordinates

Comoving coordinates in FRW spacetime are not the same as Minkowski coordinates in Minkowski spacetime. And of course neither one gives a "universal rest frame".

 

[Hiero]

Thanks for the replies.

The answer would be that in the standard model there is no answer. There is a singularity at t=0, which 1) means that the model doesn't ever reach t=0, so it can't tell you what happens there (the temporal boundary is not a part of the space-time) 2) suggests that the model stops being valid sometime before that, and one would need some additional physics to either extend the temporal scale or describe the singularity region.
From what I read, in the second category there is e.g. the Hartle-Hawking theorem, which attempts to eliminate the singularity (time stops being distinguishable from spatial dimensions at some point in the past - but it's beyond my ability to understand in more detail). In the former category, there is e.g. inflation or bounce cosmology, both of which are in some sense eternal.

This is fascinating. I look forward to learning GR quantitatively.

Anyone, please free to continue the discussions, but I will postpone my questions until I know more math.

 

[Ibix]

The way I look at co-moving coordinates is that they are physically significant in the same way the local rest frame of the surface of the Earth is physically significant: the only thing special about it is that more or less everything of interest is stationary(ish) in it, but some things (the age of the universe, the maximum speed of your car) only have simple meanings in such frames. But you aren't obligated to use it, and physical laws are no different if you use any other.

 

[Khashishi]

Naively, there would be only one (class of) reference frame(s with the same velocity) in which time starts simultaneously everywhere.

The age of the universe certainly depends on the reference frame, but it's probably not sensical to think of time as something that can start at a certain time. In the big bang model, everything in our observable universe goes to zero volume at some point in spacetime. Since it's just a single point, everything is trivially simultaneous regardless of any Lorentz boost to the coordinates.

Just keep in mind that the big bang model is almost certainly incomplete, and something weird must happen as we get close to the singular point. If the weird stuff happened over some finite volume, then it also happened under some finite time, depending on coordinates.