Did the universe Absolutely begin?

[houlahound]

No a moment is a precise unit of celestial time. There is a whole thread on it.

 

[phinds]

No a moment is a precise unit of celestial time. There is a whole thread on it.

Oh, I didn't realize it had a definition involving duration. I never think of it that way, but my usage is English, not Physics.

 

[phinds]

That statement is true.

Ah, observable universe, I'm sorry. Yes, it's true then.

Do either of you have a calculation to support this? The only calculation I've ever seen puts it at the size of a grain of sand, so very small but WAY bigger than an atom.

 

[houlahound]

The universe began when humans first observed it - I keed, I keed.

 

[andrewkirk]

It's a simple question: did the universe have a beginning?

It is anything but a simple question. I'll try to briefly explain why. There are two very vague words in the question: 'universe' and 'beginning'.

'Universe', in the context of a physics discussion, is usually taken to refer to this spacetime that we are in. So discussions of that universe beginning are about the earliest times in this spacetime manifold. But, based on its etymological roots, 'universe' means 'everything that exists'. So if there are many spacetimes (sometimes called a 'multiverse') then they are all part of the 'universe' too. Further, if there are other things that are nothing like spacetimes, they are also part of the universe under this definition (see Max Tegmark's 'Mathematical Universe' for an example of this idea taken to extremes). In fact, if there are deities or other beings that people think of as 'supernatural' then they too are part of the universe under that definition.

'Beginning' can also mean different things. For instance, what if time dimensions only exist as features of spacetimes, and there are multiple spacetimes? Then although we may be able to talk about a particular spacetime beginning, we cannot talk about a beginning of the set of all spacetimes, because there is no over-arching time dimension by reference to which we can make sense of the time-bound concept of 'beginning'.

Even when we focus on just one spacetime, the notion of beginning is a problem. We can define a universal coordinate system for the spacetime that has a time dimension, and we can identify a value of that coordinate such that no part of the spacetime has a time coordinate earlier than that. We can even adjust our scale so that that coordinate is time zero. We can then ask the question: is there any part of the spacetime that has that coordinate? Maybe there is, or maybe the times get closer and closer to zero but never reach it. If that is the case, would you count it as the universe having a beginning? Or would you only say the universe had a beginning if there is a part of the spacetime that has time coordinate zero?

Reflect deeply on what your question (which is actually about as far from a simple question as one can get) means, and you may get some insights as to the nature of the possible answers to it, or even whether an answer is even possible in theory (it may not be).

 

[russ_watters]

The answer to this question is, we don't know. We have models in which it did, and models in which it didn't. We don't have enough evidence to distinguish between them.

Maybe I'm just not sure of what "models" you speak, but would it be accurate to say the currently favored theory or collection of theories (BBT) implies a beginning?

 

[russ_watters]

I'm not sure if this was answered succinctly:

The question is not knowing what was "before" (because time would also have begun), but if at some "moment" there was no universe.

This question is - as far as we know - a self contradiction, since as you correctly point out "time" is a component of the universe and thus there can be no time (no "moment") - in the sense we know it - without it.

 

[houlahound]

If the universe means existence then there was no time existing before existence existed.

If the universe is the spacetime we inhabit then clearly it had a beginning according to all evidence.

 

[houlahound]

The CMB, Hubble shift, baryon Genesis, formation of galaxies etc all indicate this observed universe and its physics had a beginning.

Anything about any other universes I have no information on.

 

[PeterDonis]

would it be accurate to say the currently favored theory or collection of theories (BBT) implies a beginning?

I don't think so, because the current well-confirmed theory, the standard hot big bang model, does not go back further than the end of inflation (or what is assumed to be the end of inflation--the hot, dense, rapidly expanding state that is the earliest state for which we have both reasonable evidence and a well-accepted model). But the model explicitly recognizes that that state is not a "beginning"--something came before it. The open question is whether the something that came before it includes a genuine beginning or not. Some models do, as in the idealized FRW spacetime with no inflation (which has an initial singularity by the standard Hawking-Penrose singularity theorems), the original inflation model, or proposals like Hawking's "no boundary", i.e., a model in which there is a genuine finite "earliest time". Other models don't, as in eternal inflation models, bounce models, and probably others that I can't think of right now. I don't think we can say at this point which of those two types of models is going to win out; the pendulum seems to swing back and forth every few years or so and I don't see it settling yet. Others who are more knowledgeable about the latest research might be able to give better insight on that.

 

[nikkkom]

The CMB, Hubble shift, baryon Genesis, formation of galaxies etc all indicate this observed universe and its physics had a beginning.

No, it is not. These observed facts indicate that Universe was much denser, hotter, and more rapidly expanding 13.5 billion years ago.

They do not _prove_ that there was a "beginning" (i.e. that all timelike curves can only be traced back in time by a _finite_ amount). The "beginning" is merely a naive extrapolation of expansion back in time, which does not consider the very likely possibility that currently unknown physics appears at high densities and energies.

If you measure the strength and direction of Voyager 1 signals in many places across Solar system, the naive extrapolation of these observations is that there is an infinitely small, point source of radio signals approx. 137 AU from Sun; and since it's a point, it has an infinite power density, infinite EM field strength at the source, etc.

Of course, we all know that if you come really close to the Voyager 1, you'd see that the source of radio signals is not infinitely small. It's a completely normal object. No infinities.

 

[StandardsGuy]

Did the universe Absolutely begin?

Einstein's theories of relativity and the discovery that the universe appears to be expanding suggest that the universe started at a particular time (which was the beginning of time). However relativity breaks down at the "start" when the density would be infinite, so the beginning is not certain.

 

[PeterDonis]

Einstein's theories of relativity and the discovery that the universe appears to be expanding suggest that the universe started at a particular time (which was the beginning of time).

It would be more accurate to say that they suggested this when they were first discovered. Since then we have found plenty of possible models in which relativity holds and the universe is expanding now, but which do not have a beginning in the sense of a particular time at which the universe started.

However relativity breaks down at the "start" when the density would be infinite, so the beginning is not certain.

The reason we don't know whether there is a beginning is not necessarily tied to the fact that we expect GR to break down at sufficiently high densities. In some of the possible models, the density never approaches the Planck scale, so GR should be applicable, at least at the classical level, throughout the model.