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Does time stretch all the way back for infinity or was there a point when time appears to start in the universe?

I remember reading long ago somewhere that according to one theory time began shortly before the creation of the universe.

Does time have a starting point of note?

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I'm tempted to label this more as a philosophy/metaphysics question than physics, but let's see what others thing first. Suffice to say, the Big Bang theory implies time came into existences at the moment of the singularity of the cosmos (t = 0); other theories predict very different things. –  Noldorin Dec 29 '10 at 17:15
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I'm curious about this answer from a physics point of view only, but I see your point. Do we know nothing definite about the start of the universe? –  Mark Rogers Dec 29 '10 at 17:16
    
Very little I'm afraid. All accepted physical theories (QFT, GR mainly) break down when you get closer than something like $10^{-34}$ s to the Big Bang singularity. The very fact it's called a singularity means something has gone wrong with theory (quantities blow up to infinity). Still, it's an active area of research, so there's every reason to be curious. :) Still, if you'd like to flesh out your question a bit with the theories/specific aspects you're interested in, it might help. –  Noldorin Dec 29 '10 at 17:25
    
I guess I'm interested in whatever theories are most accepted by the general Physics community, especially if there was one that was accepted above others. –  Mark Rogers Dec 29 '10 at 17:31
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It is not a physical, but metaphysical question. –  Anixx Aug 4 '12 at 4:45

4 Answers 4

up vote 5 down vote accepted

In short, we don't know. There are a few indications that time started at the big bang, or at least it had some form of discontinuity. This might be wrong though.

  • According to General Relativity, there is no such thing as an absolute time. Time is always relative to an observer, without the universe there would be no corresponding concept of time. All observers within the universe would have their clocks "slowed down" the nearer they are to the big bang (nearer in time). At the big bang point, their clock would stop. This said, we know that GR doesn't apply as-is all the way to the Big Bang.

  • Some cosmological theories like CCC predict a series of aeons and some form of cyclic universe. These predict a discontinuity (CCC predicts a conformal scale change) of time at the big bang, and at the end of the universe.


As a side note: people tend to have a special fascination with time. For all we know though, time is only relatively special. From a cosmological point of view the discussion is whether space-time existed. We are pretty sure that it was very very small at some point.

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Actually you first point is not correct. All observers measure proper time, so there is no "slowing down" perceived by the observers as they approach the initial singularity. Time abruptly ends when one reaches the singularity. –  Vagelford Dec 30 '10 at 15:37
    
@Vagelford, clearly nobody would see any difference on his own clock. As a question, wouldn't an observer at infinity see the clocks of other observers slow down? –  Ebenezer Sklivvze Dec 31 '10 at 9:45
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Basically, there are no stationary observer at infinity in an FRW geometry, so you don't have the effect that you have in the gravitational field of a spherical body, where different stationary observers at different radii measure different time rates. If you assume two observers that are following the Hubble flow that are on different positions on the same spatial slice, then they measure the same time (proper time), so they would agree on the age of the universe. That means that there is a global notion of time in FRW. Of course there is a redshift but it is because of the $\dot{a}/a$. –  Vagelford Dec 31 '10 at 13:14
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to "We are pretty sure that it was very very small at some point" : I thought, that probably the universe is and was since the big bang infinite. –  jjcale Aug 29 '12 at 19:23
    
The material in the second bullet point is also not right. The analogy between CCC and other cyclic cosmologies really doesn't work at all; they are qualitatively different. Cyclical cosmologies in pure GR were ruled out by Tolman in 1934 on thermodynamic grounds (R.C. Tolman, Relativity, Thermodynamics, and Cosmology), and also can't apply to our universe due to the Hawking singularity theorem. In any case, such models don't predict a discontinuity in time. CCC is completely different. Although it may have been of interest in 2010, when this answer was written, it's no longer viable in 2013. –  Ben Crowell Aug 30 '13 at 19:00

The only well tested theory of gravity we have right now is general relativity (GR). In models based on GR, time and space only exist for $t>0$. In relativity, we use the term "event" to mean a certain position in space at a certain time. The big bang is not an event, because there is no time $t=0$. If you want to find a cause for some event happening at a given time $t>0$, there is always some earlier $t'$, with $0<t'<t$, that can supply that cause. Since the big bang isn't an event, it doesn't have a cause.

We also have fundamental reasons to believe that GR lacks self-consistency under the very dense and hot conditions at $t \lesssim 10^{-43}$ s (known as the Planck time), because of quantum-mechanical effects. If we had a theory of quantum gravity that worked under those conditions, then it might turn out that the singularity at $t=0$ was not real, and events at $t>0$ could be explained in terms of causes at $t<0$. This is what seems to happen, for example, in loop quantum cosmology. However, nobody has a theory of quantum gravity that works and has been tested against experiment, so we don't really know.

"I remember reading long ago somewhere that according to one theory time began shortly before the creation of the universe." I don't think there is any professionally researched scientific theory that says this. Theories that have time before the Big Bang generally do not have a beginning to time at all.

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There is a simple answer: time is just a way of labelling different configurations of matter and/or space with a number. Therefore, there is no time without matter or space, or without change in matter/space.

In other words, for there to be a concept of time, something has to be able to exist in different states. Imagine there was nothing but one solid sphere sitting in space, never changing (and no observer). There is no time in such a situation. The big bang singularity would be even more drastically of this sort.

(Meta-physically, some form of many-worlds theory could eliminate any autonomous meaning of time altogether, by assuming that all the different configurations of matter/space are 'there' somehow, and time just means going through them in a certain order.)

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Technically if there was no change ocurring in the 'solid sphere' there never would have been a big bang. You cannot pass from no change to change without a physical process ocurring. –  eJunior Aug 20 '13 at 22:13

Really, the question does boil down to metaphysics if taken out of the mathematical construct. It's a matter of simple metaphysical logical consistency. Time cannot have a beginning because time cannot, by definition, be a function of NO time. But it cannot be eternal (without beginning or end) either because any value of time given to an object on the timeline must logically be infinity. Also, static time is a contradiction in terms.

I am ceding that this is a metaphysical question like others have done. Down vote is puzzling. I have edited.

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protected by Qmechanic Apr 4 '13 at 7:09

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