Time Before Big Bang: Is Time Eternal?

[revo74]

You often hear that it makes no sense to talk about time before the Big Bang because time itself did not exist, however, isn't it possible and perhaps even likely that only the time that is an inherent property of our Universe did not exists before the BB, but time in another physical realm may have? Aren't we making an unwarranted assumption when we say time (in general) did not exist before the BB?

 

[Simon Bridge]

The other time-like dimension would have to come into being somehow, and then what happened before that time-like dimension came into being?
We cannot talk about a meta-time before the meta-time dimension ... but maybe there is a meta-meta time and so on ad infinitum?

Even if this meta-time did not need to come into existence (it is eternal say) there are still no space dimensions for stuff to happen in that would make having the meta-time useful.

Most of the current cosmogonies try to avoid this sort of thing:
https://www.physicsforums.com/showthread.php?p=3605530

 

[Drakkith]

There is nothing in science that says it is impossible that something existed prior to the Big Bang. It is simply that our standard model cannot look beyond that. There are other models that don't break down past that point, they simply don't have enough evidence to be accepted yet.

 

[Simon Bridge]

OP explicitly assumes a model of the big bang where time does not exist beforehand and then speculates about how one may talk about "before" when there is no time to have a "before" in - for instance, if there were another time-like dimension.

I doubt the assumption [no time before the big bang] is scientifically "unwarranted".
However, I also suspect that "big bang" may be a tad imprecise a term for this discussion. It is commonly takes as the beginning of the Universe - with nothing "before", though others may use it to mean the particular period of rapid expansion in the early Universe.

The no-boundary and tunneling proposals don't break down - but the concept of "before" requires a topological trick. Usually these models are worked out in configuration space rather than space-time right?

 

[Chronos]

Under GR, time and space are emergent properties of the gravitational field. If you could turn off gravity, time and space would cease to exist. See http://www.astronomycafe.net/gravity/gravity.html for discussion. The is no evidence of gravity prior to the BB

 

[Simon Bridge]

Or the other way around - GR treats gravity as curvature in space-time but it is mathematically ambiguous which gives rise to what.

Some non-copenhagen quantum interpretations treat classical physics, like gravity, as emergent behavior rather than an average behavior.

Ive found an accessible lecture, from Hawking as it happens, describing his kind of ideas about the beginning of time:
http://www.hawking.org.uk/index.php/lectures/62 [Broken]

I've been looking for a similar level talk about the tunneling model.

 

[skydivephil]

As I understand it,the evidence is clear that the universe was in a very hot dense 13.7 billion years. If you assume GR can describe the universe at the smallest scales then time may stop around this time.
NOW THE CAVEATS:
We have to assume GR is good to these tiny scales. Gr has never been tested down to these tiny scales and so one might argue there is no good evidence for the conclusion of no time before the big bang.

Other quanutm gravity models for example, Loop quanutm cosmology predict there was time before the big bang. see for example http://arxiv.org/abs/0812.4703

Another assumption one has to make is inflation is not eternal, accoprding to Guth most models of inflation imply its eternal, which implies our big bang is only one of many and there's no reason to think ours is the first. See:http://arxiv.org/abs/hep-th/0702178

Its also possible to construct models that have no inflation and no quantum gravity corrections that still imply our big bang is one of many eg Penropse CCC: http://en.wikipedia.org/wiki/Conformal_Cyclic_Cosmology

There are many other models too frequent to mention to be honest.

Bottom line: the idea that time began at the big bang is one conjecture out of many, at the moment we don't know if it or alternate models such as quanutm bounce, CCC, eternal inflation, ekprotic etc are correct. So we should keep an open mind.

 

[Chronos]

The 'we do not know' card is always in play. That is the nature of science. We can apply what we do know to make reasonable conclusions. I'm not saying we shouldn't keep an open mind, merely keeping the discussion focused on physics as we currently understand it.

 

[skydivephil]

Agreed, all science has doubt. But there are certian things in physics that are reasonably well understood and there are others which are not . The behaviour of the universe at the so called sinugularity is without doubt one of the things that is not well understood and I think many times the public is often misled into thinking that it is.