I Time translation symmetry and the Big Bang

[Robin04]

Hi,

As I know we now think that time translation is not a symmetry of spacetime because of the Big Bang, so we cannot say that our physical laws are applicable at every point in time. But then isn't the developing of the Big Bang theory against this asymmetry?

 

[Orodruin]

so we cannot say that our physical laws are applicable at every point in time

Why would you come to this conclusion? It is not correct. General relativity describes most of the expansion of the Universe perfectly well.

 

[Robin04]

Why would you come to this conclusion? It is not correct. General relativity describes most of the expansion of the Universe perfectly well.

But also use for example thermodynamics and particle physics to describe the properties of matter at that time.

 

[Orodruin]

But also use for example thermodynamics and particle physics to describe the properties of matter at that time.

Yes, to a very good approximation. When it comes to particle physics, we often assume that the background is Minkowski space-time when making computations. For most of the evolution of the Universe, this is a very good approximation.

 

[Robin04]

Yes, to a very good approximation. When it comes to particle physics, we often assume that the background is Minkowski space-time when making computations. For most of the evolution of the Universe, this is a very good approximation.

But how do we "know" this? This doens't seem like a simple extrapolation to me because we say that there's no time translation.

 

[Orodruin]

But how do we "know" this? This doens't seem like a simple extrapolation to me because we say that there's no time translation.

It is an assumption, and it turns out to describe observations very well. I do not understand why you think time translation non-invariance breaks this assumption (that the same physical laws should apply to all events in space-time).

 

[Robin04]

I think I missed that this assymetry doesn't necessarily mean that our laws are not applicable but simply it says that we cannot be sure about their range in which they can be used. Problem solved, I think. Thank you for your help. :)

 

[PeterDonis]

this assymetry doesn't necessarily mean that our laws are not applicable but simply it says that we cannot be sure about their range in which they can be used

No, that's not what the asymmetry implies. It implies one of two things: either (1) the laws themselves are time-asymmetric; or (2) the initial conditions of the particular solution of the laws that we live in were time asymmetric. The current mainstream belief is that (2) is the case for our universe: we live in a solution of the laws of physics in which the past is very different from the future (hot, dense, rapidly expanding Big Bang in the past, vs. increasingly dilute matter and radiation in the future). But the same laws still apply everywhere, and they are time symmetric.

The reason this can happen with time-symmetric laws of physics is that solutions come in pairs: there is another solution to the laws of physics in which the "past" looks like our future and the "future" looks like our past--i.e., a universe contracting from highly dilute matter and radiation in just the right way to form a hot, dense, rapidly contracting "Big Crunch" at the end. So the time symmetry of the laws only appears when you look at the full set of solutions; it doesn't appear when you look at just one solution by itself. But the laws still apply everywhere in all solutions.