Time Dilation & big bang Question

[Garth]

The redshifts can be explained with relativity without the need to conjure up stretching of light cones. Why do you have to stretch space-time?

The redshifts can be explained with relativity without the need to conjure up stretching of light cones. Why do you have to stretch space-time?

I presume by “relativity” you mean SR; the reason we talk about the ‘stretching’ of space-time is because that is the prediction of GR in the Robertson-Walker metric. Of course you could do away with GR and use a SR modified perhaps with a Newtonian scalar to explain gravitational forces, however such attempts are internally inconsistent (MTW pg181-6). The fact that GR ‘works’ accurately as a theory in solar system experiments gives us confidence to apply it cosmologically where it predicts cosmological red shift as a result of a space-like “stretching” of space-time or cosmic expansion.

If you look at some of my other posts you will realize that I am a maverick and personally think GR falls short in some areas of cosmology, as it requires inflation, exotic dark matter and dark energy to explain cosmological observations - none of which has yet been discovered or verified in laboratory physics even after about thirty years of intensive research, but the ‘stretching’ of space-time is not one of them.

Garth

 

[Wave's_Hand_Particle]

And I don't disagree with anything you have said except the last sentence (above). It appears to me that the expansion of space-time is a mathematical construct introduced to explain certain features of the current universe that GR mathematical solutions do not explain. There is no empirical evidence to support it, yet. While it may solve these problems mathematically, the theory appears to offer no physical explanation for space-time expansion.

I am not so concerned with the possibility of space-time expansion in the very early stages of the universe (ie. the first few pico seconds of its existence). My concern is with the concept of expanding space-time in the current universe.

Andrew Mason

This may be resolved in the near future.

T. Padmanabhan is about to make some interesting alternate suggestions with respect to these very issue's.

I believe it has to do with TWO paramiters of Constant Expansion, ie Space Expands separate from Space-Time?

I have seen a little on the ideas that all Spacetimes with matter(Galaxies) are thought to be in Contraction, and all of intervening Space external to Spacetimes, is what we actually see as Universe Expansion, but this is quite speculative.

 

[Andrew Mason]

I presume by “relativity” you mean SR; the reason we talk about the ‘stretching’ of space-time is because that is the prediction of GR in the Robertson-Walker metric. Of course you could do away with GR and use a SR modified perhaps with a Newtonian scalar to explain gravitational forces, however such attempts are internally inconsistent (MTW pg181-6). The fact that GR ‘works’ accurately as a theory in solar system experiments gives us confidence to apply it cosmologically where it predicts cosmological red shift as a result of a space-like “stretching” of space-time or cosmic expansion.

Actually I meant both SR and GR. I understand the concept of stretching of space-time due to gravity. That is a corollary to the principle of equivalence. But it is a local phenomenon. What I have problems with is the inflationary stretching of space-time for the entire universe. This is not needed to explain doppler and gravitational redshift.

Andrew Mason

 

[Garth]

Actually I meant both SR and GR. I understand the concept of stretching of space-time due to gravity. That is a corollary to the principle of equivalence. But it is a local phenomenon. What I have problems with is the inflationary stretching of space-time for the entire universe. This is not needed to explain doppler and gravitational redshift.
Andrew Mason

If GR works within the solar system, predicting the precession of Mercury's orbit etc. and is then applied, via the cosmological principle, to the universe as a whole, it predicts the space-like expansion of space-time which would lead to Hubble red shift, primordial nuclearsynthesis and the CMB, all of which have been observed.

Hence the expansion of the space-time of the universe is taken as verified scientific fact.

The going does then get tougher with unverified Inflation, and undiscovered Dark Matter and Dark Energy - but that is another story discussed in these forums elsewhere!

Garth