# The speed of light and the expansion of the universe

1. Jan 29, 2007

### Robert J. Grave

Can galaxies move faster than light, the speed limit, do to expansion of the universe?

2. Jan 29, 2007

### hellfire

They are allowed to recede faster than $c$ as for example in the standard cosmological model for galaxies located at higher redshifts than z > 1.4. The recession is not a relative velocity but a change of the geometry of space, and galaxies are at rest in the comoving coordinate system from which the universe is observed as homogeneous and isotropic. In special relativity, the speed limit of $c$ arises for motion of object relative to each other in a space with static geometry. This means also that the cosmological redshift is not a Doppler effect due to relative motion but a pure general relativistic effect due to stretching of space.

3. Jan 29, 2007

### Robert J. Grave

Thanks hellfire, galaxies moving within a changing space geometry makes sence. I can put that question to rest now.
Regarding my post about conservation, you are correct, we really do not
understand the physics at or before the big bang. My question is, if it did come from nothing, wouldn't that violate the conservation of mass/energy?
I personally would need strong evidence to accept the universe came about from nothing.

4. Jan 29, 2007

### marcus

The latest version of Big Bang theory does not have the universe coming from nothing.

there was just a 3-week workshop about this at a UC Santa Barbara institute called KITP.
The workshop was called "The Quantum Nature of Spacetime Singularities".

http://online.kitp.ucsb.edu/online/singular_m07/

World-class scientists from Europe Canada Latin America and of course the USA attended and presented papers. A lot of it is online---you can download movies of the talks.

I did not hear anybody making the supposition that the universe "came from nothing". Maybe one or two of them did (I did not watch all of the talks) but they were probably a small minority. I gather that "came from nothing" (whatever that means) is not a fashionable view among the people actually doing the research these days. The effort is to get a handle on what was going on immediately before the beginning of expansion, and there are a bunch of competing ideas.

Last edited: Jan 29, 2007
5. Jan 29, 2007

### complexPHILOSOPHY

Two popular theories, which have strong mathematical and (perhaps mostly indirect) empirical evidence, regarding the ‘big bang’ is the oscillating universe and cosmological inflationary theory, both of which avoid the ‘something out of nothing’ postulate.

If you are interested in a more abstract, philosophical discussion of causal relations and the nature of the inception of the universe, engage me in some dialogue, my friend.

6. Jan 29, 2007

### hellfire

I know from Vilenkin to have said that the universe came from nothing, but within the realm of his quantum cosmology proposal he means a vanishing (spatial) three-geometry $g_{ij} = 0$, as you can read for example below equation (8) in http://arxiv.org/gr-qc/9812027 [Broken]. This does not actually mean nothing at all, but of course something that is far from our usual notions of space and time. Probably, the wrong usage here of the term "nothing" was exported to popular articles leading to some confusion. More refined theories of quantum cosmology go even further and assume different classical regimes beyond the classical singularity (see for example the link marcus has provided or brane cosmology).

Last edited by a moderator: May 2, 2017
7. Jan 29, 2007

### Robert J. Grave

I, for one, am glad to here of the abandoment from the idea of something from nothing as I was never able to accept it. The oscillating universe makes more sence, for one idea. Watching the Hawking movie of his early days, He came up with a time direction reversal taking a black hole and reversing time making it explode back into normal matter/energy, the big bang?

Last edited: Jan 29, 2007