# Speed of Expansion

1. Apr 13, 2004

### Stonefeather

How fast is the universe expanding? I have heard it is near light speed. If this is the case does it throw a monkey wrench into Einstiens theorys? What I am really asking is if the earth is acctually moveing at near lightspeed say this . is earth. and this 0 is the sun, also say that the univers is expanding this way <- Would not light move faster with the earth in this position .0 than with it in this position 0. ?

2. Apr 13, 2004

### Blistering Peanut

I don't think the universe would be expanding in only one direction but equally in all directions.

Last edited: Apr 13, 2004
3. Apr 13, 2004

### Kurdt

Staff Emeritus
It doesn't throw a spanner in special relativity because with the earth in both positions the light from the sun would be measured as exactly c.

4. Apr 13, 2004

Staff Emeritus
The stars and planets, including sun and earth, are not actually moving through space at the expansion speed. Rather space itself is expanding. The galaxies are getting farther apart without moving. And that increase can go faster than light, and it does. They can measure it using doppler effect on the spectra of distant objects.

5. Apr 13, 2004

### Erring Flatley

The universe may not be expanding. The theory of expantion is based on the red shift observed in the light from distant galaxies. If the red shift is caused by an energy shift from wavelength to photon numbers instead of a Dopler shift, then the universe is not receeding from us, but rather standing in a stationary pattern. This kind of an energy shift could expain the comic microwave background radiation, which is found everywhere.

6. Apr 13, 2004

### Nereid

Staff Emeritus
Welcome to Physics Forums Erring Flatley!

Do you have a reference to this idea that "the red shift [of galaxies] is caused by an energy shift from wavelength to photon numbers instead of a Dopler shift"? Or more details of what this is, including some equations and numbers?

7. Apr 13, 2004

### Mike2

Then wouldn't it appear that we are moving faster than light if we are simply being carried by space as space expands FTL?

What is there to hinder the expansion, especially during inflation? It would seem that there must be some quality of space itself that prevents an infinite expansion rate.

Last edited: Apr 13, 2004
8. Apr 14, 2004

Staff Emeritus
Moving faster than light relative to what? We are in our own rest frame, and it includes much of what we see locally; we aren't speeding much with respect to the nearer stars or the milky way. We have a very small proper motion with respect to the CMB. Study up the Lineweaver site or some other resource on cosmology.

The universe has expanded in the past by size orders of $$10^{30}$$ in a very short time (inflation), amd may well do so again. It's wrong to think of space, or spacetime as a physical membrane.

9. Apr 20, 2004

### Erring Flatley

It is my own idea. It is based on my idea of photon-photon interaction under the unified field equation. Most assume that photons do not interact as they contact each other but they do! Coherence could not occur if they did not. (Please excuse me, I must sign off now, be back latter.) [/color]

10. Apr 20, 2004

### Mike2

If we are still with respect to our local space (that's expanding) and some far away galaxy is still wrt it's local space, then if space itself is expanding FTL, then the two galaxies are speeding away from each other FTL. Although there is probably no such thing as still wrt local space, still the point is made. If there is an expansion of space, then there is a frame relative to that expansion, right?

Is there some topological property where the connectedness of space with time would require a maximum speed of expansion? Would that require a metric? As I've argued before, space must have the topological property of being connected if anything is going to propagate through it. And likewise, time must be connected or there is no propagation through it. So propagation itself implies a connected property of both time and space. But how is time connected to space?

11. Apr 20, 2004

### Mike2

If the universe expanded from a singularity, then to begin with so much space was added with so much time. If each point of spacetime always has this same rate of expansion, then doesn't this imply an exponential growth in the expansion? And isn't this what we are told about inflation? Comments please?

12. Apr 20, 2004

### billy_boy_999

space-time is expanding at the speed of light, this is why light and gravity propogate at such constant, certain speeds...

13. Apr 21, 2004

### Mike2

In other words, the rate is proportional to the size, more points contributing at the same rate, then the more points there are, the faster the overall rate of expansion. Does this make sense?

14. Apr 21, 2004

### Erring Flatley

According to my reading, many decades ago when Hubble first demonstrated the cosmic red shift and gave a rate of expantion, which gave an age to the universe, Hubble's age for the universe was less than the accepted geological age of the Earth. Many astrophysicists proposed a theory called "the tired light" theory, to account for the shift to red. It was of course not a theory based on Dopler shift. But no one was able to find an explanation for light shifting its wavelength other than by Dopler shift. Other astronomers revised Hubble's estimate to be greater than the age of the Earth and with that the tired light hypothesis died. But, my study of the unified field equation would say that a shift from wavelength to photon numbers is possible and that reawakens the tired light hypothesis. The big bang theory rests heavily on the cosmic red shift being Dopler. If I am right that only leaves observations from the Hubble space telescope ultra-deep field photos of galaxies as being "young" in support of the big bang theory. And that is up to personal interpretation with so little seen so far.

15. Apr 21, 2004

### AntiMagicMan

Whenever you look at expanding spacetime you have to use General Relativity. And it is a little known fact that in the General Theory spacetime can expand faster than light.

16. Apr 21, 2004

### Nereid

Staff Emeritus
You may find the
Ned Wright cosmology website of some interest, in particular his discussion of the 'tired light' hypothesis (in a word, observations of distant supernovae are inconsistent with the tired light hypothesis), as well as this page, which summarises the observational results which support the big bang theory (in four words: the hubble expansion, the cosmic microwave background, the primordial abundance of light nuclides, and large-scale structure).

17. Apr 21, 2004

### Mike2

So if that expansion were unhindered by mass or whaterver, is there something in GR that predicts an expansion at an exponential rate?

I'm told GR requires initial conditions to predict how fast expansion takes place. What if the universe started from a singularity in the infinite past (no t=0, no initial conditions)?

18. Apr 22, 2004

Staff Emeritus
First of all, the expansion is not indifferent to mass. If there was sufficient mass the universe would reach a max and then contract. But mass is only a couple of percent of the total energy budget.

The GR explanation for the acceleration is the cosmological constant, that is, the form of Einstein's equations, not just the initial conditions. As Nereid has said, we don't know what the initial conditions were.

19. Sep 7, 2004

### Blueplanetbob

Is the dark energy density moving faster than light speed?

20. Sep 7, 2004

### Nereid

Staff Emeritus
Welcome to Physics Forums Blueplanetbob!

I'm not sure I understand your question ... 'dark energy' is just a shorthand for either a) whatever may be causing the acceleration of the expansion of the universe (i.e. a mystery), or b) one of several ideas that are NOT the cosmological constant (a la Einstein). In the former, your question has no meaning (what is the density of a mystery?); in the latter, 'dark energy density' would probably be a bulk property, so it wouldn't make sense to ask how fast it was moving!