# Speed of universe expansion

1. May 5, 2005

### michael879

does anyone know an approximate speed for the expansion of the universe? or if not, how red-shifted planets are? (with the age of universe would give speed). And someone told me its accelerating too. did he mean - acceleration? or is it actually speeding up?

2. May 6, 2005

### kleinwolf

According to Hubble's phenomenological law (a posteriori of exp. datas), the speed of something x away from you is v=Hx...where H is the Hubble constant....so the farther you are, the fastest it seems you go away...but I don't know numerical datas.

From this law you see the acceleration is exponential (it's just first order diff. equ)...but there are other cosmological models....

3. May 6, 2005

### michael879

ok what Im rly trying to ask is, is the universe expanding anywhere near the speed of light? Im talking about radial expansion, not surface expansion.

4. May 7, 2005

### yogi

That is the simple extrapolation of the Hubble's law - at the radius of the Hubble sphere centered on the observer, the recession velocity will be c.

5. May 7, 2005

### Nereid

Staff Emeritus
Ned Wright has a very useful calculator ... you can plug in your favourite parameters, and get some interesting numbers, from which you can calculate an answer to your question. (Caution: be sure to understand what you are actually asking/trying to find out!)

6. May 7, 2005

### Chronos

Hehe, why ruin the experience? There is something to be said for enjoying a moment of slack jawed amazement before attempting to understand it.

7. May 7, 2005

### DrChinese

The universe is 13.7 billion years old. It's radius is currently estimated at 78 billion lights years, i.e. a diameter of 156 billion LY. So yes, it is expanding much faster than the speed of light. Try this reference:

http://www.astro.columbia.edu/~dave/papers/lineweaver.pdf [Broken]

Last edited by a moderator: May 2, 2017
8. May 8, 2005

### meteor

After the results obtained by Cornish, in this paper,

http://arxiv.org/abs/astro-ph/0310233

he concluded that the observations excluded the possibility of a Universe with a diameter smaller than 78 Gly, but then the popular article that appeared in various webs, for example this, http://www.space.com/scienceastronomy/mystery_monday_040524.html
got the information messed up and proclamed that we live in an Universe that is at least 156 Gly wide. This is wrong, and the radius of the observable Universe is approximately 46 Gly. Read this thread where hellfire clear matters
https://www.physicsforums.com/archive/topic/t-63386_Boarder_of_universe.html

Last edited by a moderator: May 2, 2017
9. May 13, 2005

### michael879

we cant observe the whole universe though right? or else we'd be able to see our own galaxy. So how do we know the radius of the actual universe?

10. May 13, 2005

### Mike2

I believe that the assumptions of homogeneity and isotropy allow us to extropolate on what we do observe.

11. May 13, 2005

### Phobos

Staff Emeritus
The Hubble Constant is about 71 kps/Mpc (kilometers per second per megaparsec) plus or minus 5%. So something 1 megaparsec away from us appears to be expanding away from us at 71 kps and something 2 Mpc away appears to be expanding away at 142 kps, etc...

Right now, the universe is not expanding faster than the speed of light (71 kps is much less than the speed of light which is 300,000 kps), but very distant objects can seem like they are from our perspective (actually, we cannot see them because the light can't reach us due to all the expanding space in between).

I assume you mean 'galaxies', not 'planets'. The planets we can see are not expanding away from us. The expansion of the universe occurs on intergalactic scales, not interplanetary.

Yep, it's a recent discovery that the the expansion rate of the universe is increasing. It's a mystery why it's happening (caused by an unidentified 'dark energy'...where 'dark' means something we don't see, not something evil of course).

12. May 13, 2005

### Phobos

Staff Emeritus
Some would say the universe is infinite (which therefore limits our discussion of a 'radius' on the 'visible universe', i.e., the part we can see based on the speed of light, the age of the universe, and the expansion rate of the universe.)

13. May 13, 2005

### yogi

Phobos in your post 11, I think your statement regarding the 71kps must be multiplied by the size R (approx 1.5 x 10^26 meters) after you convert from parsec. For H equal to approximately 2 x 10^-18/sec and R approximately 10^26 then v = 3 x 10^8 meteres

Last edited: May 13, 2005
14. May 15, 2009

### eha

most distant galaxies are moving away with about 19 times faster than speed of light as a part of expansion of universe. in my calculation, hubble constant is 74kps per megaparsec, and we have 78 000 of these megaparsecs, since we agree the radius is 78Gly, am i right or wrong?

15. May 15, 2009

### RUTA

It looks like you forgot to convert light-years (cy) to parsecs, 3.26cy = 1pc, as I recall. So the distance to the particle horizon is ~24,000 Mpc and the speed of objects on the particle horizon would be ~6c ... with these numbers anyway.

16. May 15, 2009

### marcus

As Ruta indicates, it depends on what numbers you use. There is no one fixed speed of expansion, because the rate depends on what horizon distance you choose.

I guess one could argue that the natural choice is the particle horizon---at an estimated 46 billion lightyears.

But another good distance mark would be the surface of last scattering, where the matter is that emitted the Background radiation that we are now receiving. The usual estimate is that the Background has redshift z = 1090.

That is not quite to the particle horizon, but it is not too far from it, since the CMB light was emitted (380,000 years) less than a million years into expansion. The last scatter surface is estimated at roughly 45.5 billion lightyears. 45 point something, near 46.

You can just plug z = 1090 into Morgan's Cosmos Calculator and get the recession rate.
Google "cosmos calculator", put in .27, .73, 71 or what ever the more up-to-date parameters are.
When I use .27, .73, 71 I get that the current distance is 45.5 and the current recession rate is 3.3 c.
Also that the recession speed of that matter when the light was emitted was 56.7 c.

That all makes sense, and is what we're used to, but now with a new Hubble rate estimate of 74, instead of 71, there will be some changes.
By a few percent, but it will take a little while to settle down. The particle horizon distance will be recalculated, likewise the distance to last scatter.
Probably the CMB redshift will stay the same.

Last edited: May 15, 2009
17. May 16, 2009

### Chronos

The cosmic calculator is still viable, just plug in the new parameters.

18. May 25, 2009

### John37

no one knows we change our minds every 10 years or so!

19. May 28, 2009

### Pangaea1

20. May 30, 2009

### yogi

Harrison at page 299 gives a derived velocity of the Hubble sphere as c(1+q). Estimates for the rate of expansion of the observable universe follow from the on-line calculators - but the numbers produced by these calculators would seem to be sharply sensitive to the model used to estimate the size of the cosmic surface at last scattering -