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Conceivably it expands with the speed of light. I do not know, but curious, if there is an answer. At what velocity, does our universe expand?

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3 Answers 3

up vote 5 down vote accepted

The rate of expansion of the universe (the fact that all objects are receding from each other and more so if they're farther away from each other) is given by the Hubble constant $H_0= 69.32 ± 0.80 (km/s)/Mpc$ 1

Check out this plot from Wikipedia

On the y axis you have the velocity with which the object is receding from us and on the x axis the current distance in a common astrophysical unit called Megaparsec (parallax of one arc-second $1\text{pc} \approx 3.26$ light yrs).


the discrepancy circled in blue is due to the galaxies having additional internal motions on top of their receding due to expansion. The galaxies measured there are (as the label says) part of the virgo cluster. The internal motion will induce a Doppler shift that will influence the overall redshift of the galaxy

1: According 20th Dec 2012 the Hubble constant, as measured by NASA's Wilkinson Microwave Anisotropy Probe (WMAP)

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The Hubble constant. Ok! I found a more accurate value for it $$H_0 \approx (72 \pm 8) \ \frac{\rm km}{\rm s \cdot Mpc}$$ –  Stephan Schielke Aug 11 '11 at 12:13
well 68 is within 1$\sigma$ of 72. –  luksen Aug 11 '11 at 12:18
As far as i can see it is absolutly not clear. This speed is just the current speed and changes by time. Constant may be a bad word for it. It should be called Hubble parameter. –  Stephan Schielke Aug 11 '11 at 12:23
true. it's constant in space (presumably the same value would be measured elsewhere) but not in time. actually oftentimes it is termed the Hubble parameter –  luksen Aug 11 '11 at 12:27
@Stephan Schielke: The Hubble constant is not a speed at all. It doesn't have units of speed. The basic answer to your question is that there is no answer to your question. Space doesn't have a speed at which it expands. More info here: physicsforums.com/showthread.php?t=508610 –  Ben Crowell Aug 11 '11 at 16:19

The recessional velocity is given by Hubble's Law and it's proportional to the proper distance from the observer to the object (galaxy or other distant object):

$$v=H_0 D$$

where $H_0$ is Hubble's constant.

As you can see, the recessional velocity beyond a certain distance, known as the Hubble distance, can be much greater than the speed of light (in vacuum). This does not violate the relativity because the recession is not a movement through the space, it's the expansion of the space itself, between the objects.

For more details check the Wikipedia article: http://en.wikipedia.org/wiki/Hubble%27s_law

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Yes. And that implies the important part of the question, is how big is the universe? How many Hubble distances is it? Is the stuff beyond our spacetime horizon similar, i.e. can we simply extrapolate from what we can see, or could there be interesting variation beyond the horizon. –  Omega Centauri Aug 11 '11 at 15:54
@OmegaCentauri: I believe that question is covered [here]:[physics.stackexchange.com/questions/26613/… –  Argus May 30 '12 at 5:45

Perhaps it should be pointed out here that there is no such thing as "the expansion speed of the universe". The universe does not expand at a single speed. Instead the observed expansion speed depends on the distance, as given by the Hubble constant. That constant is 72 km/sec per Mpc (1 pc is 3.25 lightyears).

What Hubble tells us is that "the expansion speed rises 72 km/sec for every 3.25 million lightyear separation". Two galaxies that are separated by a distance X, measured in megaparsecs, will move apart at a speed of 72X km/sec. For example, if we look at a galaxy that's 100 Mpc (325 Mly) away, we will see it moving away from us at a speed of 7200 km/sec.

This also means that there is no center to this expansion. No part of the universe is standing still, with everything else moving away from it. Instead, everything moves away from everything else.

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