# Expansion of Space

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#### Ishimura_

##### Guest
Is there a formula that tells us how fast space is expanding? For example, assume we determine via several measurements that a star is 1000 ly away. Does this distance take into account that space is expanding for the years that the light has taken to reach us? And is the star currently 1000 ly away or is it something different now? I'm visualizing a thought experiment where I want to plot a course to that star travelling very close to c. I realize that the earth, the star and my space ship are all moving relative to one another. To plot this hypothetical course, would I need to factor in the expansion of space too?

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#### SpeedFreek

##### Guest
The rate of expansion averages out at around 74 km/s per 3,262,000 light-years.

But this only applies across scales larger than our local cluster of galaxies - if two galaxies are orbiting each other, bound by gravity, then the space in between them does not increase. Not does the space in between stars in galaxies increase. It is the distance between highly separated clusters of galaxies that increases.

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#### Couerl

##### Guest
Hi Speedfreek,

Is it that the space between relatively close galaxies (as well as space inside galaxies) is not expanding as well period? Or that the expansion of space between relatively close or orbiting galaxies appears to be less than 74km/s because of gravitational influence? or none of the above...

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#### ramparts

##### Guest
No expansion, period, if they're gravitationally bound.

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#### Gravity_Ray

##### Guest
the value of 74km/s sounds accurate but its only a guess based on Cepheid variables (young and massive stars whose luminosity change periodically), supernovae (explosion of a star at the end of its life) and globular clusters (dense groups of old stars found in the centre of galaxies).. Although its a pretty good guess, I doubt its real (or should I say releative). The newest hubble constant is around 72+/-8 km/s/Mpc (Mpc is Megaparsec) but I think there is a strong case that it is between 50-100 km/s/Mpc (maybe as low as 35 km/s/Mpc)

So nobody is really sure yet.

@ OP, this is a great question to show exactly how little we know about the way the universe is working. If you are planning to go between two stars your speed will have a lot to do with plotting a course with your space ship.

@ Couerl, the reason they give the hubble constant relative to a megaparsec (around 3.2 million light years) is because this value is sort of reached with an "empty" universe. If you add everything in, you get strange stuff like some stars are older than the universe and other such nonsense. remember space isnt just space, but space-time.

All I know is that I dont know.

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#### SpeedFreek

##### Guest
Gravity_Ray":2t6lqk0i said:
the value of 74km/s sounds accurate but its only a guess based on Cepheid variables (young and massive stars whose luminosity change periodically), supernovae (explosion of a star at the end of its life) and globular clusters (dense groups of old stars found in the centre of galaxies).. Although its a pretty good guess, I doubt its real (or should I say releative). The newest hubble constant is around 72+/-8 km/s/Mpc (Mpc is Megaparsec) but I think there is a strong case that it is between 50-100 km/s/Mpc (maybe as low as 35 km/s/Mpc)

I was using the newest Hubble constant of 74.2 km/sec/megaparsec, from last year.

http://hubblesite.org/newscenter/archiv ... 9/08/full/

I doubt we will have to take the expansion of the universe into account with any interstellar journey we undertake in the foreseeable future.

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#### Couerl

##### Guest
Thank you, that answers my question/s.

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#### darkmatter4brains

##### Guest
Ishimura_":2bvvs122 said:
For example, assume we determine via several measurements that a star is 1000 ly away. Does this distance take into account that space is expanding for the years that the light has taken to reach us?

Yes, this is taken into account with two different distances. The one we're all familar with, which happens to be called proper distance, doesn't take into account the expansion of space, etc. Then there is another called the comoving distance and it does take into account the fact that space is expanding. The further away an object is the larger the difference between this comoving distance and the regular distance we're all used too. Obviously the comoving distance is larger. For objects that are close to us (on galactic scales!) the difference is hardly worth mentioning though. Here's a wikipedia article on the two, although its not the best write up I've seen:

http://en.wikipedia.org/wiki/Comoving_distance

EDIT: Just remember where I saw a great write up on this: Astronomy Magazine. Maybe it could be even be found on their web page?

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#### Couerl

##### Guest
Hi and one last question.

Given that we know the expansion is 74.2 km/s/mpc and accelerating, is there a plot handy that shows what speed that might be in say, 10 m yrs, 100 m, 1/2 billion yrs etc? In short, a billion years from now will we know what that rate of expansion will be or is it too unpredictable to extrapolate?

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#### ramparts

##### Guest
We have ideas but don't know for sure. The best guess is that it's an exponential function, so try plotting e^x and you'll see what the future holds

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#### Ishimura_

##### Guest
I'm curious now, how did the measure this acceleration? 74km/s seems very small considering the distance of a megaparsec. I don't know what the earth equivalent would be... maybe measuring the speed of an ant across the ground from orbit

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#### ramparts

##### Guest
Pretty simple, really, just measure the distances and recessional speeds (that is, how fast they seem to be moving away) for different galaxies - the expansion rate (called the Hubble constant) is just the ratio of those two

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#### MeteorWayne

##### Guest
There are refined values from both the HST and WMAP. The new HST value (including Cepheids in even more distant galaxies) is 74.2 +/- 3.6, the WMAP value is 70.4 +/- 1.4. So the overlap between them is 70.6 to 71.8 km/s/megaparsec.

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