Expanding Universe

[Deset Gled]

Well then that brings up the speed of light limit.

Since you need to be more than ten thousand light years apart before the expansion speed becomes (relativistically) meaningful, you'll be dealing with a lot of relativity issues in space travel well before the rate of expansion ever plays a significant factor in your plans. At that point, keeping track of relativistic effects of Hubble's Law would essentially be standard intergalactic space travel bookkeeping.

Edit: Gahh, I am much to late the game on this one.

 

[Umbran]

Since you need to be more than ten thousand light years apart before the expansion speed becomes (relativistically) meaningful...

I think you've slipped some factors of ten, there. Ten thousand light years won't even get you outside this galaxy, much less be spanning intergalactic distances.

"Relativistic speeds," where the effects of relativity start to become meaningful in human terms, is usually taken to be about 0.1c.

Hubble's Law is v=H*D. Where v is the "speed", H is the Hubble constant (roughly 68 km/s/Mpc*), and D is the "proper distance" to the object in question.

End result, if I have my quick math right, is that before expansion starts yielding relativistic speeds in human terms, we need to be considering something that's 10^10 light years away - that's ten billion light years. Not ten thousand.



*The speed will be roughly 68 kilometers per second for every megaparsec between the points.

 

[Mustrum_Ridcully]

I found the "Big Rip" an interesting concept - one possible end of the Universe (possible in that we don't know enough about the expansion process - at least not at the time the theory was formed - to say if this could or could not happen. I think today we know this will not happen, barring any big surprises) - at some point the expansion would accelerate so fast that it would split apart galaxies, star systems and eventually even atoms.

 

[freyar]

I find it very cool that EN World has this sort of discussion, so, as always, props to Morrus and the mods for supporting it (especially to Umbran who always has very reasonable answers). I just wish I had more time to post right now, so I could participate more. Anyway, most of my work relates to cosmology, and I like talking about this stuff.

Most of this conversation is pretty spot-on, though I'd offer some extra words on a few points.

Dark energy isn't precisely responsible for the expansion of the universe. You could, for example, have a universe without any dark energy that expands perfectly well. What is true is that, in our universe (as opposed to some other hypothetical one), dark energy is responsible for the expansion speeding up. If you replaced all the dark energy in our universe with matter, for example, the universe would still be expanding but would be slowing down. As for the far future, JediSoth is right, we don't really know, though there are some big experiments trying to get information to find out. IMO, the best explanation is that the dark energy is a cosmological constant, so, in the far future, our galaxy (well, after it has merged with nearby ones) would be all alone, with its stars slowly going out. However, it is possible, say, that space will actually start to expand so fast that it rips itself apart in a finite time (this is called a Big Rip). I think there are some very strong criticisms of the physics behind models like that, though.

There's also the whole bit about Hubble's law and the speed of light. As Morrus and Umbran have said, the expansion of the universe is an effect of Einstein's general relativity (gravity), and it is really the expansion of space between objects, not the motion of objects through space. So the speed limit doesn't apply. But even if you forgot about that and wanted to think of it as a speed, Einstein's special relativity keeps the speed below the speed of light. Umbran's formula up there is actually the nonrelativistic version of Hubble's law. What we really measure is the "redshift," or amount that the color of another galaxy changes. It's really that "redshift" = H*d/c, and redshift is related to speed by "redshift"=square root of [(c+v)/(c-v)]. You can see from that formula that you can make redshift as big as you want without speed v ever getting bigger than the speed of light c.

 

[Umbran]

I found the "Big Rip" an interesting concept - one possible end of the Universe (possible in that we don't know enough about the expansion process - at least not at the time the theory was formed - to say if this could or could not happen. I think today we know this will not happen, barring any big surprises) - at some point the expansion would accelerate so fast that it would split apart galaxies, star systems and eventually even atoms.

My understanding is that today we don't know whether or not this will happen. It depends upon the nature of the dark energy that is causing the acceleration of expansion, and we've not observed enough about it to tell if it will lead to this end or not.

 

[freyar]

Mustrum ninja'ed me there on the Big Rip. I do think the basic version of the Big Rip are ruled out, but I feel like it's not all of them. I didn't find most of them that terribly sensible anyway, but that's really just my opinion.

EDIT: Went back and looked this up. A couple of papers do claim to rule out Big Rips at some level of confidence, but not most of them.

 

[El Mahdi]

deleted

 

[Umbran]

Umbran's formula up there is actually the nonrelativistic version of Hubble's law.

Yes. I applied Hawking's basic posit on science writing*, and found the non-relativistic version was close enough for government work, so to speak.

Mustrum ninja'ed me there on the Big Rip. I do think the basic version of the Big Rip are ruled out, but I feel like it's not all of them. I didn't find most of them that terribly sensible anyway, but that's really just my opinion.

The basic version of which I'm aware (which hinges on the ratio between dark energy pressure and dark energy density) I thought was still up in the air. My understanding is that experimental error is still too large to tell if the ratio in our universe was less than, equal to, or greater than -1.

In this version of the "big rip" it is not that space actually rips, in some Doctor Whovian sense. But the expansion accelerates to such a degree that distant objects keep falling outside the observable universe. First it is distant galaxies, then galaxies in the Local Group, then other stars in the Milky Way. Eventually, you get to the point where the observable universe cannot contain a single molecule or atom, and the forces that hold matter together can no longer do so. The universe doesn't really end, or rip, but everything in it is reduced to a fine mist of particles (eventually photons, which may have no size, per se) that cannot interact, because the distances between them are too great.


*It didn't really originate with Hawking, but he popularized it to modern science writers - for every equation you include, you lose half your audience.

 

[freyar]

Yes. I applied Hawking's basic posit on science writing*, and found the non-relativistic version was close enough for government work, so to speak.

The problem in this particular case is that Hubble's law in that form can't address the question of high speeds, so I always like to note that redshift isn't proportional to velocity at high speed. Just a matter of taste, in some ways, since neither is really correct.

The basic version of which I'm aware (which hinges on the ratio between dark energy pressure and dark energy density) I thought was still up in the air. My understanding is that experimental error is still too large to tell if the ratio in our universe was less than, equal to, or greater than -1.

Yes, you're right about that. My memory is a bit fuzzy on the Big Rip papers as I don't usually follow it in much detail. There have been some papers putting the Big Rip scenario out of reach, but I think my memory may just be placing too much importance on them. In any case, I just went and looked up what I'd consider the best synopsis of current data (from the Planck cosmic microwave background mission), which would allow a Big Rip but also a cosmological constant.

In this version of the "big rip" it is not that space actually rips, in some Doctor Whovian sense. But the expansion accelerates to such a degree that distant objects keep falling outside the observable universe. First it is distant galaxies, then galaxies in the Local Group, then other stars in the Milky Way. Eventually, you get to the point where the observable universe cannot contain a single molecule or atom, and the forces that hold matter together can no longer do so. The universe doesn't really end, or rip, but everything in it is reduced to a fine mist of particles (eventually photons, which may have no size, per se) that cannot interact, because the distances between them are too great.

Well, there is a legitimate metric singularity in what I've read. If that's not space ripping, I'm not sure what is. Everything being pulled apart does happen before that, though.

 

[tomBitonti]

My understanding is that the rate of expansion appears to be increasing?

But, I have always had the same question: At what scale does expansion occur? Uniform expansion implies odd effects in bound systems. On the other hand, non-uniform expansion seems to imply curvature between the expanding regions and the non-expanding regions.

Also, the scale at which gravity still has an effect, say, for a galaxy, is quite large, and never quite goes away - it's just overwhelmed at a distance by the fields from other masses. I don't understand how there wouldn't be an an expansion within some bound system.

Thx!

TomB