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<blockquote data-quote="freyar" data-source="post: 6132254" data-attributes="member: 40227">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).&nbsp; I just wish I had more time to post right now, so I could participate more. <img src="https://cdn.jsdelivr.net/joypixels/assets/8.0/png/unicode/64/1f642.png" class="smilie smilie--emoji" loading="lazy" width="64" height="64" alt=":)" title="Smile&nbsp; &nbsp; :)"&nbsp; data-smilie="1"data-shortname=":)" />&nbsp; 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.&nbsp; You could, for example, have a universe without any dark energy that expands perfectly well.&nbsp; What is true is that, in our universe (as opposed to some other hypothetical one), dark energy is responsible for the expansion speeding up.&nbsp; 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.&nbsp; 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.&nbsp; 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.&nbsp; 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).&nbsp; 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.&nbsp; 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.&nbsp; So the speed limit doesn't apply.&nbsp; 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.&nbsp; Umbran's formula up there is actually the nonrelativistic version of Hubble's law.&nbsp; What we really measure is the &quot;redshift,&quot; or amount that the color of another galaxy changes.&nbsp; It's really that &quot;redshift&quot; = H*d/c, and redshift is related to speed by &quot;redshift&quot;=square root of [(c+v)/(c-v)].&nbsp; 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.</blockquote>

[QUOTE="freyar, post: 6132254, member: 40227"] 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 [I]our[/I] universe (as opposed to some other hypothetical one), dark energy is responsible for the expansion [I]speeding up[/I]. 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. [/QUOTE]

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