Superluminal Recession

[Warren Wollman]

If a galaxy is so far away that it's speed of recession is superluminal, then what is the fate of a light wave headed toward us, i.e., under what conditions, if any, can it reach us? Is the premise of this question allowed? Any references on this topic?

[Dark Jaguar]

From what I understand, such a recessive speed isn't permitted due to that pesky universal speed limit. I must confess that that whole concept confuses me too.

[Warren Wollman]

The "pesky universal speed limit" refers to motion through space, but recessional speed is due to the expansion of space itself, hence nothing pesky.

[Warren Wollman]

From what I understand, such a recessive speed isn't permitted due to that pesky universal speed limit. I must confess that that whole concept confuses me too.

Dark Jag: The recession speed is due to the expansion of space, not motion through space, hence there is no pesky limit. The question remains, as far as I know.

[Dark Jaguar]

Thanks for that. Then again, that speed limit isn't so bad now that I understand light has a certain pressure. If it could go at an infinite speed, I'm pretty sure everything in the universe would be pulverized instantly (on top of being roasted).

If that's the case, then my vague understanding of light getting stuck in a black hole would probably apply here too. If space expands to the point where light must travel faster than itself to get here, then we'll never see it. It would basically be moving away from us at whatever that superluminal speed was - c. Light would still be travelling to us at the same speed, but the "road" would be getting yanked backwards faster than it was travelling, thus maintaining light's speed as a constant. Did I get that right?

[Dark Jaguar]

Actually, I don't think I did. I seem to recall a mention that light is still heading towards us at that constant despite the acceleration, it's just massively red shifted, at this point to the level of being just a straight line. I know I read that somewhere...

Um, it's best if someone else corrects me at this point. I think I've muddled things up enough.

[Warren Wollman]

Like you, Dark Jag, my intuition tells me we would never see the light emitted by a superluminal receding galaxy. However, I've learned that in the area of general relativity and cosmology, my intuition is probably worthless.

[Tim Thompson]

Any redshift greater than 1 is a superluminal recession. Nothing spectacular happens, but it is a common misperception, even amongst physicists, that no recession speed can surpass the speed of light. Relativity theory (both special and general) is something that has to be studied in specific detail, to be well understood; the general education that all physicists get in relativity is really a lot less useful than one might expect. The fact that the galaxy is taken for a ride by expanding spacetime is crucial.

• Expanding Confusion: Common Misconceptions of Cosmological Horizons and the Superluminal Expansion of the Universe, Tamara Davis & Charles Lineweaver, Publications of the Astronomical Society of Australia 21(1): 97-109, 2004
• Misconceptions about the Big Bang, Charles Lineweaver & Tamara Davis, Scientific American, March 2005

[Ken G]

Then again, that speed limit isn't so bad now that I understand light has a certain pressure. If it could go at an infinite speed, I'm pretty sure everything in the universe would be pulverized instantly (on top of being roasted).

Ironically, this is not the case-- if light travelled at infinite speed, rather than seeing an increase in pressure that could pulverize things, it would lose all the pressure it has. The idea is that you convert a set amount of energy E into making a photon (never mind how changes in c would affect those energy processes, let's just assume the energy is given), then the momentum of the photon would be E/c. So the fact that c is large is the reason that photons have so little pressure, and increasing it further would just make their pressure even less noticeable.