[Ichneumon]

Question: why aren't there greater consequences than the appearance of the light from these objects? Why is this motion, relative to ours, immune from the effects of special relativity?

Click on the link and read the whole article at Scientific American (and view all the sidebars, that's where most of the graphics are).

This article covers all those issues and more, and does it *very* well.

The short form is that while the distant galaxies are *separating* from us at over the speed of light, they are not *traveling* at/beyond the speed of light. "Travel" is velocity *through* space-time. But those galaxies are actually "sitting still" while space-time *itself* expands and just "carries" the galaxies with it. And Einstein's laws don't place a limit on the (relative) expansion velocity of space-time, only on the speeds of things *in* it.

For an analogy, consider a rubber sheet which has objects on it, and the rubber sheet has a high friction coefficient, so things on the sheet can't travel more than a certain speed across it because friction-based drag slows them to a crawl. But there's no limit on how fast the sheet *itself* can be stretched, separating the objects resting on it by any arbitrary velocity.

Here's one of the relevant sidebars -- the text of the article itself explains it in more detail:

And here's part of the relevant text from the body of the article (although it's best to read the whole article to get a real feel for all the interrelationships):

Notice that, according to Hubble's law, the universe does not expand at a single speed. Some galaxies recede from us at 1,000 kilometers per second, others (those twice as distant) at 2,000 km/s, and so on. In fact, Hubble's law predicts that galaxies beyond a certain distance, known as the Hubble distance, recede faster than the speed of light. For the measured value of the Hubble constant, this distance is about 14 billion light-years.

Does this prediction of faster-than-light galaxies mean that Hubble's law is wrong? Doesn't Einstein's special theory of relativity say that nothing can have a velocity exceeding that of light? This question has confused generations of students. The solution is that special relativity applies only to "normal" velocities--motion through space. The velocity in Hubble's law is a recession velocity caused by the expansion of space, not a motion through space. It is a general relativistic effect and is not bound by the special relativistic limit. Having a recession velocity greater than the speed of light does not violate special relativity. It is still true that nothing ever overtakes a light beam.

Another surprising result of this is that we can still "see" galaxies which are receding from us at well over the speed of light. One would think that the Hubble limit (the edge of the "observable universe" from where we are) would be at the point where galaxies are receding from us at the speed of light, but it's actually well *beyond* that. Again, the article explains all this in easily understandable terms if you read the whole thing.

It's well worth the time. I read it yesterday standing at the magazine rack while waiting for a prescription to be filled, and I'm glad someone posted it already -- I'd have done so myself otherwise, it's *that* good.

[PatrickHenry]

The article says:
The solution is that special relativity applies only to "normal" velocities--motion through space. The velocity in Hubble's law is a recession velocity caused by the expansion of space, not a motion through space. It is a general relativistic effect and is not bound by the special relativistic limit. Having a recession velocity greater than the speed of light does not violate special relativity. It is still true that nothing ever overtakes a light beam.

Yes, I read that before I posted my question. I understand that it's due to the expansion of space. Still, those galaxies are moving (so to speak) faster than lightspeed with respect to us. Shouldn't their mass be rather ... large?

I understand that we're moving at that speed relative to them, and I assume we don't notice such a crushing mass because, locally, we're in free fall, so we wouldn't notice it. Nor would they, locally. But we should notice it as to them, and vice versa. Or so it seems to my limited understanding.

[rock58seg]

Travel" is velocity *through* space-time. But those galaxies are actually "sitting still" while space-time *itself* expands and just "carries" the galaxies with it.

I am so happy to hear that an area that is devoid of all matter is not space until it has matter in it or approaching it?

When the explanation has to twist semantics, so badly, It is not a very elegant explanation.

[rock58seg]

Since there are galaxies colliding, is it possible there was more than one big bang, in vastly different areas of space?

Since there are supposedly galaxies out there past the 14 billion mile marker receding from at a greater velocity than the speed of light, how does one measure the red shift if the light never reaches us?

Since we can't measure that they exist, are they possibly the missing matter one hears about?