The Universe Never Expands Faster than the Speed of Light

[Chalnoth]

Really good blog post by Sean Carroll on a shockingly common misconception:

http://www.preposterousuniverse.com...never-expands-faster-than-the-speed-of-light/

Hopefully at least scientists will stop spreading this misconception soon.

 

[andrewkirk]

I agree with him on points 2 and 3, but I don't share his objection to the term 'superluminal expansion'. Applied to the current, non-inflationary, universe the term is meaningful, and handy, because it relates to which galaxies lie outside the observable region. Nor is there anything technically incorrect in it.

It comes down to having a clear understanding of the difference between
(1). a rate of increase in the proper distance between two objects; and
(2). a (four-)velocity vector

As long as a physicist doesn't use the word 'velocity' to describe (1) in relation to distant galaxies (I like the word 'rate', as even 'speed' sounds a bit too similar to 'velocity'), I don't mind. I think his point 2 is objecting to the use of the word 'velocity' in relation to expansion, and I agree with that.

 

[phinds]

I agree with him on points 2 and 3, but I don't share his objection to the term 'superluminal expansion'. Applied to the current, non-inflationary, universe the term is meaningful, and handy, because it relates to which galaxies lie outside the observable region.

Actually, it doesn't, unless I am misunderstanding you. Objects at the outer reaches of the Observable Universe have a recession velocity of about 3c, and recession velocities of c start fairly well inside the sphere of the Observable Universe.

 

[andrewkirk]

Sure, I agree. That's why I said 'relates to', to avoid the more specific, and incorrect, suggestion that whether we can see a galaxy is determined by whether its 'recession rate' exceeds c.

 

[phinds]

Sure, I agree. That's why I said 'relates to', to avoid the more specific, and incorrect, suggestion that whether we can see a galaxy is determined by whether its 'recession rate' exceeds c.

Fair enough.

 

[Chalnoth]

I agree with him on points 2 and 3, but I don't share his objection to the term 'superluminal expansion'. Applied to the current, non-inflationary, universe the term is meaningful, and handy, because it relates to which galaxies lie outside the observable region. Nor is there anything technically incorrect in it.

No, it isn't meaningful, because the velocities of far-away galaxies aren't well-defined. There is no sense in which "superluminal" is sensible as applied to the limits of the observable universe. Worse, it is usually a term that is applied to inflation, where it makes even less sense as this isn't a meaningful way in which the expansion differs from today's expansion.

 

[andrewkirk]

No, it isn't meaningful, because the velocities of far-away galaxies aren't well-defined.

That's why the word 'velocity' should not be used (see last part of my post). It's using the word 'velocity' in this context that contributes confusion.

 

[Chalnoth]

That's why the word 'velocity' should not be used (see last part of my post). It's using the word 'velocity' in this context that contributes confusion.

Except the word "superluminal" only makes sense in relation to either speed or velocity.

 

[andrewkirk]

I find it perfectly reasonable to say that a galaxy's recession rate is superluminal if the proper distance to it is increasing at a faster rate than the distance to a wavefront on a laser I have pointed away from me and switched on. But what seems reasonable to one may not seem reasonable to another.

 

[Chalnoth]

I find it perfectly reasonable to say that a galaxy's recession rate is superluminal if the proper distance to it is increasing at a faster rate than the distance to a wavefront on a laser I have pointed away from me and switched on. But what seems reasonable to one may not seem reasonable to another.

But that doesn't have much of anything to do with the limits of the observable universe. Most observable galaxies are "superluminal" by that definition. More to the point, the post was about "superluminal expansion," which definitely has nothing to do with the situation you just described.

 

[andrewkirk]

But that doesn't have much of anything to do with the limits of the observable universe. Most observable galaxies are "superluminal" by that definition. More to the point, the post was about "superluminal expansion," which definitely has nothing to do with the situation you just described.

Actually, most of the blog is about misuse of the word 'velocity' (issue 2 of 3) and other physicists implying that superluminal recession can only occur during inflation (issue 3 of 3).
I agree with Carroll on both those points. As regards point 1, there is plenty of room to differ. We are not talking about a theorem or an equation here, so there is nothing 'definite' about it one way or the other - no 'right answer'. It's just a question of what words people like to use to describe, in the easygoing way that natural language is used, certain phenomena that can only be accurately described by equations.

Even Charlie Lineweaver and Tamara Davis, whom Carroll mentions with special praise in his post, use the term, viz this from p9 of their marvellous Scientific American article ('Misconceptions about the Big Bang'):

'The galaxy they came from, though, may continue to recede superluminally.'​

In their Expanding Confusion paper, which is more academic and mathematical (and which Carroll links in his blog post), they go even further and refer to 'recession velocities' right there in the abstract. 'Recession velocity', or any phrase using 'velocity' to describe this phenomenon, is a term that I personally regard as unfortunate, as discussed above.

 

[Chalnoth]

The post was talking about the term, "superluminal expansion," not the use of superluminal in general, or the use of superluminal recession velocity (which is unrelated to the particle horizon that bounds the observable universe), and is a separate issue from the expansion itself.