Ask Ethan #80: Can space expand faster than the speed of light? (Synopsis)

“If everything seems under control, you’re not going fast enough.” -Mario Andretti

One of the toughest things to wrap your mind around in the natural world is the idea of special relativity: the faster you move, the closer you get to the speed of light, the more difficult it becomes to increase your speed at all. While you might approach the speed of light arbitrarily and asymptotically, you'll never reach it.

Image credit: user Fx-1988 of deviantART. Image credit: user Fx-1988 of deviantART.

And yet, we have the Universe, expanding all the time, where the expansion rate itself is even speeding up. You might wonder, then, if these distant galaxies -- the farther and farther away you look -- might ever be seen moving away from us faster than the speed of light?

Surprisingly and mind-bendingly, the answer is yes.

How do we reconcile this? Find out on this week's Ask Ethan!

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Doesn't this imply that at any given point that there is a "preferred" frame of reference, whereby the reference frame is stationary wrt. the local spacetime? I suspect we might even be able to estimate our local velocity by carefully observing the CMB. And isn't that counter to the basic assumption of SR, that there is no "preferred" reference frame?

By Omega Centauri (not verified) on 20 Mar 2015 #permalink

No it doesn't. About the best you can do is a preferred direction of time. Which classical mechanics had a good enough answer for years ago.

Omega Centauri,

The way I understand it is that the CMB does provide a reference frame, but that reference frame is not a preferred reference frame in the sense that physical laws in that reference frame are exactly the same as they are in any other reference frame. Any other reference frame is equally valid.

Where the confusion might come in is that the CMB could provide a convenient, mutually agreeable, reference frame for hypothetical observers anywhere in the universe. When we do thought experiments about how we would communicate with a hypothetical alien race, for example, we can well imagine using the CMB as a convenient agreed upon reference frame for measuring the motions of other bodies. That no more makes it a preferred frame, though, than the common practice of using the locally flat surface of the earth for measuring the speed of automobiles makes that reference frame a preferred one.

I did mean APPROXIMATELY locally flat in reference to the earth's surface, of course. I am well aware that the earth's surface is not mathematically flat even locally. For most practical measurements over a small portion of that surface, though, it's close enough to flat to ignore the curvature.

I find the question a bit nebulous:
when we ask "does the rabbit go faster than the tortoise", we imagine lining them up on the starting line, shouting 'GO!'

with expanding space vs a photon, there is no particle of space to measure against it. Yes, you can measure against some remote region of the universe, but that seems to miss the sense of the original question.

By MobiusKlein (not verified) on 20 Mar 2015 #permalink

Yes, Mobius, it is. The problem is that English isn't a precision tool for containing ideas.

It's not even the best one we've got, but we're stuck with it for historical reasons, and it's not *too* bad.

I posted over on slashdot what may be a useful way of thinking of space:

[S]pace is the distance between things.

When traveling between two things separated by space, we call that speed, and it cannot be faster than light in a vacuum.

However, that distance between those two things doesn't have to be constant and doesn't have to move, therefore it doesn't have to increase below some maximum threshold.

A quick question about extraterrestrial life. Based on the comment made in the article that 97% of the universe is unreachable or even potentially unobservable could this provide some explaination as to the reason the universe appears so void of intelligent life?

By Matt Done (not verified) on 20 Mar 2015 #permalink

Over time does an expanding Universe, where the expansion is speeding up ever resemble inflation?

re #8: the huge size of the galaxy is entirely sufficient to explain why we've not seen any extraterrestrials. There's no reason to come here, and the distances so huge that it takes lifetimes to get here, therefore even a colonisation would take thousands of generations, even if they had rapid "extraterraforming" and practically unlimited energy.

re #9: No, because inflation is naming something different. What will happen to what we see at the edge is "things are disappearing faster". Adding accelerating space expansion and you get something that will rip atoms apart, since the visible horizon of the nucleus is smaller than the distance to the electrons. More expansion means the nucleons no longer see each other. And so on.

Inflation isn't entirely the same, in cosmology, because the scenario "starting" is so very different.

I thought I read that scientists say the universe expanded faster than the speed of light in the first instants after the Big Bang.

By See Noevo (not verified) on 21 Mar 2015 #permalink

They did say that, and you may well have read it. I don't know your reading material.

Why did you think it necessary to say that, though?

@9: to expand (heh) on Wow''s point: inflation causes an exponential expansion. Assuming dark energy is a cosmological constant, then just as its name implies, its causing a constant expansion instead. V proportional to delta x between two objects for all times t, versus inflation's V proportional to (delta x)^t.
I guess its possible that two really widely spaced objects in our universe might, in the future, be expanding away from each other as fast as two closer spaced objects did at the beginning of the inflationary period. IIRC this is similar or identical to the concept of "the big rip." But (a) no the rate will never "resemble" inflation in terms of how it behaves over time, and (b) I'm too lazy to work out the math as to how far separated those objects would have to be. :)

What's it expanding into? Kind of like dimensions > 4, hard to imagine.

By Andy Eppink (not verified) on 24 Mar 2015 #permalink

It's not expanding into anything, it's just expanding.

Hey guys was wondering...Why is the expansion rate of the universe speeding up? shouldn't it be slowing down??

@Altair #18: I would encourage you to browse the right hand sidebar. Ethan has written many, MANY articles about exactly your question, so you should have no trouble at all finding an answer.

By Michael Kelsey (not verified) on 25 Mar 2015 #permalink

We can see that the universe is expanding through a phenomenon called Red shift, it occurs when things move away from one another. Therefor the expansion rate is compared to movement of light and for that the movement cannot be faster than the speed of light. Most things is relevant to another except for the speed of light that is set.

By Ilze Dreyer u1… (not verified) on 06 Apr 2015 #permalink

Ilze that is very interesting. I did not know that. The speed of light is very fast so I believe nothing would ever be that fast, but it is truly mind blowing!!!

By N, Tshabalala (not verified) on 07 Apr 2015 #permalink

Fascinating that we now see evidence everywhere that the space between the stars is expanding. Somehow it seems intuitive that something is filling the space between the stars ... dark energy, what have you. And this new stuff creates a more or less steady state continuous matter density - its a thought experiment, but perhaps there are ways to measure unpredicted new matter between the stars somehow. If so then the standard 'ultimate low temperature heat death' far future metaphor is less compelling than a universe that is boiling with new matter between the ever receding stars.