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ETpro's avatar

What happens when the expansion of the Universe reaches the speed of light?

Asked by ETpro (34605points) October 18th, 2010

We once thought the Universe would expand from the momentum of the Big Bang, but that gravity would slow and ultimately reverse that expansion, leading to a big crunch where all space-time collapsed back into itself. Recent observations have rather definitively disproved that. The Universe is still expanding 13.75 billion years after the Big Bang—and more surprisingly, its rate of expansion is still accelerating. You don’t see things flying out from an exposition continue to accelerate.

We do not yet know what force is causing the acceleration of expansion. It runs counter to all forces and physical attributes of the Universe that we have been able to measure. Science has postulated dark mass and dark energy as currently undetectable but responsible for the behavior we observe. The search is on to either prove of falsify dark matter and energy.

If the driving force does turn out to be a constant, then the acceleration will continue till the most distant parts of the Universe are racing away from us at the speed of light or greater.

Is that possible? What then?

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28 Answers

YoBob's avatar

The short answer is: It can’t.

The observation of speed is relative, not absolute.

lucillelucillelucille's avatar

You may then unfasten your seatbelts and move freely about the cabin ;)

ETpro's avatar

@YoBob Actually, it can and if acceleration continues, it will. The observation of speed is indeed relative. So for an observer at our point in the Universe, the most distant object currently visible would dimply no longer be visible when it exceeded the speed of light. But that would not mean it was no longer out there.

iamthemob's avatar

If we accept that the acceleration will continue without some counterforce acting against it eventually, is one possibility that the process will result in the conversion of matter completely into energy (I think, specifically, photonic energy)?

Rarebear's avatar

The expansion of the universe is actually happening much faster than the speed of light. I’ll find you the reference later.

iamthemob's avatar


Your statement made me realize that I was assuming that the acceleration of space was the same as the acceleration of matter through space, which is not the case. My above statement is more than likely, therefore, mooted.

ETpro's avatar

The early measurements oh H sub 0 (Acceleration of expansion) were were around 500 Km/Sec but a huge host of recent measurements all all clustered at between 50 and 100 Mm/Sec. The site says “Note the very recent convergence to values near 65 +/- 10 km/sec/Mpc (about 13 miles per second per million light-years).” See here

And @Rarebear I did find this

ETpro's avatar

Oops To late to edit. Make that 50 to 100 Km/Sec. and not Mm/Sec. Best estimate now is 70 Km/Sec.

Rarebear's avatar

@iamthemob Exactly. Acceleration of matter through space is limited by c. There is no such limitation on the expansion of space itself.

gasman's avatar

The short answer is: It already has!

The universe is about 14 billion years old, which (without expansion of space) should put an upper limit of its diameter at about 28 billion light years. The actual diameter of the observable universe is about 93 billion light years, however, because distant objects are dragged faster and farther by the expansion of space.

Relativity requires that nothing can move faster than light speed through space but does not prohibit space itself from moving more quickly. Mind-boggling stuff.

MissA's avatar

Well, we won’t have all of those long drives without bathroom stops. There’ll be lots of bedroom communities within the universe. (just dreamin’)

AstroChuck's avatar

Our hats fly off.

MissA's avatar


What about your fly?

talljasperman's avatar

then were doomed dooooooooommmmmmmmmmed muhhahah

hiphiphopflipflapflop's avatar

I’ve seen the notion of the end of the universe brought about by accelerating expansion called The Big Rip.

crazyivan's avatar

The expansion of space time cannot be considered in the same manner as matter or paricles moving through space. In fact, the “speed” of the universe’s expansion is a relative matter anyway because the universe isn’t expanding into anything.

My firm belief is that someday we’re going to discover something that brings the whole “red shift” measurement into question and a new theory will emerge that makes way more sense.

@gasman Sorry… answered before I saw your answer. You did way better than me!

iamthemob's avatar

@crazyivan – I too wonder why the red shift measurement isn’t really called into question before we start inventing “dark matter” and “dark energy” to make the math work. Of course, it’s the model we have to work with for now…

crazyivan's avatar

Well Dark Matter is an extrapolation from the tendency of light to react to more gravity than is observable in matter but it wouldn’t surprise me at all if a future discovery tied the two together.

ETpro's avatar

@crazyivan Great comment. My next Strange Universe question (not today’s but tomorrow’s) will dig further into that very issue.

CMaz's avatar

I hope it is ok with all of you that I masturbated while reading this.

crazyivan's avatar

@ChazMaz So long as you do it at the speed of light.

CMaz's avatar

Too late.

crazyivan's avatar

Then it was fast enough.

ETpro's avatar

@ChazMaz Ha! That’s an interesting reaction. Am I going to need to keep churning out this Strange Universe porn to keep you happy?

Rarebear's avatar

Better not. His keyboard might get sticky.

ETpro's avatar

@Rarebear Disturbing word picture. But I don’t know what sort of question might be any less sexy than discussion cosmology.

Hypocrisy_Central's avatar

Holy moly, in spite of all the numbers and calulations it is really more speculation than anything. At some point whateve expansion there is might stop and things will take a different course. However no one here will be living, so, eh….... When I was a kid we were told the Solar system was completely dead, the only planet that had any life or activity was Earth, then along cam Io, one of the moons of Jupiter and that blew holes in everything up to then. 15 decades from now all we think we know might not have been what we thought at all.

ETpro's avatar

@Hypocrisy_Central Yes I am sure that our only interest in it is academic. None of it will transpire in the foreseeable future.

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