Question of the Week

What happens as the universe expands?

Sun, 28th Nov 2010

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Fr Gerry Drummond asked:

The universe is expanding at an ever-increasing rate. a) How fast is that now? b) Presumably the limit will be the speed of light - what will happen then? Will the rate of acceleration slow down as we approach the speed of light or just suddenly stop? Will the universe begin to contract after that or just keep on expanding?



We put this to Carolyn Crawford, from the Institute of Astronomy at Cambridge University... 

Carolyn -   The most straightforward part of that question is, how fast the universe is expanding now because this is something we actually measure.  We call the current rate of expansion something called the Hubble constant, and that is really just a measure of how fast bits of the universe are moving away from us as a function of their distance.  As you go further out away from the Earth, bits of the universe are travelling ever faster.  For example if you go to 30 million light years away, maybe youíve got a bit of universe receding away from us about 700 kilometres per second, go 10 times further to 300 million light years away, itís receding at 7,000 kilometres per second, and if you carried on going further and further away, yes, you do get a point where you're far enough away and that bit of the universe is moving away from us at speeds greater than the speed of light.  So it is not a fundamental speed limit.

Hubble_deep_field_imageThe key thing here to realise is that the expansion of the universe is caused by stretching of space and that can be at any speed it likes, much greater than the light speed.  The thing that is constrained by the light speed is the rate that things can move through space.  So the expansion of the universe is the stretching of space, and itís carrying the galaxies along for the ride, but any light, any signal or information from those galaxies can't travel faster than the speed of light.

In fact, the expansion of space is not constrained by how fast itís moving.  Itís constrained by how much stuff is in it, and thatís going to determine whether the universe will contract or carry on expanding forever.

Diana -   If space objects are moving away from us at a speed faster than light then the lights from them cannot travel fast enough to reach us.

Carolyn -   All the evidence currently is that this expansion is getting ever faster and the universe is going to carry on expanding forever.  The speed at which it expands isnít affecting the eventual fate of the universe, but what it does affect is what we can see of that universe, and how much of it we can see because if a galaxy is moving away from us faster than the speed of light, that means any photons of light leaving it can't travel across the space fast enough.  It can't outrun the expansion of space.  And so we have something called the observable horizon.  Anything that's expanding further than the speed of light away from us, we will never see the light from those objects.  And as the universe gets bigger and bigger, that horizon is going to shrink and weíre going to see less and less of the universe, and thatís really what's going to be affected by this expansion speed.

Diana -   But what about that crunch theory?

Carolyn -   Whether or not the universe actually collapses back down on itself depends on how much matter there is in the universe and how much stuff there is to pull it back together under gravity, and really, the current cosmological thinking is that with the presence of dark matter and dark energy, the universe is not going to contract.  Itís going to carry on and expand.  Itís going to expand faster and faster, and never go back to that big crunch at the end.

Diana -   Current thought is, the universe will just keep expanding.  To sum up, objects are moving away from us in space as space stretches, meaning that they can travel away from us faster than the speed of light.  But those objects cannot move through space faster than the speed of light.


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Big Bang
Big Crunch
Big Bounce
Big Rip

The universe must be from Texas, where everything is big. galesteven, Wed, 24th Nov 2010

Yes, the universe is expanding. 

The rate at which galaxies are moving away from us depends on how distant they are already (ie the further away they are, the faster they are moving away from us); this ratio of change of distance against distance is the famous hubble constant (well the ratio needs to be squared to be H. Edit no it does not - wasn't thinking)). 

The speed that very distant galaxies are receding from us will be greater than the speed of light!  This doesn't really break any universal laws as they are so far away and accelerating that we can have no contact/commnication with them, and they are not locally moving with greater than speed of light - it should be thought of that the space between us is expanding so quickly that the distance between us is increasing faster than light can travel.

Unless we have got our sums wrong, the universe will keep expanding (and at an exponential rate to do with vacuum energy).  In several hundred billion years our galaxy and the merged andromeda/magellenic clouds will be the only thing it is possible to see from the remains of the earth;  the rest of today's universe will be cut off from us by the vast distances/speeds that not even light can ever bridge. 

Matthew Newell imatfaal, Wed, 24th Nov 2010

Thank you Matthew Newell for explaining the first stage of the Big Rip Theory.

Since gravity has a strong distance component, according to the Big Rip theory the expanding universe will also reduce the effects of gravity.

After galaxies are separated from each other gravity would become too weak to hold  individual galaxies together. Eventually the solar system would be gravitationally unbound, then stars and planets would be ripped apart, and finally atoms would be ripped apart.

But what about the other theories, like the Big Crunch and the Big Bounce, which have the universe reversing course and shrinking?

Steve Gale galesteven, Wed, 24th Nov 2010

Hi Steve,  I am afraid that I cannot agree with your point  . Whilst the accelerating expansion will separate those distant objects like distinct galaxies - at present the maths tends to show that these forces will not overcome even the gravitational attraction within a galaxy let alone the short range forces that hold objects together.  The big rip just will not happen ; galaxies will stay as galaxies, molecules as molecules. 

I think there is even more evidence against big crunch and bounce in the future; cosmologists are now fairly certain that the universe will continue to expand and get very cold and lonely.  imatfaal, Wed, 24th Nov 2010

You asked "What happens as the universe expands?" Flowers grow on earth, new starts are formed, you and I are alive, etc. etc.

:-) (-: kowalskil, Fri, 26th Nov 2010

Yes, it looks like dark energy is not increasing in the way it would need to get a big rip:
wolfekeeper, Wed, 1st Dec 2010

According to my point of view about the theory that matter is basically construct exclusively of photons, we may be inside a black hole. A black hole would be a wave of photons rotating and precessing inside the event horizon. It would produce a gravitational field inside and outside of the black hole toward the wave near the event horizon... The BigBang would be a collision of two black holes and there sizes at the BigBang would explain a supposed faster than light expansion after the BigBang... Recent data analysis from the WMAP (cosmic background radiation) tend to support my theory... CPT ArkAngel, Thu, 2nd Dec 2010

CPT - Please don't turn this thread into yet another debate about the nature of everything.

Geezer, Thu, 2nd Dec 2010

Hi wolfekeeper and imatfaal!

I asked one of the authors of the original "Big Rip" paper if his theory has been disproved.  He responded:

So although less likely, it is still on the table of possibilities.


Original paper reference: Caldwell, Robert R.; Kamionkowski, Marc and Weinberg, Nevin N. (2003). "Phantom Energy and Cosmic Doomsday". "Physical Review Letters", 91,: 071301 galesteven, Wed, 5th Jan 2011

Hi Steve - the maths is beyond me, and I am not sure that the evidential data is strong but I based my comments on a Leonard Susskind cosmology lecture in which he dismissed it.  I think everything is still on table - but some are more likely than others.  the way Susskind explained the action of a cosmological constant acting on space does tally with the notion that it will never be strong enough to challenge nuclear and electromagnetic over small distances. 

But as dark energy is still a completely fluid concept you are wise to never say never. imatfaal, Wed, 5th Jan 2011

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