The Fate of the Universe -- Expansion or Collapse?

[stilt]

TL;DR

Expansion over collapse

If galaxies are now expanding away faster than light, then how would we know if this has now started to slow, stop and or reverse ? Is there a chance we may never know and the universe collapse won't be seen coming ?

 

[Ibix]

Welcome to Physics Forums.

In our cosmological models the expansion rate is the same everywhere. If galaxies we can see are still receding then so are the ones we can't see. And our current best estimate is that we aren't in the kind of universe that collapses anyway.

 

[stilt]

Thanks for your reply. It was only recently we discovered galaxies are expanding out against certain theories. Let's say the gravitational effect starts to kick in and they do start to accelerate back into a collapsed universe.

 

[PeterDonis]

It was only recently we discovered galaxies are expanding out against certain theories.

What are you referring to here?

Lets say the gravitational effect starts to kick in

What "gravitational effect" are you talking about? Galaxies right now are responding to all "gravitational effects".

 

[Ibix]

It was only recently we discovered galaxies are expanding out against certain theories.

Nearly a century ago isn't all that recent...

Lets say the gravitational effect starts to kick in and they do start to accelerate back into a collapsed universe.

There isn't any gravitational effect that we are aware of that isn't already included in our models, so there's nothing to "kick in" that hasn't already.

If you are asking what would happen if something we haven't yet discovered happens then the answer is we won't know until we've discovered it.

 

[kimbyd]

Summary:: Expansion over collapse

If galaxies are now expanding away faster than light, then how would we know if this has now started to slow, stop and or reverse ? Is there a chance we may never know and the universe collapse won't be seen coming ?

Beyond a certain distance, every galaxy we can observe is now and always has been receding at faster than the speed of light. This includes most of the observable galaxies.

This sounds weird, but the fact is that in General Relativity, there is no well-defined notion of relative speed between different locations. Which means that the speed you get depends critically upon how you define your terms. In this case, the speed being talked about is rate of change in proper distance.

And we can observe galaxies that are now and always have been moving faster than the speed of light simply because they were much closer when that light was emitted, and the rate of expansion has slowed down over time. As a photon started to travel towards us from such a galaxy, at first the distance between us and that photon grew: it would move in our direction, but slower than the universe between us and the photon would expand. Eventually, the rate of expansion slowed to the point that the photon started making ground instead.

The photon by this time will have traveled a good distance away from its source galaxy. With the larger distance between us, the expansion is carrying that galaxy away faster than the now-closer photon. So the galaxy can keep moving away at faster than light while the photon eventually does reach us.

Note that in the far future, when the universe's expansion is dominated by the cosmological constant, the expansion itself will be a constant so this will no longer occur: once a galaxy starts receding at faster than light in such a universe, we no longer receive any new light from that galaxy. We continue to see light from the galaxy for a long time, but none of it from after the point it crossed that horizon.

Our universe is not currently dominated by the cosmological constant, but there is still just such a horizon (it's just way more complicated to calculate). And all of the galaxies which have always been moving faster than the speed of light away from us have, I'm pretty sure, moved past that horizon long ago. So we can never see those galaxies age beyond a certain point. Their light will just redshift away until we can't observe them any longer, similar to what happens when an object enters a black hole.