[thedoc (OP)]

Alan asked the Naked Scientists:
   
What is the force universal force holding up the galaxies and stars, or is the universe falling at a constant rate?
What do you think?

[evan_au]

It is inertia that keeps galaxies and stars moving on their courses; for galaxies, this is generally away from each other.
Inertia was analyzed by Isaac Newton, and enshrined in his laws of motion.

It is gravity that pulls galaxies and stars together.
Gravity was analyzed by Isaac Newton, and enshrined in his law of universal gravitation.

These two fight each other. And cosmologists fought each other about which one would win.
See: https://en.wikipedia.org/wiki/Metric_expansion_of_space#Theoretical_basis_and_first_evidence

It is Dark Energy that causes galaxies to accelerate away from each other.
This was awarded a Nobel Prize in 2011.

Now most cosmologists think that Dark Energy will win, although there is no real agreement about what it is.

[Bill S]

Now most cosmologists think that Dark Energy will win,

Is there any concensus as to whether this means that the Universe will expand for ever?

What would happen if/when entropy reached equilibrium?

[timey]

Bill - Roger Penrose has a theory that takes the universe in this direction that he describes in his book "Cycles of Time".  It's a good read!

[yor_on]

Heh

For a universe to be 'falling' you will need another place in where 'it' can do so. Every action is relative something, with the possible exception of a acceleration that can be felt 'intrinsically'. But as all accelerations we find/make exist relative SpaceTime it's very hard to prove. If you think of it in those terms gravity might be able to exist, according to the equivalence principle, 'on its own' no universe needed, which is a very weird thought.

[Bill S]

Bill - Roger Penrose has a theory that takes the universe in this direction that he describes in his book "Cycles of Time".  It's a good read!

Thanks; I have it, just need to get round to reading it.

[evan_au]

Is there any consensus as to whether this means that the Universe will expand for ever?

Dark Energy is rather a mystery. It describes an effect, rather than a cause.

If Dark Energy continues its current trend, the feeling is that the universe will continue to expand at an accelerating rate.

However, some models suggest that Dark Energy might someday increase enormously in strength, leading to a Big Rip, which would pull apart galaxies, solar systems, planets and even atoms!

Or perhaps Dark Energy might reverse, and then the universe might contract?

See: https://en.wikipedia.org/wiki/Big_Rip

[yor_on]

I don't think so Evan, the idea reminds me of the one in where everything 'shrinks' relative a vacuum. We're bound by the 'forces' keeping matter together, and I will trust in that, for the moment that is