[holment (OP)]

The universe is expanding, which means that if you go back in time, all the mass in the universe was closer together. So how did it continue to expand? If you have x amount of distance between molecules at time t-1, shouldn't this distance be smaller at time t due to gravity? Is this related to dark energy?
Apologies if this was a stupid question
Best
Tor

[Kryptid]

Gravity isn't the only force at work in the Universe. Since the expansion rate of the Universe seems to be accelerating, dark energy does indeed play a role in preventing gravity from collapsing it. Even without dark energy, the Universe would still be expected to expand for some period of time after the Big Bang before gravity put the brakes on it. I suppose you could liken this to throwing a baseball up into the air. The ball has enough momentum to fight gravity for a little while, but it eventually slows down and comes to a stop before falling back down.

[chris]

Physicist Fran Day discussed the expansion of the Universe on a recent Naked Scientists episode.

[Janus]

I suppose you could liken this to throwing a baseball up into the air. The ball has enough momentum to fight gravity for a little while, but it eventually slows down and comes to a stop before falling back down.

Unless the ball left your hand moving at or exceeding escape velocity.  Then while it does continue to slow as it climbs, it will never come to a full stop and fall back.
With the universe,  Something equivalent can happen. if the expansion is fast enough, the energy involved can exceed the gravitational binding energy of the universe, so while the expansion slows over time, it still continues forever.
If however, it were below a given threshold, the expansion would eventually stop, and things would collapse back together.
The study that first provided evidence for dark energy was meant to determine which of the above cases was the right one.
They measured the red-shift of galaxies at various distances and plotted it against the distance.  Now as we look further out into space, we are also looking further and further back in time.  Thus  this gave them a way to plot the expansion rate of the universe over time. 
What they expected to find was a universe for which the expansion had slowed over time, and what they wanted to know was whether it had slowed enough to indicate that it would someday come to a stop and begin to collapse.
The unexpected result that they got indicated that the expansion had actually sped up over time.  Something other than the initial Big Bang was still driving the expansion. ( It was like someone had a attached a rocket engine to Kryptid's ball so that it continued to accelerate upward after you realeased it)

The term settled on for whatever caused this was "dark energy".

The point is that, even without dark energy, you could still have a universe that expands forever.

[alancalverd]

The Observable Universe is finite, but there's no reason why the Actual Universe should not be a lot bigger, or even infinite. If the AU is made of the same stuff as the OU (and why not?) then there is more stuff outside the OU than inside it, hence gravity will pull the edges of the OU outwards, whilst the stuff in any small sample of the OU will tend to coalesce.

[PmbPhy]

Gravity isn't the only force at work in the Universe. Since the expansion rate of the Universe seems to be accelerating, dark energy does indeed play a role in preventing gravity from collapsing it. Even without dark energy, the Universe would still be expected to expand for some period of time after the Big Bang before gravity put the brakes on it. I suppose you could liken this to throwing a baseball up into the air. The ball has enough momentum to fight gravity for a little while, but it eventually slows down and comes to a stop before falling back down.

If the ball is tossed up with an energy greater than zero it will keep going forever at a speed depending on the energy. Einstein added a cosmological constant to allow for what he thought was a static universe. Later if was found that the universe was expanding at an accelerating rate and viola, the notion of dark energy was invented. Dark matter acts like a negative gravitational mass, aka antigravity.

[alancalverd]

Newton's shell theorem shows that the outside will have no effect on the OU given a reasonably uniform distribution of material in all directions as described.

The shell theorem depends on the gravitational field being constant. This is reasonably true for small objects but as far as we know gravitation propagates at about the speed of light, so at very large distances (say the diameter of the OU) and speeds approaching c, the gravitational field at the inner surface of an expanding shell is not calculable from a static inverse square law: If the shell radius is expanding at c/2 the field contribution from a  diametrically opposite element is zero.   

[PmbPhy]

Gravity isn't the only force at work in the Universe. Since the expansion rate of the Universe seems to be accelerating, dark energy does indeed play a role in preventing gravity from collapsing it.

Even so gravity is at work with an accelerating expansion. Dark energy is merely the source of the field playing the role of a negative active gravitational mass density.

[alancalverd]

Pete: Have you considered the possibilities of negative mass conjugates of observable particles?

No energy is required to spontaneously create an atom and a negatom. Hence a Big Bang literally ex nihilo. Negatoms are attracted to each other but repel atoms, so the expansion of the observable universe is inevitable despite both atoms and negatoms forming clumps like stars and negastars - the clumps are driven apart by intervening negaclumps.  Thanks to the inverse square law there  is no possibility of galaxies and negagalaxies actually colliding though they might pass through each other's fringes and disperse a bit.

Not sure how negative mass would interact with photons. Presumably some inverse lensing, but since lensing is a very subtle  phenomenon and only observed on rare occasions when we know where to look, maybe we've just never noticed it? 

At worst, this idea could spawn a bit of sci-fi where an interstellar rocket goes mysteriously off course "almost as if we're being pushed away from something, Captain" and a leathery bush pilot (Cambridge's answer to Mr Spock) explains it all. Feasting, merrymaking, a Nobel Prize, and the Voice of God saying "bugger, they've worked it out" as the credits roll.