How does dark energy allow the universes expansion to accelerate?

Question

I was wondering how dark energy is effecting the rate at which the universes expansion occurs.

Answer 1

I suspect this may not make much sense to non-GR heads, but the Einstein equation relates the curvature of spacetime to an object called the stress-energy tensor.

The stress-energy tensor describes the properties of the matter/energy that is causing the curvature. In most cases we're only interested in the amount of matter/energy present i.e. its density, but the pressure of the matter/energy also contributes to the curvature. In other words some matter that has been highly pressurised produces a greater gravitational force than the same density of matter that isn't under any pressure. Pressure leads to an attractive gravitational force.

The curious thing about dark energy is that it behaves as if it has a negative pressure. I mentioned above that normal (positive) pressure causes an attractive gravitational force, and the corollary is that negative pressure causes a repulsive gravitational force.

And this is why dark energy is causing the expansion of the universe to accelerate. It's because it has a negative pressure and the negative pressure produces a gravitational repulsion.

Answer 2

How does dark energy allow the universes expansion to accelerate?

I hope that it is clear to the questioner and the readers that the horse pulling dark energy is the experimental observation that the expansion of the universe is accelerating. Dark energy is proposed as the reason why the expansion is accelerating. It is called "dark" because it is not interacting with normal matter, but only with the space time structure.

Take a three dimensional explosion in space. The fragments will fly off and steadily expand from each other. If one observed that their expansion was accelerating, it would mean that extra energy was appearing in the system ( explosives in the fragments? ). The same reasoning applies. The geodesics in space were supposed to follow the Big Bang (after the inflationary period) expanding at a steady rate imposed by the initial impetus, but in an accurate model decelerating slowly because of the weak effect of gravity which is attractive. The observation that the expansion is accelerating introduced the simple concept of extra energy entering the local four dimensional space , and called "dark energy". It is still a matter for research.

Answer 3

Nobody has any certain answers to this, but IMHO there's an obvious issue with the cosmological constant, which is "the value of the energy density of the vacuum of space". If it's really constant, we've got energy being continually created as the universe expands. That goes against the grain of conservation of energy. I'm not happy with that because I don't know of any situation where energy is not conserved. But I do know that the expanding universe is sometimes likened to an inflating balloon:

^{Image courtesy of the one-minute astronomer}.

IMHO it's worth looking into this. If you have a balloon in a vacuum, the pressure of the air inside is balanced by the tension in the skin. You can make the balloon expand by blowing in more air. But since energy is pressure x volume, that takes us right back to the breach of conservation of energy which I'm not happy with. However there is another way to make the balloon bigger. Not by increasing the pressure, but by reducing the tension. This sounds impossible until you think bubble-gum. As the balloon expands, the skin gets thinner and weaker, and less able to resist the expansion. So it expands further, so the skin gets weaker, and so on. The pressure drops, the volume increases, but energy is conserved. This fits with what you read about negative pressure. Tension is negative pressure. Interestingly you can find references here and there to the strength of space which suggest this general idea might have some merit. Milgrom mentions it on page five of http://arxiv.org/abs/0912.2678:

"We see that the modification of GR entailed by MOND does not enter here by modifying the ‘elasticity’ of spacetime (except perhaps its strength), as is done in f(R) theories and the like".

I would hazard a guess that dark energy will turn out to be related to the above in some way. It isn't something mysteriously increasing and pushing space to expand faster and faster. Instead it's something that is becoming less effective at slowing down the expansion of space.