The last 2 days Physorg.com has had articles about repulsive gravity being a possible candidate to replace Dark Energy.

Repulsive gravity as an alternative to dark energy (Part 1: In voids)

Repulsive gravity as an alternative to dark energy (Part 2: In the quantum vacuum)

In Part 1 the idea is that antimatter is concentrated in the cosmic voids where we don't see any stars. The thought is that the repulsion of the antimatter is what keeps the visible stars and galaxies out of the voids and causes the accelerated expansion of the universe. My first problem with this is: what is the antimatter? if it is just matter that is made up of antiparticles then I should think that the voids would be full of stars and galaxies, because the antimatter would work the same as regular matter.

In part 2 the idea is that the quantum vacuum is full of virtual particles and antiparticles that would produce enough repulsion to account for the accelerated expansion of the universe. The problem with this one and in a way with the first one is that nobody knows that antiparticles have the opposite gravitational sign from normal particles. There are currently tests going to to see if the repulsion can be detected.

Both of the ideas do have consequences on the expansion of the universe. With these ideas the acceleration may be tapering off and so the end of the universe will be quite different from what current theory says.

Bill Gill

I agree, and that's what I've been hearing all along. I don't know enough to speculate reasonably, but space is expanding, and the expanded space is expanding, so there would surely be a steady increase in the recessional velocities. What's happening is exactly what I'd expect to happen: a constant rate of expansion per unit of volume, but an ever increasing volume. So I'm obviously missing some crucial fact.

The usual analogies such as the expanding balloon are misleading. They lead one to imagine a steady, liniar rate of expansion of the balloon's surface. That's not a correct analogy. The fabric of the balloon is expanding, so the amount of fabric expanding is constantly increasing. Result: accelerated mutual recessional velocity of all points on the fabric.

Please tell me, what is it I'm not getting?

The thing is that the acceleration is greater than expected. That was what threw the cosmologists for a loop and led to the idea of dark energy.

Bill Gill

I understand that expansion of space has been known since Hubble. Then in 1998 it was discovered, by observations of supernovae, that the expansion was accelerating - the popular explanation being a kind of negative pressure (dark energy), possibly vacuum energy. That, as far as I knew, was the unexpected part of it. Now you're telling me that the acceleration is greater than expected. Oh boy, weirderer and weirderer

Part 2 suggests that there would be a repulsive force between virtual particles and antiparticles that would cause them to escape from each other, thus becoming real particles of matter and antimatter.

Obviously, this would be very different from the scenario in which matter and antimatter virtual particles annihilate almost as soon as they are formed. Would this not play havoc with conservation laws?

Isn’t this continuous “creation” of matter and antimatter reminiscent of the steady state Universe?

Well, there you have another problem with the scenario. I mentioned one or 2, but didn't think of that one. QM requires that the 2 particles annihilate each other within a very short time, but if they were repulsive then the time would be lengthened, possibly longer than is required by current theory. It seems to me that would have a major impact on how the universe works.

I don't think that the scenario calls for any increase in the time that the virtual particles exist. It appears to be just a matter of the existence of the repulsive force during the brief time they do exist.

In any case I don't think that either idea will be jumped on with cries of glee by the physics community.

Bill Gill

“If particles and antiparticles have gravitational charges of the opposite sign, a sufficiently strong gravitational field can convert a virtual pair into a real one,”

Wouldn't being converted into real particles cause them to become permanent residents, thus very considerably increasing the they were around?

It's an interesting idea but I would agree with many of the detractors ... why don't we see antimatter galaxies being one of the biggest.

Fortunately this theory will either fly or die very fast because CERN are setting up to see which way antimatter particles fall

http://www.bbc.co.uk/news/science-environment-16756457
http://www.dailygalaxy.com/my_weblog/2012/01/cern-zeroes-in-on-the-great-antimatter-mystery.html

Do we know how antimatter reacts with photons?

It would have to act in accordance with General Relativity. The sign might be negative, but that would mean that light passing a galaxy would be deflected outwards instead of inwards, and that would be an obvious indication that something odd was going on.

Bill Gill

But you have to know where the antipartciles were coming from ... that is you would need to know the source ... and since we don't see antiparticle suns or antiparticle galaxies thats a little hard.

Basically you would need alot of luck.

Is there an "antiphoton"?

The antigalaxies would exhibit reverse lensing. The light would be spread as it passed it. I think that would be readily detectable.

Bill Gill

Are you sure about that, Bill?

"The term antimatter was first used by Arthur Schuster in two rather whimsical letters to Nature in 1898, in which he coined the term...and like the previous ideas, differed from the modern concept of antimatter in that it possessed negative gravity." http://en.wikipedia.org/wiki/Antimatter

"Under general relativity any form of energy couples with spacetime to create the geometries that cause gravity. A longstanding question was whether or not these same equations applied to antimatter. The issue was considered solved in 1960 with the development of CPT symmetry, which demonstrated that antimatter follows the same laws of physics as "normal" matter, and therefore has positive energy content and also causes (and reacts to) gravity like normal matter" http://en.wikipedia.org/wiki/Anti-gravity

My book says "no". An photon is it's own antiparticle, i.e, it's the same thing.

Normally it is believed that antimatter would have normal gravity, but the start of this discussion was based on suggestions that it had negative gravity. Therefore an antimatter galaxy would have a negative lensing effect.

Bill Gill

Yes, I see Bill, sry, hadn't followed it. It seems that if anyone's going to seriously argue the case for antimatter, they have to show that the accepted CPT symmetry theorem is wrong.