[Geezer (OP)]

I see the results of some recent extensive analysis are significantly adding to the case for Dark Energy. Does dark energy act as a sort of "anti-gravity" in that it only acts to repel actual matter, or does it simply expand space irregardless of whether there is matter in it or not?

[Kryptid]

I have seen an argument that if antimatter has antigravity, then it could explain dark energy without having to invoke a new force. I see a problem with that idea myself. Imagine that all of the matter and antimatter in the Universe repel each other. Over time as the Universe expands, the force of this repulsion should decrease as the distance between matter and antimatter increases. This should result in a deccelerating expansion of the Universe.

However, the expansion of the Universe is accelerating, not deccelerating. That is difficult to explain in terms of a vector field which should weaken with distance.

[Geezer (OP)]

Thanks SC. Yes, that seems to make more sense, although hoping to make sense with this stuff may be a bit overly optimistic  []

That would mean that it's independent of the presence of matter and we are only using the presence of matter to observe the effect. If it was a function of actual matter, I suppose it would be referred to as some sort of anti-gravity.

I'm sort of thinking the acceleration aspect of it is essential. I'm assuming that the same effect is observed from any point in the Universe (it would be a bit strange if it was only observed from Earth!). So, it seems to me that would require acceleration rather than constant speed. If it was constant speed, uniform expansion would only be observed at one particular location.

[imatfaal]

personally the idea that grips my imagination is the background growth - space is getting bigger.  for every unit of space, a little later you have slightly over a unit.  the forces involved are miniscule so can only be seen in the vast reaches between galaxies - over a human/stellar level gravity/em/strong/weak will easily keep us in shape. 

SC - yeah I have seen that one as well.  Could you not say tho - if antimatter gives antigravity, then its mass must be somehow anti-mass and its energy antienergy.  But when it annihilates with matter it does not cancel out - it blasts out (two or three gamma rays per electron/positron).  Additionally if there were any large deposits of antimatter we would see the boundary between normal and anti lit up like a gamma ray christmas tree

[imatfaal]

For those who want to read more on the latest stuff Geisa was referring to

http://www.bbc.co.uk/news/science-environment-13462926

The write up is pretty poor - but it also contains arxiv links for the brave of heart

[Geezer (OP)]

For those who want to read more on the latest stuff Geisa was referring to

Hey! That's Mr Geyser to you laddie.

[Kryptid]

@imatfaal: Antimatter would still have a positive mass/energy content. The justification for the antigravity field had something to do with CPT invariance and Relativity. I can't pretend to understand it, though.

Also, if antimatter did have an antigravity field in addition to a positive mass/energy value, it would mean that black holes are "hairier" than we previously thought. A black hole made from collapsed matter and a black hole made from collapsed antimatter would repel each other. So mass, charge, and spin would no longer be sufficient to describe all of a black hole's properties.

[imatfaal]

Sc - ooh hadn't heard that one - will chase down a reference, sounds intriguing