FWIW, here's some stuff I wrote yesterday on another forum in response to this article. The guy who posted the article was particularly puzzled by the remark in the article that "the gravitational repulsion between matter and antimatter is a prediction of general relativity".
> In this case, the repulsive gravity could stem from antimatter hiding in voids.
Well, it's *possible* that antimatter has the opposite gravitational charge to normal matter, but this is *extremely* hard to test without access to macroscopic quantities of antimatter. The consensus in the field is that antimatter doesn't have the opposite gravitational charge to normal matter, but at this stage it hasn't been ruled out by theory or experiment. If antimatter turns out to behave like this it would be fairly easy to incorporate it into GR, but to say that "the gravitational repulsion between matter and antimatter is a prediction of general relativity" is somewhat misleading IMHO.
> When scientists discovered in 1998 that the Universe is expanding at an accelerating rate, the possibility that dark energy could explain the observation was intriguing. But because there has been little progress in figuring out exactly what dark energy is, the idea has since become more of a problem than a solution for some scientists.
> One physicist, Massimo Villata of the National Institute for Astrophysics (INAF) in Pino Torinese, Italy, describes dark energy as “embarrassing,” saying that the concept is an ad hoc element to standard cosmology and is devoid of any physical meaning.
Not really - assuming that the cosmological constant is exactly equal to zero is just as ad hoc as assigning a non-zero value to it. It's not totally devoid of any physical meaning - it represents a global tension acting on the whole of spacetime. It would be nice to be able to predict the value of the CC from first principles, but that may turn out to be impossible, and the best we'll get is a range of "legal" CC values that lead to a reasonably stable universe.
> “Cosmic voids (and in particular the nearby Local Void) are observationally very well known and constitute the largest structures of which our Universe is composed,” Villata told PhysOrg.com.
> “The problem is whether they are really empty or contain the repulsive antimatter.”
> In Villata’s paper, which will soon be published in Astrophysics and Space Science, he suggests that antimatter could be hiding in these large voids, separated from matter by mutual gravitational repulsion.
Why is it hiding? If antimatter is just like matter, then in large enough concentrations it should form stars.
> As he explained previously, the gravitational repulsion between matter and antimatter is a prediction of general relativity.
No. The gravitational repulsion between matter and antimatter is consistent with GR, it's certainly not a prediction of GR, which has little to say about the nature of matter. All GR cares about in this regard is that matter has mass, and it postulates that the gravitational mass of a body is the same magnitude as its inertial mass.
> The gravitational repulsion between matter and antimatter could be so powerful, in fact, that Villata has calculated that it could be responsible for the accelerated expansion of the Universe, eliminating the need for dark energy and possibly dark matter.
It would be nice for a single theory to simultaneously account for both dark energy and dark matter. I'm glad he's done some calculations using this theory, but I'd be interested to know if his reasoning and calculations have been independently verified.
> Unlike the first two components that are attractive, the third component could be repulsive, according to Villata. In support of this possibility, he notes that the Leo Spur galaxies, which would be located in between the Local Sheet and the attractive area, appear to be at rest with respect to this motion. Villata suggests that the origins of the third component may be on the opposite side, repelling the Local Sheet instead of attracting it. He calculates that a reasonable antimatter mass, located in a particular void, could account for the local velocity anomaly by the mechanism of repulsive gravity.
> In this way, the antimatter would act like dark energy in our local neighborhood.
> On a large scale, numerous antimatter voids could drive the expansion of the Universe without the need for dark energy, and possibly even without the need for an explosive Big Bang (perhaps implying a cyclic Universe). The theory also implies that we live in a Universe with equal amounts of matter and antimatter, as expected by standard theories. To Villata, these results make repulsive gravity an alluring alternative to dark energy.
Grrr. I wish people would stop saying the BB was some kind of explosion - it leads to misconceptions about the nature of the BB. Yes, there was expansion during the BB, especially during the (postulated) inflation phase, but that expansion was very smooth and symmetrical, not chaotic, like a typical explosion. If you blow a soap bubble with a bubble-pipe, the bubble certainly expands, but you'd hardly call that an explosion.
The other thing with typical explosions is that they involve a rapid release of energy, which causes the exploding material to heat up. But the BB was at its hottest at its onset and thenceforth proceeded to cool down, with various types of force and matter "freezing out" as the temperature became cold enough for them to persist.