Analog
Lifer
(PhysOrg.com) -- 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. Villata is one of many scientists who are looking for new explanations of the Universes accelerating expansion that involve some form of repulsive gravity. In this case, the repulsive gravity could stem from antimatter hiding in voids.
In Villatas 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. As he explained previously, the gravitational repulsion between matter and antimatter is a prediction of general relativity. In this scenario, matter has a positive gravitational charge while antimatter has a (hypothetical) negative gravitational charge. As a result, both matter and antimatter are gravitationally self-attractive, yet mutually repulsive. 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.
Repulsive gravity of this form could even theoretically explain some observations that dark energy cannot, even theoretically, explain. Recently, scientists observed an anomalous motion of the Local Sheet, the part of the Universe that includes the Milky Way and other nearby galaxies, which has its own peculiar velocity distinct from other parts of the Universe. Astronomers have identified three components that contribute to the velocity of the Local Sheet: one is due to the well-known attraction to the nearby dense Virgo Cluster; the second component, although its origin is less clear, is thought to be due to the attraction to the Centaurus Cluster; and the third component is what astronomers call the local velocity anomaly because the force is not directed toward any significant structure.
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.
http://www.physorg.com/news/2012-01-repulsive-gravity-alternative-dark-energy.html
