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  • Roger L. Bagula
    http://tech.groups.yahoo.com/group/physical_sciences/message/6389 Repulsive gravity as an alternative to dark energy (Part 1) January 31st, 2012 in Physics /
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      http://tech.groups.yahoo.com/group/physical_sciences/message/6389

      Repulsive gravity as an alternative to dark energy (Part 1)
      January 31st, 2012 in Physics / General Physics
      virgo superclusterThe Local Sheet, which includes the Local Galactic
      Group and other nearby galaxies, has a peculiar velocity that can
      theoretically be explained by the repulsive gravity of antimatter in a
      large void, among other components. Image credit: Wikimedia Commons (CC
      BY-SA 3.0)

      (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 Universe’s
      accelerating expansion that involve some form of repulsive gravity. In
      this case, the repulsive gravity could stem from antimatter hiding in voids.

      “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. 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.

      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.

      “Dark energy is thought to be uniformly permeating, so it can explain
      (formally, not physically) the global acceleration,” Villata said. “But
      it cannot explain either the strong repulsive effect on the Local Sheet
      nor the extreme emptiness of the Local Void and several other properties
      of our extragalactic neighborhood, while the proposed antimatter ‘dark
      repulsor’ in the Local Void can account for all these things and, at the
      global level, with antimatter hidden in all cosmic voids, can explain
      the overall accelerated expansion (and other Universe features) without
      dark energy and the funny initial explosion.”

      Villata hopes these ideas might be tested by experiments, although such
      tests would be difficult.

      “Some people may think that my analysis of general relativity predicting
      antigravity is not correct or appropriate,” he added. “In this case, a
      further, definitive test is mentioned in my last paper: the
      antigravitational lensing effect. In principle, if we had a good 3D map
      of galaxy clusters lying beyond the voids, it would be relatively easy
      to analyze whether some of them have shapes squeezed around the line of
      sight, which would mean that they are aligned with large concentrations
      of antimatter in the intervening void. But the problem is that there is
      another concurrent effect, which strongly distorts the distribution of
      galaxies in the radial direction, due to the peculiar motions affecting
      the redshift measurements: the finger-of-god effect, which stretches the
      shape of clusters along the line of sight. It is thus very difficult to
      distinguish whether a cluster already severely stretched by this effect
      is further thinned by antigravitational lensing.”

      The second part of this article will be published tomorrow, February 1,
      2012.

      More information: Massimo Villata. “’Dark Energy’ in the Local Void.”
      Astrophysics and Space Science. DOI: 10.1007/s10509-012-0994-9 and
      arXiv:1201.3810v1 [astro-ph.CO]

      Website of Massimo Villata (comments welcome): link

      © 2011 PhysOrg.com

      "Repulsive gravity as an alternative to dark energy (Part 1)." January
      31st, 2012.
      http://www.physorg.com/news/2012-01-repulsive-gravity-alternative-dark-energy.html

      Posted by
      Robert Karl Stonjek
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