of this type is the Bullet Cluster 14 (below), where the mass of the colliding galaxies has separated into a gas cloud and two clusters; surprisingly most of the gravity is in the galaxies whereas most of the visible matter is in the gas cloud, suggesting that something else is there. 15
NOVA science NOW, Dark Matter Mystery PBS https://www.youtube.com/watch?v=mRtGUCLjQ3w
The usual explanation for dark matter is that there are some massive particles that we cannot see 16 , either because they only interact weakly with regular matter, Weakly Interacting Massive Particles, or because they do not glow brightly enough, Massive Compact Halo Objects. The problem with this approach is twofold, what are these particles and why have we not detected them? One solution to this problem is known as Modified Newtonian Dynamics (MOND) 17 , which proposes that we can explain dark matter in terms of a modification of Newton’s Laws of motion. This can even help explain the bullet cluster which, according to Milgrom 18 can be explained by MOND as long as there is a small amount of additional regular but unobserved matter. Antigravity could provide a mechanism for MOND. We currently believe that the vacuum of space is not actually empty but full of particles and their corresponding antiparticles that flicker in and out of existence as they are produced and then annihilated. If antigravity were correct then this would create a gravitational dipole, rather like a magnet with a north and a south pole. Unlike a magnet, however, this dipole would be repulsive and therefore would augment the strength of the surrounding gravitational field 19 . This could provide a mechanism for the alteration of Newton’s Laws and would do so in such a way as to be consistent with some MOND theories. This increase in the strength of the gravitational field could explain why galaxies rotate faster than we observe without employing dark matter. In conclusion, antigravity offers a potentially convincing explanation for Dark Energy, through providing a repulsive force to accelerate the universe’s expansion. Moreover, antigravity could help explain why gravity behaves differently at larger distances within the framework of MOND in order to explain Dark Matter. These successes emphasize the importance of determining, either theoretically or experimentally, whether the gravitational interaction between matter and antimatter is attractive or repulsive.
14 Conversation with H. Brow, 27/04/14 15 NOVA science NOW, Dark Matter Mystery PBS https://www.youtube.com/watch?v=mRtGUCLjQ3w 27/08/2014 16 K. Griest, The search for the Dark Matter: WIMPs and MACHOs, Cornell University Library Online, 13/03/1993 17 M. Milgrom, (1983). ‘A modification of the Newtonian dynamics as a possible alternative to the hidden mass hypothesis’. Astrophysical Journal 18 Milgrom's perspective on the Bullet Cluster, http://www.astro.umd.edu/~ssm/mond/moti_bullet.html 13/06/2013 19 D. S. Hajdukovic (2011). ‘Is dark matter an illusion created by the gravitational polarization of the quantum vacuum?’, Astrophysics and Space Science (Online)
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