Repulsive Gravity?

• Baryogenesis: MAR would allow the universe to be made up of equal proportions of matter (+) and antimatter (-). The intermingled islands of opposite sign would repel each other. Thus, the missing annihilation radiation, a strong argument in favor of a matter-alone universe, would have a natural explanation.
• Accelerated cosmic expansion: A simple symmetric arrangement of matter and antimatter can be envisaged to correspond to a sodium chloride crystal, with the sodium atoms representing matter islands, the chlorine atoms antimatter islands, and electrostatics replacing gravity. The atomic crystal is held together by electrostatic forces. These are of either sign but the interspersed arrangement of atoms makes the crystal hold together. However, upon changing the sign in the Coulomb law, which would correspond to MAR-gravity, the crystal would fly apart in an ever accelerated expansion. This is reminiscent of what has recently been found from supernova distance measurements for the expansion of the universe. A less regular arrangement of islands is not expected to spoil the essence of the analogy.
• Flatness Problem: In a mix of equal amounts, matter and antimatter would compensate each others gravitational fields, MAR assumed. The average 'gravitating density' would be zero and kept at that value owing to matter-antimatter symmetry. No fine tuning of density would be needed. However, to make this precise, it would have to be possible to extend general relativity theory such that it could cope with MAR.
• Horizon Problem: The horizon problem originates in the small-time solutions of the Friedmann equation, in which the expansion speed grows without limit for near-zero times, because the density diverges too fast. However, it is actually the 'gravitating' density which is relevant for expansion. An equal mix of matter and antimatter would, assuming MAR, add up to zero gravitating density. Thus, the small-time solution would be well behaved, a 'Slow Bang' so to speak. Expansion would only result from segregation of matter and antimatter in corresponding islands. Radiation (made up of particles that are their own antiparticles) would have to have no gravitating property, though it would travel along geodesics (gravitational lensing).
• Density Perturbations: In the absence of the horizon problem, it is plausible that thermal equilibration could have been operative at photon-decoupling time, in accord with the high isotropy of the cosmic background radiation. Perturbation modes segregating matter and antimatter would, most probably, obtain fast growing rates.