The baryon asymmetry problem in physics refers to the fact that there is an imbalance in baryonic matter and antibaryonic matter in the observable universe. Neither the standard model of particle physics, nor the theory of general relativity provides an obvious explanation for why this should be so, and it is a natural assumption that the universe be neutral with all conserved charges. The Big Bang should have produced equal amounts of matter and antimatter. Since this does not seem to be the case, it is likely some physical laws must have acted differently or did not exist for matter and antimatter. There are several competing hypotheses to explain the imbalance of matter and antimatter that resulted in baryogenesis, but there is as of yet no one consensus theory to explain the phenomenon.
CP (charge parity) violations
Most explanations involve modifying the standard model of particle physics, to allow for some reactions (specifically involving the weak nuclear force) to proceed more easily than their opposite. This is called "violating CP symmetry" in weak interactions. Such a violation could allow matter to be produced more commonly than antimatter in conditions immediately after the Big Bang. In 2013 LHCb announced discovery of CP violation in B meson decays, so did BaBar and Belle scientists in 2015.
Regions of the universe where antimatter dominates
Another possible explanation of the apparent baryon asymmetry is that there are regions of the universe in which matter is dominant, and other regions of the universe in which antimatter is dominant, and these are widely separated. The problem then becomes a matter/antimatter separation problem, rather than a creation imbalance problem. Antimatter atoms would appear from a distance indistinguishable from matter atoms, as both matter and antimatter atoms would produce light (photons) in the same way. Only in the border between a matter dominated region and an antimatter dominated region would the antimatter's presence be detectable, as only there would matter/antimatter annihilation (and the subsequent production of gamma radiation) occur. How easy such a boundary would be to detect would depend on its distance and what the density of matter and antimatter is along it. Presumably such a boundary would lie (almost by necessity) in deep intergalactic space, and the density of matter in intergalactic space is reasonably well established at about one atom per cubic metre. Assuming this is the typical density of both matter and antimatter near a boundary, the gamma ray luminosity of the boundary interaction zone is easily calculated. Approximately 30 years of scientific research have placed boundaries on how far away, at a minimum, any such boundary interaction zone would have to be, as no such zones have been detected. Hence, it is now considered very unlikely that any region within the observable universe is dominated by antimatter.
Yet another possibility is that antimatter repels ordinary matter rather than attracting it gravitationally. This would prevent observable interactions (see Motivations for antigravity). However, this idea is in conflict with general relativity. Einstein's field equations state that the energy–momentum tensor is the source of the gravitational field, which implies that gravity is attractive for antimatter. Furthermore, there are no astronomical observations that suggest the existence of a repulsive gravitational force between any two galaxies or galaxy clusters other than that caused by the overall accelerated expansion of the universe, and the vast majority of scientists believe that matter and antimatter attract each other gravitationally (see Antimatter gravity debate).
Electric dipole moment
The presence of an electric dipole moment (EDM) in any fundamental particle would violate both parity (P) and time (T) symmetries. As such, an EDM would allow matter and antimatter to decay at different rates leading to a possible matter-antimatter asymmetry as observed today. Many experiments are currently being conducted to measure the EDM of various physical particles. All measurements are currently consistent with no dipole moment. However, the results do place rigorous constraints on the amount of symmetry violation that a physical model can permit. The most recent EDM limit, published in 2014, was that of the ACME Collaboration, which measured the EDM of the electron using a pulsed beam of thorium monoxide (ThO) molecules.
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