Bose-Einstein Condensate Dark Matter

A recent preprint sketches out the notion of a Bose-Einstein condensate as a dark matter candidate and finds that it is a better fit to the data than traditional cold dark matter models because it produces a mass distribution closer to that of observed rotation curves as opposed to the cuspy mass distributions found in cold dark matter models. The alignment of the quantum states of Bose-Einstein condensate matter creates an effectively repulsive force between the particles that counteracts the clumpiness one would see with gravity alone, allowing for a spread out mass distribution, as a result of the fundamental property of fermions that fermions of the same quantum state cannot co-exist at the same point in space. Settled physics describes these properties of Bose-Einstein condensates, and the low temperature environment of deep space is a natural place for this state of matter of persist.

The pre-print finds that the temperatures in deep space are cold enough for condensates to have arisen in the process of the formation process of the universe, and determines that only negligable error is introduced by approximating the behavior of a Bose-Einstein condensate in thermal equilibrium with the cosmic background radiation temperature of about 2.73 degrees kelvin as a zero temperature Bose-Einstein condensate at observationally viable scales.

The study concludes (although only somewhat obliquely) that the masses of the dark matter particles within this condensate, constrained by factors such as the data from the bullet cluster collision, should have about the same order of magnitude as neutrinos (10^-3 eV to 1 eV), which would conveniently dispense with the need to discover some new fundamental particle or type of interaction to explain dark matter effects at the galactic scale, while providing a non-baryonic dark matter candidate (since it is quark free) consistent with prior data.

The pre-print addresses earlier criticisms of Bose-Einstein condensate dark matter by claiming that the critics have not done a good job of modeling the way that Bose-Einstein condensates would behave.

Of course, this implies a univers with a whole lot of neutrinos in it, and through the notion of this dark matter as a relic of the early universe that experiences Bose-Einstein condensation when the universe finally gets cool enough, escapes the usual assumption that neutrino dark matter should be "hot" (i.e. move at relativistic speeds), which would eliminate galactic structure. The pre-print refrains from suggesting a leptogenesis mechanism that could create that many neutrinos, and likewise refrains from even making a definitive association between the predicted Bose-Einstein condensate dark matter and massive neutrinos.

If improved census estimates of the amount of baryonic matter in the universe are accurate and the ordinary matter to dark matter ratio is about 1-1, it suggests a baryogenesis/leptogenesis model in which the total mass of all of the leptons in the universe and the total mass of all the hadrons in the universe is equal.

A similar idea with right handed composite neutrinos with keV masses is explored here. Another similar theory is explored here. A beta decay test of the viability of this kind of scenario has been proposed. Another discussion of experimental evidence in astronomy for non-relativistic neutrinos as dark matter can be found here.

This paper is notable for being one of the few to address how leptogenesis could arise without violatig B-L symmetry or resorting the Majorana mass related methods, although it still requires "new physics."

It isn't obvious to me, however, that leptogenesis needs beyond the Standard Model physics to produce an excess of neutrinos over charged leptons and quarks. For example, if you have a Z boson that decays to a W+ and W- boson which in turn decay to a positron, a neutrino, an electron and an antineutrino, and if the positron and electron then annihilate into a photon or Z boson, but the neutrino and antineutrino do not (since they can couple to a Z boson, but not to a photon and could be prevented by matter-energy conservation from creating a final state with anything but neutrinos in it). Moreover, the photon or Z boson can repeat the cycle by creating a W+ and W- boson again until there isn't sufficient matter-energy in the system for the cycle to repeat itself. Charge conservation limits the number of charged leptons and is suggestive of a close link between net charged lepton number and baryon number, and the instability of systems with a lepton and its antilepton in the same vicinity prevents the actual number of charged leptons from being much greater than total baryon number in the long term. But, no similar bound impacts neutrino number,