Cosmic axion thermalization

Abstract

Axions differ from the other cold dark matter candidates in that they form a degenerate Bose gas. It is shown that their huge quantum degeneracy and large correlation length cause cold dark matter axions to thermalize through gravitational self-interactions when the photon temperature reaches approximately 500 eV. When they thermalize, the axions form a Bose-Einstein condensate. Their thermalization occurs in a regime, herein called the `condensed regime', where the Boltzmann equation is not valid because the energy dispersion of the particles is smaller than their interaction rate. We derive analytical expressions for the thermalization rate of particles in the condensed regime, and check the validity of these expressions by numerical simulation of a toy model. We revisit axion cosmology in light of axion Bose-Einstein condensation. It is shown that axions are indistinguishable from ordinary cold dark matter on all scales of observational interest, except when they thermalize or rethermalize. The rethermalization of axions that are about to fall in a galactic potential well causes them to acquire net overall rotation as they go to the lowest energy state consistent with the total angular momentum they acquired by tidal torquing. This phenomenon explains the occurrence of caustic rings of dark matter in galactic halos. We find that photons may reach thermal contact with axions and investigate the implications of this possibility for the measurements of cosmological parameters.Comment: 38 pages, 1 figur

Attractive versus repulsive interactions in the Bose-Einstein condensation dynamics of relativistic field theories

“…[45]). 15 As already mentioned, our considered initial conditions generate no Oð2Þ charge. However, with a slight modification of the initial conditions, which on the level of a single simulation consists in correlating the initial oscillation phases of each of the field components, a nonvanishing charge Q can be generated.…”

“…As can be seen, most of the system's charge is concentrated in an almost spherical object, 14 Similar to the single-component case, a nonzero bare mass further suppresses particle number-changing processes [42]. 15 For the case of axions, long-range gravitational interactions are supposed to be the dominant attractive interaction. In this case, it is not a priori clear if the number-changing processes are similarly effective or if there exists an approximate particle number conservation.…”

“…[7][8][9]) and would leave distinct imprints in direct detection experiments [10][11][12]. While many arguments have focused on the formation rate of a condensate [13][14][15][16][17][18][19], a qualitatively even more important point is the attractive nature of the relevant interactions [20,21] that tends to favor localized structures instead of a spatially constant condensate [21]. Inspired by this, it is one of our main aims to study the impact of attractive interactions and to delineate the differences to the repulsive case.…”

Cosmological perturbations in coherent oscillating scalar field models

“…This subject has been mainly JHEP03(2016)013 studied for harmonic potential models that mimic the standard dark matter case [65][66][67][68][69][70][71], as it happens for the axion field [72,73]. It has been proved by using the linear perturbation theory that the axion was equivalent to CDM for high enough masses [74][75][76][77].…”

The New BBN Model with the Photon Cooling, X Particle, and the Primordial Magnetic Field

“…A first model is the photon cooling. Considering the photon cooling, the predicted baryonto-photon ratio by the BEC model [14] is smaller by a factor of (2/3) 3/4 at the BBN epoch: BBN = 4.57 0.10 10 . In this model, the abundances of D, 3 He and 6 Li increase, while, abundances of 4 He and 7 Li decrease.…”