The relic density of dark matter in the ΛCDM model restricts the parameter space for a cosmological axion field, constraining the axion decay constant, the initial amplitude of the axion field and the axion mass. It is shown via lattice simulations how the relic density of axion-like particles with masses close to the one of the QCD axion is affected by axion-gauge field interactions and by initial axion inhomogeneities. For pre-inflationary axions, once the Hubble parameter becomes smaller than the axion mass, the latter starts to oscillate, and part of its energy density is spent producing gauge fields via parametric resonance. If the gauge fields are dark photons and Standard Model photons, the energy density of dark photons becomes higher than the one of the axion, while the high conductivity of the primordial plasma damps the oscillations of the photon field. Such a scenario allows for the production of small-scale, primordial magnetic fields, and it is found that the relic density of axions with a low decay constant are within the bounds set by the ΛCDM model, while GUT-scale axions are far too abundant. It is also shown that initial inhomogeneities of the axion field can change substantially the gauge field production, boosting or suppressing (depending on the axion parameters and couplings) the magnetogenesis mechanism with respect to an homogeneous axion field. It is found that when the axion mass is far lighter than the QCD axion model and the initial axion field is inhomogeneous, weak but cosmologically relevant magnetic field seeds can be generated on scales of the order of 0.1 kpc.

中文翻译：

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轴子和原始磁发生：初始轴子不均匀性的作用


ΛCDM模型中暗物质的遗迹密度限制了宇宙轴子场的参数空间，限制了轴子衰变常数、轴子场的初始振幅和轴子质量。通过晶格模拟显示了质量接近 QCD 轴子的类轴子粒子的残余密度如何受到轴子规范场相互作用和初始轴子不均匀性的影响。对于暴胀前的轴子，一旦哈勃参数变得小于轴子质量，轴子质量就开始振荡，其部分能量密度用于通过参数共振产生规范场。如果规范场是暗光子和标准模型光子，则暗光子的能量密度变得高于轴子的能量密度，而原始等离子体的高电导率会抑制光子场的振荡。这种情况允许产生小规模的原始磁场，并且发现具有低衰减常数的轴子的遗迹密度在ΛCDM模型设定的范围内，而GUT尺度的轴子过于丰富。还表明，轴子场的初始不均匀性可以显着改变规范场的产生，增强或抑制（取决于轴子参数和耦合）相对于均匀轴子场的磁生机制。研究发现，当轴子质量远轻于QCD轴子模型且初始轴子场不均匀时，可以在0.1 kpc量级上产生微弱但与宇宙学相关的磁场种子。