Late-Time Magnetogenesis Driven by Axionlike Particle Dark Matter and a Dark Photon.

We propose a mechanism generating primordial magnetic fields after the e^{+}e^{-} annihilations. Our mechanism involves an ultralight axionlike particle (ALP) which constitutes the dark matter and a dark U(1)_{X} gauge boson introduced to bypass the obstacle placed by the conductivity of cosmic plasma. In our scheme, a coherently oscillating ALP amplifies the dark photon field, and part of the amplified dark photon field is concurrently converted to the ordinary magnetic field through the ALP-induced magnetic mixing. For the relevant ALP mass range 10^{-21} eV≲m_{ϕ}≲10^{-17} eV, our mechanism can generate B∼10^{-24} G(m_{ϕ}/10^{-17} eV)^{5/4} with a coherent length λ∼(m_{ϕ}/10^{-17} eV)^{-1/2} kpc, which is large enough to provide a seed of the galactic magnetic fields. The mechanism also predicts a dark U(1)_{X} electromagnetic field E_{X}∼B_{X}∼80 nG(m_{ϕ}/10^{-17} eV)^{-1/4}, which can result in interesting astrophysical or cosmological phenomena by inducing the mixings between the ALP, ordinary photon, and dark photon states.

Self-Heating Dark Matter via Semiannihilation.

The freeze-out of dark matter (DM) depends on the evolution of the DM temperature. The DM temperature does not have to follow the standard model one, when the elastic scattering is not sufficient to maintain the kinetic equilibrium. We study the temperature evolution of the semiannihilating DM, where a pair of the DM particles annihilate into one DM particle and another particle coupled to the standard model sector. We find that the kinetic equilibrium is maintained solely via semiannihilation until the last stage of the freeze-out. After the freeze-out, semiannihilation converts the mass deficit to the kinetic energy of DM, which leads to nontrivial evolution of the DM temperature. We argue that the DM temperature redshifts like radiation as long as the DM self-interaction is efficient. We dub this novel temperature evolution as self-heating. Notably, the structure formation is suppressed at subgalactic scales like keV-scale warm DM but with GeV-scale self-heating DM if the self-heating lasts roughly until the matter-radiation equality. The long duration of the self-heating requires the large self-scattering cross section, which in turn flattens the DM density profile in inner halos. Consequently, self-heating DM can be a unified solution to apparent failures of cold DM to reproduce the observed subgalactic scale structure of the Universe.