We study the effects of ultralight vector field (ULVF) dark matter on gravitational wave propagation. We find that the coherent oscillations of the vector field induce an anisotropic suppression of the gravitational wave amplitude as compared to the standard cosmology prediction. The effect is enhanced for smaller vector field masses and peaks for modes aroundThe suppression is negligible for astrophysically generated gravitational waves but could be sizable for primordial gravity waves. We discuss the possibility of detecting such an effect on the tensor power spectrum with future cosmic microwave background B-mode polarization detectors. We find that for the sensitivity of the upcoming LiteBIRD mission, the correction to the tensor power spectrum at decoupling time could be distinguishable from that of ΛCDM for ULVF masses m ≲ 10 −26 eV and sufficiently large abundances.
We consider conformal vector models which could play the role of a cosmological dark
radiation component. We analyse the propagation of gravitational waves in the presence of this
vector background and find a suppression in the tensor transfer function at large scales. We also
find that although the cosmological background metric is isotropic, anisotropies are imprinted in
the tensor power spectrum. In addition, the presence of the background vector fields induces a net
polarization of the gravitational wave background and, for certain configurations of the vector
field, a linear to circular polarization conversion. We also show that this kind of effects are
also present for vector models with more general potential terms.
In this work we make the observation that the gravitational leptogenesis mechanism can be implemented without invoking new axial couplings in the inflaton sector. We show that in the perturbed Robertson-Walker background emerging after inflation, the spacetime metric itself breaks parity symmetry and generates a non-vansihing Pontryagin density which can produce a matter-antimatter asymmetry. We analyze the inflationary and reheating scenarios in which the produced asymmetry could be compatible with observations.
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