Perturbations of ultralight vector field dark matter

Understand

We study the dynamics of cosmological perturbations in models of dark matter based on ultralight coherent vector fields.

• Very much as for scalar field dark matter, we find two different regimes in the evolution: for modes with $k^2\ll {\cal H}ma$, we have a particle-like behaviour indistinguishable from cold dark matter, whereas for modes with $k^2\gg {\cal H}ma$, we get a wave-like behaviour in which the sound speed is non-vanishing and of order $c_s^2\simeq k^2/m^2a^2$.
• This implies that, also in these models, structure formation could be suppressed on small scales.
• However, unlike the scalar case, the fact that the background evolution contains a non-vanishing homogeneous vector field implies that, in general, the evolution of the three kinds of perturbations (scalar, vector and tensor) can no longer be decoupled at the linear level.