Perturbations of ultralight vector field dark matter

Abstract

Abstract: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 Hma, we have a particle-like behaviour indistinguishable from cold dark matter, whereas for modes with k 2Hma, we get a wave-like behaviour in which the sound speed is non-vanishing and of order c 2 s k 2 /m 2 a 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. More specifically, in the particle regime, the three types of perturbations are actually decoupled, whereas in the wave regime, the three vector field perturbations generate one scalar-tensor and two vector-tensor perturbations in the metric. Also in the wave regime, we find that a non-vanishing anisotropic stress is present in the perturbed energy-momentum tensor giving rise to a gravitational slip of order (Φ − Ψ)/Φ ∼ c 2 s . Moreover in this regime the amplitude of the tensor to scalar ratio of the scalar-tensor modes is also h/Φ ∼ c 2 s . This implies that small-scale density perturbations are necessarily associated to the presence of gravity waves in this model. We compare their spectrum with the sensitivity of present and future gravity waves detectors.

Condensate of massive graviton and dark matter

“…For the calculations, we use the adiabatic expansion in terms of the graviton mass inverse m −1 (see Section 2.5 in [44] for calculations in the case of the scalar condensate and [45] for the vector case). We set the orders of both g µν (t) and δg µν (t, x) as O(m 0 ).…”

Constraints on anharmonic corrections of fuzzy dark matter

“…Although the work on ultralight fields has been mainly concerned with scalar fields, there are recent results on higher spin fields. It has been shown that abelian vectors at the background [22] and perturbation level [23], non-abelian vectors [24] and arbitrary-spin fields [25] behave in a similar way. Interestingly, the results of [25] show that it is possible to achieve an isotropic model of higher-spin dark matter as long as it is rapidly oscillating.…”

Dark photon searches with atomic transitions

“…In fact, although the coherent state of the DP may have a particular polarization, it has associated an isotropic average energy-momentum tensor [9,12] and supports isotropic cosmologies. In any case, bounds on the DP mass can be obtained from supporting a proper structure formation in the early Universe [13,14].…”