Isotropy theorem for arbitrary-spin cosmological fields

2017

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.

Section: Jhep02(2017)064mentioning

confidence: 88%

Section: Discussionmentioning

confidence: 99%

Section: Jhep02(2017)064mentioning

confidence: 99%

“…Let us consider a massive abelian vector field in an expanding universe [57]. The corresponding action reads…”

Section: Jhep02(2017)064 3 Massive Vector Cosmologymentioning

confidence: 99%

See 2 more Smart Citations

Perturbations of ultralight vector field dark matter

2017

“…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.…”

Section: Jhep08(2018)073mentioning

confidence: 99%

Constraints on anharmonic corrections of fuzzy dark matter

et al. 2018

The cold dark matter (CDM) scenario has proved successful in cosmology. However, we lack a fundamental understanding of its microscopic nature. Moreover, the apparent disagreement between CDM predictions and subgalactic-structure observations has prompted the debate about its behaviour at small scales. These problems could be alleviated if the dark matter is composed of ultralight fields m ∼ 10 −22 eV, usually known as fuzzy dark matter (FDM). Some specific models, with axion-like potentials, have been thoroughly studied and are collectively referred to as ultralight axions (ULAs) or axion-like particles (ALPs). In this work we consider anharmonic corrections to the mass term coming from a repulsive quartic self-interaction. Whenever this anharmonic term dominates, the field behaves as radiation instead of cold matter, modifying the time of matter-radiation equality. Additionally, even for high masses, i.e. masses that reproduce the cold matter behaviour, the presence of anharmonic terms introduce a cut-off in the matter power spectrum through its contribution to the sound speed. We analyze the model and derive constraints using a modified version of class and comparing with CMB and large-scale structure data.

Cosmological perturbations in coherent oscillating scalar field models

2016

The fact that fast oscillating homogeneous scalar fields behave as perfect fluids in average and their intrinsic isotropy have made these models very fruitful in cosmology. In this work we will analyse the perturbations dynamics in these theories assuming general power law potentials V (φ) = λ|φ| n /n. At leading order in the wavenumber expansion, a simple expression for the effective sound speed of perturbations is obtained c 2 eff = ω = (n − 2)/(n + 2) with ω the effective equation of state. We also obtain the first order correction in k 2 /ω 2 eff , when the wavenumber k of the perturbations is much smaller than the background oscillation frequency, ω eff . For the standard massive case we have also analysed general anharmonic contributions to the effective sound speed. These results are reached through a perturbed version of the generalized virial theorem and also studying the exact system both in the super-Hubble limit, deriving the natural ansatz for δφ; and for sub-Hubble modes, exploiting Floquet's theorem.