Isotropy theorem for cosmological Yang-Mills theories

et al. 2018

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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.

Section: Jhep08(2018)073mentioning

confidence: 95%

Constraints on anharmonic corrections of fuzzy dark matter

et al. 2018

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“…His results can be recovered by means of a generalization of the virial theorem [60]. Recently, it has been shown that a fast oscillating abelian vector [61], non-abelian vector [62,63] or arbitrary spin field [64] will behave in a very similar way.…”

Section: Introductionmentioning

confidence: 90%

Cosmological perturbations in coherent oscillating scalar field models

2016

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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.

“…The main problem which arises in the case of vectors or higher spin fields is that coherent homogeneous fields typically break isotropy. However it has been recently shown (see [54,55] for abelian vector fields, [56] for nonabelian theories and [57] for arbitrary spin) that for rapidly oscillating coherent fields, even though the field evolution is generically anisotropic, the average energy-momentum tensor is not. In particular, for massive fields it is straightforward to show that the average energy density scales as a −3 .…”

Section: Jhep02(2017)064mentioning

confidence: 99%

Perturbations of ultralight vector field dark matter

2017

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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.