An ultralight free scalar field with mass around 10 −23 − 10 −22 eV is a viable dark mater candidate, which can help to resolve some of the issues of the cold dark matter on sub-galactic scales. We consider the gravitational field of the galactic halo composed out of such dark matter. The scalar field has oscillating in time pressure, which induces oscillations of gravitational potential with amplitude of the order of 10 −15 and frequency in the nanohertz range. This frequency is in the range of pulsar timing array observations. We estimate the magnitude of the pulse arrival time residuals induced by the oscillating gravitational potential. We find that for a range of dark matter masses, the scalar field dark matter signal is comparable to the stochastic gravitational wave signal and can be detected by the planned SKA pulsar timing array experiment. arXiv:1309.5888v1 [astro-ph.CO]

Momentum dissipation is an important ingredient in condensed matter physics that requires a translation breaking sector. In the bottom-up gauge/gravity duality, this implies that the gravity dual is massive. We start here a systematic analysis of holographic massive gravity (HMG) theories, which admit field theory dual interpretations and which, therefore, might store interesting condensed matter applications. We show that there are many phases of HMG that are fully consistent effective field theories and which have been left overlooked in the literature. The most important distinction between the different HMG phases is that they can be clearly separated into solids and fluids. This can be done both at the level of the unbroken spacetime symmetries as well as concerning the elastic properties of the dual materials. We extract the modulus of rigidity of the solid HMG black brane solutions and show how it relates to the graviton mass term. We also consider the implications of the different HMGs on the electric response. We show that the types of response that can be consistently described within this framework is much wider than what is captured by the narrow class of models mostly considered so far.

The existence of light particles with spin during inflation is prohibited by the Higuchi bound. This conclusion can be evaded if one considers states with a sizeable coupling with the inflaton foliation, since this breaks the de Sitter isometries. The action for these states can be constructed within the Effective Field Theory of Inflation, or using a CCWZ procedure. Light particles with spin have prescribed couplings with soft inflaton perturbations, which are encoded in consistency relations. We study the phenomenology of light states with spin 2. These mix with the graviton changing the tensor power spectrum and can lead to sizeable tensor non-Gaussianities. They also give rise to a scalar bispectrum and trispectrum with a characteristic angle-dependent non-Gaussianity.

We apply phase-space density considerations to obtain lower bounds on the mass of sterile neutrino as dark matter candidate. The bounds are different for non-resonant production, resonant production in the presence of lepton asymmetry and production in decays of heavier particles. In the former case our bound is comparable to, but independent of the Lyman-α bound, and together with X-ray upper limit it disfavors non-resonantly produced sterile neutrino dark matter. An interesting feature of the latter case is that warm dark matter may be composed of heavy particles.One candidate for dark matter particle is sterile neutrino. In the first place, the existence of sterile neutrinos is favored by the discovery of neutrino oscillations. Furthermore, by adding sterile neutrinos to the Standard Model and fine-tuning parameters in the neutrino sector, one is able to explain all known facts in high energy physics and cosmology without introducing any other new fields except for inflaton [1,2,3].The sterile neutrino dark matter should satisfy cosmological and astrophysical constraints. These constraints depend on the way sterile neutrinos are produced in the early Universe. There are several mechanisms capable of generating sterile neutrino in right amount. These include the non-resonant production via active-sterile neutrino mixing [4], resonant production in the presence of lepton asymmetry [5,6], production in decays of heavier particles (see, e.g., Refs. [3,7] in the context of νMSM model) and production in scattering (see e.g., Ref. [8]). In this paper we consider astrophysical constraints on sterile neutrino dark matter generated by the non-resonant production mechanism. We comment on other production mechanisms towards the end of this paper.Sterile neutrino is coupled to active ones through mixing only. Thus, the mixing angle controls the decay rate of sterile neutrino into active neutrino and photons. These decays in galactic halos would produce X-ray emission lines, which are severely constrained [9] by 1

We make use of the phase space density approach to discuss gravitino as a warm dark matter candidate. Barring fine tuning between the reheat temperature in the Universe and superparticle masses, we find that warm gravitinos have both appropriate total mass density, ΩG = Ω DM ≃ 0.2, and suitable primordial phase space density at low momenta provided that their mass is in the range 1 keV mG 15 keV, the reheat temperature in the Universe is low, T R 10 TeV, and masses of some of the superparticles are sufficiently small, M 350 GeV. The latter property implies that the gravitino warm dark matter scenario will be either ruled out or supported by the LHC experiments.