Magnetic fields in galaxies are produced via the amplification of seed magnetic fields of unknown nature. The seed fields, which might exist in their initial form in the intergalactic medium, were never detected. We report a lower bound B > or = 3 x 10(-16) gauss on the strength of intergalactic magnetic fields, which stems from the nonobservation of GeV gamma-ray emission from electromagnetic cascade initiated by tera-electron volt gamma rays in intergalactic medium. The bound improves as lambdaB(-1/2) if magnetic field correlation length, lambdaB, is much smaller than a megaparsec. This lower bound constrains models for the origin of cosmic magnetic fields.

We review the possible mechanisms for the generation of cosmological magnetic fields, discuss their evolution in an expanding Universe filled with the cosmic plasma and provide a critical review of the literature on the subject. We put special emphasis on the prospects for observational tests of the proposed cosmological magnetogenesis scenarios using radio and gammaray astronomy and ultra high energy cosmic rays. We argue that primordial magnetic fields are observationally testable. They lead to magnetic fields in the intergalactic medium with magnetic field strength and correlation length in a well defined range.We also state the unsolved questions in this fascinating open problem of cosmology and propose future observations to address them.

Context. Attenuation of the TeV γ-ray flux from distant blazars through pair production with extragalactic background light leads to the development of electromagnetic cascades and subsequent, lower energy, GeV secondary γ-ray emission. Due to the deflection of VHE cascade electrons by extragalactic magnetic fields (EGMF), the spectral shape of this arriving cascade γ-ray emission is dependent on the strength of the EGMF. Thus, the spectral shape of the GeV-TeV emission from blazars has the potential to probe the EGMF strength along the line of sight to the object. Constraints on the EGMF previously derived from the gamma-ray data suffer from an uncertainty related to the non-simultaneity of GeV and TeV band observations. Aims. We investigate constraints on the EGMF derived from observations of blazars for which TeV observations simultaneous with those by Fermi telescope were reported. We study the dependence of the EGMF bound on the hidden assumptions it rests upon. Methods. We select blazar objects for which simultaneous Fermi/LAT GeV and Veritas, MAGIC or HESS TeV emission have been published. We model the development of electromagnetic cascades along the gamma-ray beams from these sources using Monte Carlo simulations, including the calculation of the temporal delay incurred by cascade photons, relative to the light propagation time of direct γ-rays from the source. Results. Constraints on the EGMF could be derived from the simultaneous GeV-TeV data on the blazars RGB J0710+591, 1ES 0229+200, and 1ES 1218+304. The measured source flux level in the GeV band is lower than the flux of the expected cascade component calculated under the assumption of zero EGMF. Assuming that the reason for the suppression of the cascade component is the extended nature of the cascade emission, we find that B 10 −15 G (assuming an EGMF correlation length of ≥1 Mpc) is consistent with the data. Alternatively, the assumption that the suppression of the cascade emission is caused by the time delay of the cascade photons the data are consistent with B 10 −17 G for the same correlation length.

Abstract. We present a comprehensive review of keV-scale sterile neutrino Dark Matter, collecting views and insights from all disciplines involved -cosmology, astrophysics, nuclear, and particle physics -in each case viewed from both theoretical and experimental/observational perspectives. After reviewing the role of active neutrinos in particle physics, astrophysics, and cosmology, we focus on sterile neutrinos in the context of the Dark Matter puzzle. Here, we first review the physics motivation for sterile neutrino Dark Matter, based on challenges and tensions in purely cold Dark Matter scenarios. We then round out the discussion by critically summarizing all known constraints on sterile neutrino Dark Matter arising from astrophysical observations, laboratory experiments, and theoretical considerations. In this context, we provide a balanced discourse on the possibly positive signal from X-ray observations. Another focus of the paper concerns the construction of particle physics models, aiming to explain how sterile neutrinos of keV-scale masses could arise in concrete settings beyond the Standard Model of elementary particle physics. The paper ends with an extensive review of current and future astrophysical and laboratory searches, highlighting new ideas and their experimental challenges, as well as future perspectives for the discovery of sterile neutrinos.

We explore potential of current and next-generation γ-ray telescopes for the detection of weak magnetic fields in the intergalactic medium. We demonstrate that using two complementary techniques, observation of extended emission around point sources and observation of time delays in γ-ray flares, one would be able to probe most of the cosmologically and astrophysically interesting part of the "magnetic field strength" vs. "correlation length" parameter space. This implies that γ-ray observations with Fermi and ground-based Cherenkov telescopes will allow to (a) strongly constrain theories of the origin of magnetic fields in galaxies and galaxy clusters and (b) discover, constrain or rule out the existence of weak primordial magnetic field generated at different stages of evolution of the Early Universe.