Gravitational effects of condensed dark matter on strange stars [CL]

In the present work we study the gravitational effects of condensed dark matter on strange stars. We consider self-interacting dark matter particles with properties consistent with current observational constraints, and dark matter inside the star is modelled as a Bose-Einstein condensate. We integrate numerically the Tolman-Oppenheimer-Volkoff equations in the two-fluid formalism assuming that strange stars are made of up to 4 per cent of dark matter. It is shown that for a mass of the dark matter particles in the range $50 MeV-160 MeV$ strange stars are characterized by a maximum mass and radius similar to the ones found for neutron stars.

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G. Panotopoulos and I. Lopes
Fri, 23 Jun 17

Comments: Two-column REVTEX, 5 pages, 3 figures

Thermal Dark Matter Below an MeV [CL]

We consider a class of models in which thermal dark matter is lighter than an MeV. If dark matter thermalizes with the Standard Model below the temperature of neutrino-photon decoupling, equilibration and freeze-out cools and heats the Standard Model bath comparably, alleviating constraints from measurements of the effective number of neutrino species. We demonstrate this mechanism in a model consisting of fermionic dark matter coupled to a light scalar mediator. Thermal dark matter can be as light as a few keV, while remaining compatible with existing cosmological and astrophysical observations. This framework motivates new experiments in the direct search for sub-MeV thermal dark matter and light force carriers.

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A. Berlin and N. Blinov
Fri, 23 Jun 17

Comments: 5 pages, 4 figures

Are eikonal quasinormal modes linked to the unstable circular null geodesics? [CL]

In Phys. Rev. D 79, 064016 (2009) it was claimed that quasinormal modes which any stationary, spherically symmetric and asymptotically flat black hole emits in the eikonal regime are determined by the parameters of the circular null geodesic: the real and imaginary parts of the quasinormal mode are multiples of the frequency and instability timescale of the circular null geodesics respectively. We shall consider asymptotically flat black hole in the Einstein-Lovelock theory, find analytical expressions for gravitational quasinormal modes in the eikonal regime and analyze the null geodesics. Comparison of the both phenomena shows that the expected link between the null geodesics and quasinormal modes is violated in the Einstein-Lovelock theory. Nevertheless, the correspondence exists for a number of other cases and here we formulate its actual limits.

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R. Konoplya and Z. Stuchlik
Fri, 23 Jun 17

Comments: 8 pages, revtex, refereed version, accepted for publication in Physics Letters B

Modification of magnetohydrodynamic waves by the relativistic Hall effect [CL]

This study shows that a relativistic Hall effect significantly changes the properties of wave propagation by deriving a linear dispersion relation for relativistic Hall magnetohydrodynamics (HMHD). Whereas, in non- relativistic HMHD, the phase and group velocities of fast magnetosonic wave become anisotropic with an increasing Hall effect, the relativistic Hall effect brings upper bounds to the anisotropies. The Alfve\'{e}n wave group velocity with strong Hall effect also becomes less anisotropic than non-relativistic case. Moreover, the group velocity surfaces of Alfv\'{e}n and fast waves coalesce into a single surface in the direction other than near perpendicular to the ambient magnetic field. It is also remarkable that a characteristic scale length of the relativistic HMHD depends on ion temperature, magnetic field strength, and density while the non-relativistic HMHD scale length, i.e., ion skin depth, depends only on density. The modified characteristic scale length increases as the ion temperature increases and decreases as the magnetic field strength increases.

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Y. Kawazura
Fri, 23 Jun 17

Comments: N/A

Wormholes and masses for Goldstone bosons [CL]

There exist non-trivial stationary points of the Euclidean action for an axion particle minimally coupled to Einstein gravity, dubbed wormholes. They explicitly break the continuos global shift symmetry of the axion in a non-perturbative way, and generate an effective potential that may compete with QCD depending on the value of the axion decay constant. In this paper, we explore both theoretical and phenomenological aspects of this issue. On the theory side, we address the problem of stability of the wormhole solutions, and we show that the spectrum of the quadratic action features only positive eigenvalues. On the phenomenological side, we discuss, beside the obvious application to the QCD axion, relevant consequences for models with ultralight dark matter, black hole superradiance, and the relaxation of the electroweak scale. We conclude discussing wormhole solutions for a generic coset and the potential they generate.

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R. Alonso and A. Urbano
Fri, 23 Jun 17

Comments: 50 pages, 15 figures

Effect of collisions on neutrino flavor inhomogeneity in the early universe [CL]

We investigate the stability, with respect to spatial inhomogeneity, of a two-dimensional dense neutrino gas. The system exhibits growth of seed inhomogeneity due to nonlinear coherent neutrino self-interactions. In the absence of incoherent collisional effects, we observe a dependence of this instability growth rate on the neutrino mass spectrum: the normal neutrino mass hierarchy exhibits spatial instability over a larger range of neutrino number density compared to that of the inverted case. We further consider the effect of elastic incoherent collisions of the neutrinos with a static background of heavy, nucleon-like scatterers. At small scales, the growth of flavor instability can be suppressed by collisions. At large length scales we find, perhaps surprisingly, that for inverted neutrino mass hierarchy incoherent collisions fail to suppress flavor instabilities, independent of the coupling strength.

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V. Cirigliano, M. Paris and S. Shalgar
Fri, 23 Jun 17

Comments: 10 pages, 6 figures

Infused Ice can Multiply IceCube's Sensitivity [CL]

The IceCube Neutrino Observatory is preparing for a major upgrade that may increase its sensitivity by an order of magnitude. A promising technological innovation under investigation for this upgrade is wavelength shifting optics, which could increase the photo-collection area of IceCube’s digital optical modules (DOMs), and shift photon wavelength to where DOMs are more sensitive. Here we investigate the use of IceCube’s drill holes themselves as wavelength shifting optics. We calculate the sensitivity enhancement due to (i) increasing the ice’s refractive index in the holes, and (ii) infusing wavelength-shifting substrate into the ice. We find that, with adequate wavelength-shifter infusion, every ~0.05 increase in the ice’s refractive index will increase IceCube’s sensitivity by 100%, opening the possibility for the substantial, cost effective expansion of IceCube’s reach.

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I. Bartos, Z. Marka and S. Marka
Thu, 22 Jun 17

Comments: 4 pages, 3 figures