Enhanced stellar neutrino emissivities in annihilating Coy Dark Matter [CL]


We calculate neutrino emissivities for annihilating Dark Matter with pseudoscalar-mediated couplings to ordinary matter in dense stellar environments. We show that for Dark Matter models compatible with current direct detection limits and cosmological relic abundance, the neutrino production rates from channels $\chi \chi \rightarrow \nu \nu$ or via pseudoscalar mediators $\chi \chi \rightarrow a a$ with subsequent decay $ a \rightarrow \nu \nu$, can be locally much stronger than the modified URCA processes at early stages of star cooling. In light of these results we discuss the parameter ranges and thermodynamical stellar conditions that could give rise to inner temperature profiles warmer than those obtained for standard scenarios, having thus an impact on possible superfluid components in the inner core.

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M. Cermeno, M. Perez-Garcia and R. Lineros
Fri, 19 May 17

Comments: 5 pages, 2 figures, and 1 table. Comments welcome

On the Inflationary Perturbations of Massive Higher-Spin Fields [CL]


Cosmological perturbations of massive higher-spin fields are generated during inflation, but they decay on scales larger than the Hubble radius as a consequence of the Higuchi bound. By introducing suitable couplings to the inflaton field, we show that one can obtain statistical correlators of massive higher-spin fields which remain constant or decay very slowly outside the Hubble radius. This opens up the possibility of new observational signatures from inflation.

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A. Kehagias and A. Riotto
Fri, 19 May 17

Comments: 22 pages

Is Patience a Virtue? Cosmic Censorship of Infrared Effects in de Sitter [CL]


While the accumulation of long wavelength modes during inflation wreaks havoc on the large scale structure of spacetime, the question of even observability of their presence by any local observer has lead to considerable confusion. Though it is commonly agreed that infrared effects are not visible to a single sub-horizon observer at late times, we argue that the question is less trivial for a \emph{patient observer} who has lived long enough to have a record of the state before the soft mode was created. Though classically there is no obstruction to measuring this effect locally, we give several indications that quantum mechanical uncertainties censor the effect, rendering the observation of long modes ultimately forbidden.

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R. Ferreira, M. Sandora and M. Sloth
Fri, 19 May 17

Comments: 7 pages, awarded honorable mention in the 2017 Gravity Research Foundation Essays on Gravitation competition

A proof of the weak gravity conjecture [CL]


The weak gravity conjecture suggests that, in a self-consistent theory of quantum gravity, the strength of gravity is bounded from above by the strengths of the various gauge forces in the theory. In particular, this intriguing conjecture asserts that in a theory describing a U(1) gauge field coupled consistently to gravity, there must exist a particle whose proper mass is bounded (in Planck units) by its charge: $m/m_{\text{P}}<q$. This beautiful and remarkably compact conjecture has attracted the attention of physicists and mathematicians over the last decade. It should be emphasized, however, that despite the fact that there are numerous examples from field theory and string theory that support the conjecture, we still lack a general proof of its validity. In the present Letter we prove that the weak gravity conjecture (and, in particular, the mass-charge upper bound $m/m_{\text{P}}<q$) can be inferred directly from Bekenstein’s generalized second law of thermodynamics, a law which is widely believed to reflect a fundamental aspect of the elusive theory of quantum gravity.

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S. Hod
Fri, 19 May 17

Comments: 7 pages. This essay is awarded 4th Prize in the 2017 Essay Competition of the Gravity Research Foundation

The Cosmological Constant and its Problems: A Review of Gravitational Aether [CL]


In this essay we offer a comprehensible overview of the gravitational aether scenario. This is a possible extension of Einstein’s theory of relativity to the quantum regime via an effective approach. Quantization of gravity usually faces several issues including an unexpected high vacuum energy density caused by quantum fluctuations. The model presented in this paper offers a solution to the so-called cosmological constant problems.
As its name suggests, the gravitational aether introduces preferred reference frames, while it remains compatible with the general theory of relativity. As a rare feature among quantum gravity inspired theories, it can predict measurable astronomical and cosmological effects. Observational data disfavor the gravitational aether scenario at $2.6\text{-}5\,\sigma$. This experimental feedback gives rise to possible refinements of the theory.

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M. Wondrak
Fri, 19 May 17

Comments: 10 pages, 2 figures. To appear in the collection of the fifth international conference on “Experimental Search for Quantum Gravity”, held at FIAS, Frankfurt am Main (Germany), 19-23 September 2016, Springer

On the evolution of mass density power-law index in strong gravitational lensing: cosmological model independent constraints [CL]


Strong gravitational lensing is a powerful cosmological tool, furnishing angular diameter distances independent of local calibrators and cosmic transparency. However, a crucial point in the strong gravitational lensing science is the knowledge of exact matter distribution of lens. Nowadays, studies have shown that slopes of density profiles of individual galaxies exhibit a non-negligible scatter from the simplest model, the singular isothermal sphere ($\rho \propto r^{-2}$), and a spherically symmetric power-law mass distribution has been assumed ($\rho \propto r^{-\gamma}$) as a generalization, including a possible time-evolution of the $\gamma$ parameter. In this work, by using strong gravitational lensing observations, SNe Ia data and the cosmic distance duality relation validity, we propose a cosmological model independent method to explore if the $\gamma$ parameter is time-dependent. As is largely known, a $\gamma$ evolution may play a crucial role on galaxy structure. We use three different parametrizations for $\gamma(z)$, namely: $\gamma(z)=\gamma_0+\gamma_1 z$, $\gamma(z)=\gamma_0+\gamma_1 z/(1+z)$ and $\gamma(z)=\gamma_0+\gamma_1 \ln(1+z)$. No significant evidence for $\gamma$ evolution was verified with present data.

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R. Holanda, S. Pereira and D. Jain
Fri, 19 May 17

Comments: 6 pages, 6 figures

The Shape of Bouncing Universes [CL]


What happens to the most general closed oscillating universes in general relativity? We sketch the development of interest in cyclic universes from the early work of Friedmann and Tolman to modern variations introduced by the presence of a cosmological constant. Then we show what happens in the cyclic evolution of the most general closed anisotropic universes provided by the Mixmaster universe. We show that in the presence of entropy increase its cycles grow in size and age, increasingly approaching flatness. But these cycles also grow increasingly anisotropic at their expansion maxima. If there is a positive cosmological constant, or dark energy, present then these oscillations always end and the last cycle evolves from an anisotropic inflexion point towards a de Sitter future of everlasting expansion.

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J. Barrow and C. Ganguly
Fri, 19 May 17

Comments: Awarded Honorable Mention in the 2017 Gravity Research Foundation Essay competition; 4 pages, 6 figures