Action functional of the Cardassian universe [CL]

It is known that the Cardassian universe is successful in describing the accelerated expansion of the universe, but its dynamical equations are hard to get from the action principle. In this paper, we establish the connection between the Cardassian universe and $f(T, \mathcal{T})$ gravity, where $T$ is the torsion scalar and $\mathcal{T}$ is the trace of the matter energy-momentum tensor. For dust matter, we find that the modified Friedmann equations from $f(T, \mathcal{T})$ gravity can correspond to those of Cardassian models, and thus, a possible origin of Cardassian universe is given. We obtain the original Cardassian model, the modified polytropic Cardassian model, and the exponential Cardassian model from the Lagrangians of $f(T,\mathcal{T})$ theory. Furthermore, by adding an additional term to the corresponding Lagrangians, we give three generalized Cardassian models from $f(T,\mathcal{T})$ theory. Using the observation data of type Ia supernovae, cosmic microwave background radiation, and baryon acoustic oscillations, we get the fitting results of the cosmological parameters and give constraints of model parameters for all of these models.

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X. Zhai, R. Lin, C. Feng, et. al.
Mon, 29 May 17

Comments: 7 pages, 3 figures

Relativistic wide-angle galaxy bispectrum on the light-cone [CEA]

Given the important role that the galaxy bispectrum has recently acquired in cosmology and the scale and precision of forthcoming galaxy clustering observations, it is timely to derive the full expression of the large-scale bispectrum going beyond approximated treatments which neglect integrated terms or higher-order bias terms or use the Limber approximation. On cosmological scales, relativistic effects that arise from observing on the past light-cone alter the observed galaxy number counts, therefore leaving their imprints on N-point correlators at all orders. In this paper we compute for the first time the bispectrum including all general relativistic, local and integrated, effects at second order, the tracers’ bias at second order, geometric effects as well as the primordial non-Gaussianity contribution. This is timely considering that future surveys will probe scales comparable to the horizon where approximations widely used currently may not hold; neglecting these effects may introduce biases in estimation of cosmological parameters as well as primordial non-Gaussianity.

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D. Bertacca, A. Raccanelli, N. Bartolo, et. al.
Mon, 29 May 17

Comments: 65 pages, no figures

Barnacles and Gravity [CL]

Theories with more than one vacuum allow quantum transitions between them, which may proceed via bubble nucleation; theories with more than two vacua posses additional decay modes in which the wall of a bubble may further decay. The instantons which mediate such a process have $O(3)$ symmetry (in four dimensions, rather than the usual $O(4)$ symmetry of homogeneous vacuum decay), and have been called `barnacles’; previously they have been studied in flat space, in the thin wall limit, and this paper extends the analysis to include gravity. It is found that there are regions of parameter space in which, given an initial bubble, barnacles are the favoured subsequent decay process, and that the inclusion of gravity can enlarge this region. The relation to other heterogeneous vacuum decay scenarios, as well as some of the phenomenological implications of barnacles are briefly discussed.

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J. Scargill
Fri, 26 May 17

Comments: 31 pages, 11 figures

Effective Temperatures and Radiation Spectra for a Higher-Dimensional Schwarzschild-de-Sitter Black-Hole [CL]

The absence of a true thermodynamical equilibrium for an observer located in the causal area of a Schwarzschild-de Sitter spacetime has repeatedly raised the question of the correct definition of its temperature. In this work, we consider five different temperatures for a higher-dimensional Schwarzschild-de Sitter black hole: the bare $T_0$, the normalised $T_{BH}$ and three effective ones given in terms of both the black hole and cosmological horizon temperatures. We find that these five temperatures exhibit similarities but also significant differences in their behaviour as the number of extra dimensions and the value of the cosmological constant are varied. We then investigate their effect on the energy emission spectra of Hawking radiation. We demonstrate that the radiation spectra for the normalised temperature $T_{BH}$ — proposed by Bousso and Hawking over twenty years ago — leads to the dominant emission curve while the other temperatures either support a significant emission rate only at a specific $\Lambda$ regime or they have their emission rates globally suppressed. Finally, we compute the bulk-over-brane emissivity ratio and show that the use of different temperatures may lead to different conclusions regarding the brane or bulk dominance.

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P. Kanti and T. Pappas
Fri, 26 May 17

Comments: 31 pages, PDFLatex, 8 Figures, 8 Tables

$\bar{D3}$ Induced Geometric Inflation [CL]

Effective supergravity inflationary models induced by anti-D3 brane interaction with the moduli fields in the bulk geometry have a geometric description. The K\”ahler function carries the complete geometric information on the theory. The non-vanishing bisectional curvature plays an important role in the construction. The new geometric formalism, with the nilpotent superfield representing the anti-D3 brane, allows a powerful generalization of the existing inflationary models based on supergravity. They can easily incorporate arbitrary values of the Hubble parameter, cosmological constant and gravitino mass. We illustrate it by providing generalized versions of polynomial chaotic inflation, T- and E-models of $\alpha$-attractor type, disk merger and cascade inflation models.

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R. Kallosh, A. Linde, D. Roest, et. al.
Fri, 26 May 17

Comments: 22 pages, 7 figures

A no-short scalar hair theorem for rotating Kerr black holes [CL]

If a black hole has hair, how short can this hair be? A partial answer to this intriguing question was recently provided by the ‘no-short hair’ theorem which asserts that the external fields of a spherically-symmetric electrically neutral hairy black-hole configuration must extend beyond the null circular geodesic which characterizes the corresponding black-hole spacetime. One naturally wonders whether the no-short hair inequality $r_{\text{hair}}>r_{\text{null}}$ is a generic property of all electrically neutral hairy black-hole spacetimes? In this paper we provide evidence that the answer to this interesting question may be positive. In particular, we prove that the recently discovered cloudy Kerr black-hole spacetimes — non-spherically symmetric non-static black holes which support linearized massive scalar fields in their exterior regions — also respect this no-short hair lower bound. Specifically, we analytically derive the lower bound $r_{\text{field}}/r_+>r_+/r_-$ on the effective lengths of the external bound-state massive scalar clouds (here $r_{\text{field}}$ is the peak location of the stationary bound-state scalar fields and $r_{\pm}$ are the horizon radii of the black hole). Remarkably, this lower bound is universal in the sense that it is independent of the physical parameters (proper mass and angular harmonic indices) of the exterior scalar fields. Our results suggest that the lower bound $r_{\text{hair}}>r_{\text{null}}$ may be a general property of asymptotically flat electrically neutral hairy black-hole configurations.

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S. Hod
Fri, 26 May 17

Comments: 9 pages. Invited contribution to the Focus Issue on “Hairy black holes”, Classical and Quantum Gravity

Probing Primordial-Black-Hole Dark Matter with Gravitational Waves [CEA]

Primordial black holes (PBHs) have long been suggested as a candidate for making up some or all of the dark matter in the Universe. Most of the theoretically possible mass range for PBH dark matter has been ruled out with various null observations of expected signatures of their interaction with standard astrophysical objects. However, current constraints are significantly less robust in the 20 M_sun < M_PBH < 100 M_sun mass window, which has received much attention recently, following the detection of merging black holes with estimated masses of ~30 M_sun by LIGO and the suggestion that these could be black holes formed in the early Universe. We consider the potential of advanced LIGO (aLIGO) operating at design sensitivity to probe this mass range by looking for peaks in the mass spectrum of detected events. To quantify the background, which is due to black holes that are formed from dying stars, we model the shape of the stellar-black-hole mass function and calibrate its amplitude to match the O1 results. Adopting very conservative assumptions about the PBH and stellar-black-hole merger rates, we show that ~5 years of aLIGO data can be used to detect a contribution of >20 M_sun PBHs to dark matter down to f_PBH<0.5 at >99.9% confidence level. Combined with other probes that already suggest tension with f_PBH=1, the obtainable independent limits from aLIGO will thus enable a firm test of the scenario that PBHs make up all of dark matter.

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E. Kovetz
Fri, 26 May 17

Comments: 5 pages, 2 figures