Screening in perturbative approaches to LSS [CEA]

A specific value for the cosmological constant, \Lambda, can account for late-time cosmic acceleration. However, motivated by the so-called cosmological constant problem(s), several alternative mechanisms have been explored. To date, a host of well-studied dynamical dark energy and modified gravity models exists. Going beyond \Lambda CDM often comes with additional degrees of freedom (dofs). For these to pass existing observational tests, an efficient screening mechanism must be in place. The linear and quasi-linear regimes of structure formation are ideal probes of such dofs and can capture the onset of screening. We propose here a semi-phenomenological treatment to account for screening dynamics on LSS observables, with special emphasis on Vainshtein-type screening.

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M. Fasiello and Z. Vlah
Wed, 26 Apr 17

Comments: 7 pages, two figures

Primordial magnetogenesis from vector Galileons [CEA]

Like Scalar Galileons, Einstein-Hilbert action and the Lovelock extensions contain higher order derivatives in action, however their equations of motion are second order. We are lead to ask: Can there exist a corresponding action for spin-1 or electromagnetic fields? By demanding three conditions – theory be described by vector potential $A^\mu$ and its derivatives, Gauge invariance be satisfied, and equations of motion be linear in second derivatives of vector potential – we construct a higher derivative electromagnetic action which does not have ghosts and preserve gauge invariance. We show that the action breaks conformal invariance explicitly and leads to generation of magnetic field during inflation. One unique feature of our model is that appreciable magnetic fields are generated at small wavelengths while tiny magnetic fields at large wavelengths that are consistent with current observations.

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D. Nandi and S. Shankaranarayanan
Tue, 25 Apr 17

Comments: 10 pages, 0 figure

Electroweak Vacuum Instability and Renormalized Vacuum Field Fluctuations in Adiabatic or Non-adiabatic Cosmological Background [CL]

In this work, we investigate the electroweak vacuum instability in the adiabatic or non-adiabatic cosmological background. In the general cosmological background, the vacuum field fluctuations $\left< { \delta \phi }^{ 2 } \right>$ grow in proportional to the cosmological scale. The large vacuum fluctuations of the Higgs field can destabilize the effective Higgs potential, or generate the catastrophic AdS domains or bubbles. These unwanted phenomena cause the catastrophic collapse of the Universe. By using the adiabatic (WKB) expansion or the adiabatic regularization methods, we obtain the exact renormalized vacuum fluctuations $\left< { \delta \phi }^{ 2 } \right>{\rm ren}$ of the Higgs field in the adiabatic and the non-adiabatic cosmological background. The non-adiabatic Higgs vacuum fluctuations generally cause the catastrophic phenomena. On the other hand, the adiabatic Higgs vacuum fluctuations have little effect on the Higgs vacuum stability. However, in the slowly-varying background by another scalar field $S$, the adiabatic Higgs vacuum fluctuations can destabilize the effective Higgs potential and provide the upper bound of the mass of the background scalar field $S$ as $m{S} \lesssim 10^{13}\ {\rm GeV}$ where we assume the instability scale $\Lambda_{I} \approx 10^{11}\ {\rm GeV}$.

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K. Kohri and H. Matsui
Tue, 25 Apr 17

Comments: 16 pages

On (in)stabilities of perturbations in mimetic models with higher derivatives [CL]

Usually when applying the mimetic model to the early universe, higher derivative terms are needed to promote the mimetic field to be dynamical. However such models suffer from the ghost and/or the gradient instabilities and simple extensions cannot cure this pathology. We point out in this paper that it is possible to overcome this difficulty by considering the direct couplings of the higher derivatives of the mimetic field to the curvature of the spacetime.

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Y. Zheng, L. Shen, Y. Mou, et. al.
Tue, 25 Apr 17

Comments: 8 pages, 1 figure

Starobinsky-like Inflation, Supercosmology and Neutrino Masses in No-Scale Flipped SU(5) [CL]

We embed a flipped ${\rm SU}(5) \times {\rm U}(1)$ GUT model in a no-scale supergravity framework, and discuss its predictions for cosmic microwave background observables, which are similar to those of the Starobinsky model of inflation. Measurements of the tilt in the spectrum of scalar perturbations in the cosmic microwave background, $n_s$, constrain significantly the model parameters. We also discuss the model’s predictions for neutrino masses, and pay particular attention to the behaviours of scalar fields during and after inflation, reheating and the GUT phase transition. We argue in favor of strong reheating in order to avoid excessive entropy production which could dilute the generated baryon asymmetry.

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J. Ellis, M. Garcia, N. Nagata, et. al.
Tue, 25 Apr 17

Comments: 51 pages, 13 figures

Quasinormal modes as a distinguisher between general relativity and f(R) gravity [CL]

Quasi-Normal Modes (QNM) or ringdown phase of gravitational waves provide critical information about the structure of compact objects like Black Holes. Thus, QNMs can be a tool to test General Relativity (GR) and possible deviations from it. In the case of GR, it is known for a long time that a relation between two types of Black Hole perturbations: scalar (Zerilli) and vector (Regge-Wheeler), leads to an equal share of emitted gravitational energy. With the direct detection of Gravitational waves, it is now natural to ask: whether the same relation (between scalar and vector perturbations) holds for modified gravity theories? If not, whether one can use this as a way to probe deviations from General Relativity. As a first step, we show explicitly that the above relation between Regge-Wheeler and Zerilli breaks down for a general f (R) model, and hence the two perturbations do not share equal amounts of emitted gravitational energy. We discuss the implication of this imbalance on observations and the no-hair conjecture.

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S. Bhattacharyya and S. Shankaranarayanan
Tue, 25 Apr 17

Comments: 9 pages, 2 figures

The Memory Effect for Plane Gravitational Waves [CL]

We give an account of the “gravitational memory effect” in the presence of an exact plane wave solution of Einstein’s vacuum equations. This allows an elementary but exact description of the soft gravitons and how their presence may be detected by observing the motion of freely falling particles.

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P. Zhang, C. Duval, G. Gibbons, et. al.
Mon, 24 Apr 17

Comments: 7 pages, 2 figures