Observers in Kerr spacetimes: the ergoregion on the equatorial plane [CL]

We perform a detailed analysis of the properties of stationary observers located on the equatorial plane of the ergosphere in a Kerr spacetime, including light-surfaces. This study highlights crucial differences between black hole and the super-spinner sources. In the case of Kerr naked singularities, the results allow us to distinguish between “weak” and “strong” singularities, corresponding to spin values close to or distant from the limiting case of extreme black holes, respectively. We derive important limiting angular frequencies for naked singularities. We especially study very weak singularities as resulting from the spin variation of black holes. We also explore the main properties of zero angular momentum observers for different classes of black hole and naked singularity spacetimes.

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D. Pugliese and H. Quevedo
Fri, 19 Jan 18

Comments: 20 pages, 13 multi-panels figures, 2 tables

Constraints on the radiation temperature before inflation [CL]

We consider the short period of cosmic expansion ranging from the end of the Planck era to the beginning of inflation and set upper and lower limits on the temperature of the radiation at the commencement of the inflationary phase.

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R. Herrera, D. Pavon and J. Saavedra
Fri, 19 Jan 18

Comments: 10 pages, 1 eps figure, key words: cosmology, early Universe, inflation, thermodynamics

Modulus D-term Inflation [CL]

We propose a new model of single-field D-term inflation in supergravity, where the inflation is driven by a single modulus field which transforms non-linearly under the U(1) gauge symmetry. One of the notable features of our modulus D-term inflation scenario is that the global U(1) remains unbroken in the vacuum and hence our model is not plagued by the cosmic string problem which can exclude most of the conventional D-term inflation models proposed so far due to the CMB observations.

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K. Kadota, T. Kobayashi, I. Saga, et. al.
Fri, 19 Jan 18

Comments: 15 pages, 5 figures

Imprints of the redshift evolution of double neutron star merger rate on the signal to noise ratio distribution [CL]

Proposed third generation gravitational wave (GW) interferometers such as Einstein Telescope will have the sensitivity to observe double neutron star (DNS) mergers up to a redshift of $\sim 2$ with good signal to noise ratios. We argue that the measurement of {\it redshifted signal to noise ratio} defined by $\sigma=\rho (1+z)^{1/6}$, where $\rho$ is the signal to noise ratio (SNR) of a detected GW event and $z$ is its redshift can be used to study the distribution of DNS mergers. We show that if the DNS binaries are distributed uniformly within the co-moving volume, the distribution of redshifted SNR, $\sigma$, will be inversely proportional to the fourth power of $\sigma$, $p(\sigma)\propto \frac{1}{\sigma^4}$. We argue that the redshift evolution of DNS mergers will leave imprints on the distribution of $\sigma$ and hence this may provide a method to probe their redshift evolution. Using various parametric models for evolution of co-moving merger rate density as a function of redshift and assuming the sensitivity of Einstein Telescope, we discuss the distinguishability of the $\sigma$ distributions of these models with that of constant co-moving number density of the mergers.

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S. Kastha and K. Arun
Fri, 19 Jan 18

Comments: 7 pages, 5 figures

Fluid and gyrofluid modeling of low-$β_e$ plasmas: phenomenology of kinetic Alfvén wave turbulence [CL]

Reduced fluid models including electron inertia and ion finite Larmor radius corrections are derived asymptotically, both from fluid basic equations and from a gyrofluid model. They apply to collisionless plasmas with small ion-to-electron equilibrium temperature ratio and low $\beta_e$, where $\beta_e$ indicates the ratio between the equilibrium electron pressure and the magnetic pressure exerted by a strong, constant and uniform magnetic guide field. The consistency between the fluid and gyrofluid approaches is ensured when choosing ion closure relations prescribed by the underlying ordering. A two-field reduction of the gyrofluid model valid for arbitrary equilibrium temperature ratio is also introduced, and is shown to have a noncanonical Hamiltonian structure. This model provides a convenient framework for studying kinetic Alfv\’en wave turbulence, from MHD to sub-$d_e$ scales (where $d_e$ holds for the electron skin depth). Magnetic energy spectra are phenomenologically determined within energy and generalized helicity cascades in the perpendicular spectral plane. Arguments based on absolute statistical equilibria are used to predict the direction of the transfers, pointing out that, within the sub-ion range associated with a $k_\perp^{-7/3}$ transverse magnetic spectrum, the generalized helicity could display an inverse cascade if injected at small scales, for example by reconnection processes.

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T. Passot, P. Sulem and E. Tassi
Fri, 19 Jan 18

Comments: 18 pages, 4 figures

Vacuum Polarization and Photon Propagation in an Electromagnetic Plane Wave [CL]

The QED vacuum polarization in external monochromatic plane-wave electromagnetic fields is calculated with spatial and temporal variations of the external fields being taken into account. We develop a perturbation theory to calculate the induced electromagnetic current that appears in the Maxwell equations, based on Schwinger’s proper-time method, and combine it with the so-called gradient expansion to handle the variation of external fields perturbatively. The crossed field, i.e., the long wavelength limit of the electromagnetic wave is first considered. The eigenmodes and the refractive indices as the eigenvalues associated with the eigenmodes are computed numerically for the probe photon propagating in some particular directions. In so doing, no limitation is imposed on the field strength and the photon energy unlike previous studies. It is shown that the real part of the refractive index becomes less than unity for strong fields, the phenomenon that has been known to occur for high-energy probe photons. We then evaluate numerically the lowest-order corrections to the crossed-field resulting from the field variations in space and time. It is demonstrated that the corrections occur mainly in the imaginary part of the refractive index.

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A. Yatabe and S. Yamada
Thu, 18 Jan 18

Comments: 50 pages, 17 figures, accepted for publication in Progress of Theoretical and Experimental Physics

Initial conditions for Inflation in an FRW Universe [CL]

We examine the class of initial conditions which give rise to inflation. Our analysis is carried out for several popular models including: Higgs inflation, Starobinsky inflation, chaotic inflation, axion monodromy inflation and non-canonical inflation. In each case we determine the set of initial conditions which give rise to sufficient inflation, with at least $60$ e-foldings. A phase-space analysis has been performed for each of these models and the effect of the initial inflationary energy scale on inflation has been studied numerically. This paper discusses two scenarios of Higgs inflation: (i) the Higgs is coupled to the scalar curvature, (ii) the Higgs Lagrangian contains a non-canonical kinetic term. In both cases we find Higgs inflation to be very robust since it can arise for a large class of initial conditions. One of the central results of our analysis is that, for plateau-like potentials associated with the Higgs and Starobinsky models, inflation can be realized even for initial scalar field values which lie close to the minimum of the potential. This dispels a misconception relating to plateau potentials prevailing in the literature. We also find that inflation in all models is more robust for larger values of the initial energy scale.

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S. Mishra, V. Sahni and A. Toporensky
Wed, 17 Jan 18

Comments: 46 pages, 29 figures