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

Reconstructing warm inflation [CL]

The reconstruction of a warm inflationary universe model from the scalar spectral index $n_S(N)$ and the tensor to scalar ratio $r(N)$ as a function of the number of e-folds $N$ is studied. Under a general formalism we find the effective potential and the dissipative coefficient in terms of the cosmological parameters $n_S$ and $r$ considering the weak and strong dissipative stages under the slow roll approximation. As a specific example, we study the attractors for the index $n_S$ given by $n_{S}-1\propto N^{-1}$ and for the ratio $r\propto N^{-2}$, in order to reconstruct the model of warm inflation. Here, expressions for the effective potential $V(\phi)$ and the dissipation coefficient $\Gamma(\phi)$ are obtained.

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R. Herrera
Wed, 17 Jan 18

Comments: 23 pages and 2 figures

Singularities in Spherically Symmetric Solutions with Limited Curvature Invariants [CL]

We investigate static, spherically symmetric solutions in gravitational theories which have limited curvature invariants, aiming to remove the singularity in the Schwarzschild space-time. We find that if we only limit the Gauss-Bonnet term and the Ricci scalar, then the singularity at the origin persists. Moreover we find that the event horizon can develop a curvature singularity. We also investigate a new class of theories in which all components of the Riemann tensor are bounded. We find that the divergence of the quadratic curvature invariants at the event horizon is avoidable in this theory. However, other kinds of singularities due to the dynamics of additional degrees of freedom cannot be removed, and the space-time remains singular.

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D. Yoshida and R. Brandenberger
Wed, 17 Jan 18

Comments: 17 pages, 17 figures

Primordial Black Holes – Perspectives in Gravitational Wave Astronomy – [CEA]

This is a review article on the primordial black holes (PBHs), with particular focus on the massive ones ($\gtrsim 10^{15}{\rm g}$) which have not evaporated by the present epoch by the Hawking radiation. By the detections of gravitational waves by LIGO, we have gained a completely novel tool to observationally search for PBHs complementary to the electromagnetic waves. Based on the perspective that gravitational-wave astronomy will make a significant progress in the next decades, a purpose of this article is to give a comprehensive review covering a wide range of topics on PBHs. After discussing PBH formation as well as several inflation models leading to PBH production, we summarize various existing and future observational constraints. We then present topics on formation of PBH binaries, gravitational waves from PBH binaries, various observational tests of PBHs by using gravitational waves.

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M. Sasaki, T. Suyama, T. Tanaka, et. al.
Wed, 17 Jan 18

Comments: 87 pages, 23 figures, invited topical review article published in Classical and Quantum Gravity

Random projections in gravitational wave searches of compact binaries [CL]

Random projection (RP) is a powerful dimension reduction technique widely used in analysis of high dimensional data. We demonstrate how this technique can be used to improve the computational efficiency of gravitational wave searches from compact binaries of neutron stars or black holes. Improvements in low-frequency response and bandwidth due to detector hardware upgrades pose a data analysis challenge in the advanced LIGO era as they result in increased redundancy in template databases and longer templates due to higher number of signal cycles in band. The RP-based methods presented here address both these issues within the same broad framework. We first use RP for an efficient, singular value decomposition inspired template matrix factorization and develop a geometric intuition for why this approach works. We then use RP to calculate approximate time-domain correlations in a lower dimensional vector space. For searches over parameters corresponding to non-spinning binaries with a neutron star and a black hole, a combination of the two methods can reduce the total on-line computational cost by an order of magnitude over a nominal baseline. This can, in turn, help free-up computational resources needed to go beyond current spin-aligned searches to more complex ones involving generically spinning waveforms.

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S. Kulkarni, K. Phukon, A. Reza, et. al.
Tue, 16 Jan 18

Comments: N/A

Constraining the range of Yukawa gravity interaction from S2 star orbits III: improvement expectations for graviton mass bounds [CL]

Recently, the LIGO-Virgo collaboration discovered gravitational waves and in their first publication on the subject the authors also presented a graviton mass constraint as $m_g < 1.2 \times 10^{-22}$ eV Abbott et al. (2016).
In the paper we analyze a potential to reduce upper bounds for graviton mass with future observational data on trajectories of bright stars near the Galactic Center.Since gravitational potentials are different for these two cases, expressions for relativistic advance for general relativity and Yukawa potential are different functions on eccentricity and semimajor axis, it gives an opportunity to improve current estimates of graviton mass with future observational facilities. In our considerations of an improvement potential for a graviton mass estimate we adopt a conservative strategy and assume trajectories of bright stars and their apocenter advance will be described with general relativity expressions and it gives opportunities to improve graviton mass constraints. In contrast with our previous studies, where we present current constraints on parameters of Yukawa gravity (Borka et al., 2013) and graviton mass (Zakharov et al. 2016) from observations of S2 star, in the paper we express expectations to improve current constraints for graviton mass, assuming the GR predictions about apocenter shifts will be confirmed with future observations. We concluded that if future observations of bright star orbits during around fifty years will confirm GR predictions about apocenter shifts of bright star orbits it give an opportunity to constrain a graviton mass at a level around $5 \times 10^{-23}$ eV or slightly better than current estimates obtained with LIGO observations.

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A. Zakharov, P. Jovanovic, D. Borka, et. al.
Tue, 16 Jan 18

Comments: 15 pages, 1 Figure, 1 Table

Conformal invariant cosmological perturbations via the covariant approach: multicomponent universe [CL]

In recent years there are lots of interests to discuss frame dependences/independences of the cosmological perturbations under the conformal transformations. This problem was investigated in terms of the covariant approach for a single component universe, and it was found that the covariant approach is very powerful to pick out the perturbative variables which are both gauge and conformal invariant. In this work, we extend the covariant approach to the universe with multicomponent fluids. We find that similar results can be derived as expected. In addition, some other interesting perturbations are also identified to be conformal invariant, such as entropy perturbation between two different components.

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Y. Zheng, Y. Mou, H. Rao, et. al.
Tue, 16 Jan 18

Comments: 10 pages, to be published in Chinese Physics C