Primordial anisotropies from cosmic strings during inflation [CEA]

http://arxiv.org/abs/1703.05714

In this work we study the imprints of a primordial cosmic string on inflationary power spectrum. Cosmic string induces two distinct contributions on curvature perturbations power spectrum. The first type of correction respects the translation invariance while violating isotropy. This generates quadrupolar statistical anisotropy in CMB maps which is constrained by the Planck data. The second contribution breaks both homogeneity and isotropy, generating a dipolar power asymmetry in variance of temperature fluctuations with its amplitude falling on small scales. We show that the strongest constraint on the tension of string is obtained from the quadrupolar anisotropy and argue that the mass scale of underlying theory responsible for the formation of string can not be much higher than the GUT scale. The predictions of string for the diagonal and off-diagonal components of CMB angular power spectrum are presented.

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S. Jazayeri, A. Sadr and H. Firouzjahi
Fri, 17 Mar 17
3/50

Comments: 25 pages, 6 figures

Constraining holographic cosmology using Planck data [CEA]

http://arxiv.org/abs/1703.05385

Holographic cosmology offers a novel framework for describing the very early Universe in which cosmological predictions are expressed in terms of the observables of a three dimensional quantum field theory (QFT). This framework includes conventional slow-roll inflation, which is described in terms of a strongly coupled QFT, but it also allows for qualitatively new models for the very early Universe, where the dual QFT may be weakly coupled. The new models describe a universe which is non-geometric at early times. While standard slow-roll inflation leads to a (near-)power-law primordial power spectrum, perturbative superrenormalizable QFT’s yield a new holographic spectral shape. Here, we compare the two predictions against cosmological observations. We use CosmoMC to determine the best fit parameters, and MultiNest for Bayesian Evidence, comparing the likelihoods. We find that the dual QFT should be non-perturbative at the very low multipoles ($l \lesssim 30$), while for higher multipoles ($l \gtrsim 30$) the new holographic model, based on perturbative QFT, fits the data just as well as the standard power-law spectrum assumed in $\Lambda$CDM cosmology. This finding opens the door to applications of non-perturbative QFT techniques, such as lattice simulations, to observational cosmology on gigaparsec scales and beyond.

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N. Afshordi, E. Gould and K. Skenderis
Fri, 17 Mar 17
9/50

Comments: 25 pages, 10 figures

Charged massive scalar field configurations supported by a spherically symmetric charged reflecting shell [CL]

http://arxiv.org/abs/1703.05333

The physical properties of bound-state charged massive scalar field configurations linearly coupled to a spherically symmetric charged reflecting shell are studied {\it analytically}. To that end, we solve the Klein-Gordon wave equation for a static scalar field of proper mass $\mu$, charge coupling constant $q$, and spherical harmonic index $l$ in the background of a charged shell of radius $R$ and electric charge $Q$. It is proved that the dimensionless inequality $\mu R<\sqrt{(qQ)^2-(l+1/2)^2}$ provides an upper bound on the regime of existence of the composed charged-spherical-shell-charged-massive-scalar-field configurations. Interestingly, we explicitly show that the {\it discrete} spectrum of shell radii $\{R_n(\mu,qQ,l)\}_{n=0}^{n=\infty}$ which can support the static bound-state charged massive scalar field configurations can be determined analytically. We confirm our analytical results by numerical computations.

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S. Hod
Fri, 17 Mar 17
11/50

Comments: 8 pages

Lectures on the Infrared Structure of Gravity and Gauge Theory [CL]

http://arxiv.org/abs/1703.05448

This is a redacted transcript of a course given by the author at Harvard in spring semester 2016. It contains a pedagogical overview of recent developments connecting the subjects of soft theorems, the memory effect and asymptotic symmetries in four-dimensional QED, nonabelian gauge theory and gravity with applications to black holes. The lectures may be viewed online at https://goo.gl/3DJdOr. Please send typos or corrections to strominger@physics.harvard.edu.

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A. Strominger
Fri, 17 Mar 17
18/50

Comments: 154 pages, 21 figures

Pulsar Timing Constraints on Physics Beyond the Standard Model [CL]

http://arxiv.org/abs/1703.05331

We argue that massive quantum fields source low-frequency long-wavelength metric fluctuations through the quantum fluctuations of their stress-energy, given reasonable assumptions about the analytic structure of its correlators. This can be traced back to the non-local nature of the gauge symmetry in General Relativity, which prevents an efficient screening of UV scales (what we call the cosmological non-constant problem). We define a covariant and gauge-invariant observable which probes line-of-sight spacetime curvature fluctuations on an observer’s past lightcone, and show that current pulsar timing data constrains any massive particle to $m\lesssim 600$ GeV. This astrophysical bound severely limits the possibilities for physics beyond the standard model below the scale of quantum gravity.

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N. Afshordi, H. Kim and E. Nelson
Fri, 17 Mar 17
20/50

Comments: 5 pages, 2 figures; comments welcome

Study of statistical properties of hybrid statistic in coherent multi-detector compact binary coalescences Search [CL]

http://arxiv.org/abs/1512.00655

In this article, we revisit the problem of coherent multi-detector search of gravitational wave from compact binary coalescence with Neutron stars and Black Holes using advanced interferometers like LIGO-Virgo. Based on the loss of optimal multi-detector signal-to-noise ratio (SNR), we construct a hybrid statistic as a best of maximum-likelihood-ratio(MLR) statistic tuned for face-on and face-off binaries. The statistical properties of the hybrid statistic is studied. The performance of this hybrid statistic is compared with that of the coherent MLR statistic for generic inclination angles. Owing to the single synthetic data stream, the hybrid statistic gives low false alarms compared to the multi-detector MLR statistic and small fractional loss in the optimum SNR for a large range of binary inclinations. We have demonstrated that for a LIGO-Virgo network and binary inclination, \epsilon < 70 deg. and \epsilon > 110 deg., the hybrid statistic captures more than 98% of network optimum matched filter SNR with low false alarm rate. The Monte-Carlo exercise with two distributions of incoming inclination angles namely, U[cos(\epsilon)] and more realistic distribution proposed by B. F. Schutz are performed with hybrid statistic and gave ~5% and ~7% higher detection probability respectively compared to the two stream multi-detector MLR statistic for a fixed false alarm probability of 10^-5.

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K. Haris and A. Pai
Fri, 17 Mar 17
42/50

Comments: Published in Phys. Rev. D

Yet another introduction to relativistic astrophysics [HEAP]

http://arxiv.org/abs/1703.05575

Late Winter Lecture Notes, Short Course (10 hours) of Relativistic Astrophysics held at the Department of Physics and Astronomy of the University of Padova, March 13-17, 2017.

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L. Foschini
Fri, 17 Mar 17
46/50

Comments: 132 pages

A search for sterile neutrinos with the latest cosmological observations [CEA]

http://arxiv.org/abs/1703.04884

We report the result of a search for sterile neutrinos with the latest cosmological observations. Both cases of massless and massive sterile neutrinos are considered in the $\Lambda$CDM cosmology. The cosmological observations used in this work include the Planck 2015 temperature and polarization data, the baryon acoustic oscillation data, the Hubble constant direct measurement data, the Planck Sunyaev-Zeldovich cluster counts data, the Planck lensing data, and the cosmic shear data. We find that the current observational data give a hint of the existence of massless sterile neutrino (as dark radiation) at the 1.44$\sigma$ level, and the consideration of an extra massless sterile neutrino can indeed relieve the tension between observations and improve the cosmological fit. For the case of massive sterile neutrino, the observations give a rather tight upper limit on the mass, which implies that actually a massless sterile neutrino is more favored. Our result is consistent with the recent result of neutrino oscillation experiment done by the Daya Bay and MINOS collaborations, as well as the recent result of cosmic ray experiment done by the IceCube collaboration.

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L. Feng, J. Zhang and X. Zhang
Thu, 16 Mar 17
19/92

Comments: 15 pages, 4 figures

Gravitational Waves from Core-Collapse Supernovae [HEAP]

http://arxiv.org/abs/1703.04633

Gravitational waves are a potential direct probe for the multi-dimensional flow during the first second of core-collapse supernova explosions. Here we outline the structure of the predicted gravitational wave signal from neutrino-driven supernovae of non-rotating progenitors from recent 2D and 3D simulations. We sketch some quantitative dependencies that govern the amplitudes of this signal and its evolution in the time-frequency domain.

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B. Muller
Thu, 16 Mar 17
53/92

Comments: 4 pages, invited contribution prepared for the minisymposium “Gravitational Waves: Sources and Detection” at the 13th International Conference on Mathematical and Numerical Aspects of Wave Propagation, Minnesota, 2017

On the post-Keplerian corrections to the orbital periods of a two-body system and their application to the Galactic Center [GA]

http://arxiv.org/abs/1703.04947

Detailed numerical analyses of the orbital motion of a test particle around a spinning primary are performed. They aim to investigate the possibility of using the post-Keplerian (pK) corrections to the orbiter’s periods (draconitic, anomalistic and sidereal) as a further opportunity to perform new tests of post-Newtonian (pN) gravity. As a specific scenario, the S-stars orbiting the Massive Black Hole (MBH) supposedly lurking in Sgr A$^\ast$ at the center of the Galaxy is adopted. We, first, study the effects of the pK Schwarzchild, Lense-Thirring and quadrupole moment accelerations experienced by a target star for various possible initial orbital configurations. It turns out that the results of the numerical simulations are consistent with the analytical ones in the small eccentricity approximation for which almost all the latter ones were derived. For highly elliptical orbits, the size of all the three pK corrections considered turn out to increase remarkably. The periods of the observed S2 and S0-102 stars as functions of the MBH’s spin axis orientation are considered as well. The pK accelerations considered lead to corrections of the orbital periods of the order of 1-100d (Schwarzschild), 0.1-10h (Lense-Thirring) and 1-10^3s (quadrupole) for a target star with a=300-800~AU and e ~ 0.8, which could be possibly measurable by the future facilities.

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L. Iorio and F. Zhang
Thu, 16 Mar 17
61/92

Comments: Accepted for publication in ApJ

General dynamical properties of cosmological models with nonminimal kinetic coupling [CL]

http://arxiv.org/abs/1703.04966

We consider cosmological dynamics in the theory of gravity with the scalar field possessing the nonminimal kinetic coupling to curvature given as $\eta G^{\mu\nu}\phi_{,\mu}\phi_{,\nu}$, where $\eta$ is an arbitrary coupling parameter, and the scalar potential $V(\phi)$ which assumed to be as general as possible. With an appropriate dimensionless parameterization we represent the field equations as an autonomous dynamical system which contains ultimately only one arbitrary function $\chi (x)= 8 \pi G |\eta| V(x/\sqrt{8 \pi G})$. Then, assuming the rather general properties of $\chi(x)$, we analyze stationary points and their stability, as well as all possible asymptotical regimes of the dynamical system. It has been shown that for a broad class of $\chi(x)$ there exist attractors representing three accelerated regimes of the Universe evolution, including de Sitter expansion (or late-time inflation), the Little Rip scenario, and the Big Rip scenario. As the specific examples, we consider a power-law potential $V(\phi)=M^4(\phi/\phi_0)^\alpha$, Higgs-like potential $V(\phi)=\frac{\lambda}{4}(\phi^2-\phi_0^2)^2$, and exponential potential $V(\phi)=M^4 e^{-\phi/\phi_0}$.

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J. Matsumoto and S. Sushkov
Thu, 16 Mar 17
65/92

Comments: 26 pages, 3 figures, 3 tables

Redshift drift of gravitational lensing [CEA]

http://arxiv.org/abs/1703.05142

We investigate the effect of the redshift drift in strong gravitational lensing. The redshift drift produces a time variation of $i)$ the apparent position of a lensed source and $ii)$ the time delay among incoming signals from different images. We dub these effects as angular drift and time delay drift, respectively, and analyze their relevance in cosmology.

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O. Piattella and L. Giani
Thu, 16 Mar 17
67/92

Comments: 4 pages, 1 figure

Priors on the effective Dark Energy equation of state in scalar-tensor theories [CEA]

http://arxiv.org/abs/1703.05297

Constraining the Dark Energy (DE) equation of state, w, is one of the primary science goals of ongoing and future cosmological surveys. In practice, with imperfect data and incomplete redshift coverage, this requires making assumptions about the evolution of w with redshift z. These assumptions can be manifested in a choice of a specific parametric form, which can potentially bias the outcome, or else one can reconstruct w(z) non-parametrically, by specifying a prior covariance matrix that correlates values of w at different redshifts. In this work, we derive the theoretical prior covariance for the effective DE equation of state predicted by general scalar-tensor theories with second order equations of motion (Horndeski theories). This is achieved by generating a large ensemble of possible scalar-tensor theories using a Monte Carlo methodology, including the application of physical viability conditions. We also separately consider the special sub-case of the minimally coupled scalar field, or quintessence. The prior shows a preference for tracking behaviors in the most general case. Given the covariance matrix, theoretical priors on parameters of any specific parametrization of w(z) can also be readily derived by projection.

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M. Raveri, P. Bull, A. Silvestri, et. al.
Thu, 16 Mar 17
83/92

Comments: 13 pages, 5 figures

Constructing Gravitational Waves from Generic Spin-Precessing Compact Binary Inspirals [CL]

http://arxiv.org/abs/1703.03967

The coalescence of compact objects is one of the most promising sources of gravitational waves for ground-based interferometric detectors, such as advanced LIGO and Virgo. Generically, com- pact objects in binaries are expected to be spinning with spin angular momenta misaligned with the orbital angular momentum, causing the orbital plane to precess. This precession adds rich structure to the gravitational waves, introducing such complexity that an analytic closed-form description has been unavailable until now. We here construct the first closed-form frequency- domain gravitational waveforms that are valid for generic spin-precessing quasicircular compact binary inspirals. We first construct time-domain gravitational waves by solving the post-Newtonian precession equations of motion with radiation reaction through multiple scale analysis. We then Fourier transform these time-domain waveforms with the method of shifted uniform asymptotics to obtain closed-form expressions for frequency-domain waveforms. We study the accuracy of these analytic, frequency-domain waveforms relative to waveforms obtained by numerically evolving the post-Newtonian equations of motion and find that they are suitable for unbiased parameter estimation for 99.2%(94.6%) of the binary configurations we studied at a signal-to-noise ratio of 10(25). These new frequency-domain waveforms could be used for detection and parameter estimation studies due to their accuracy and low computational cost.

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K. Chatziioannou, A. Klein, N. Yunes, et. al.
Tue, 14 Mar 17
8/74

Comments: 21 pages, submitted to Phys. Rev. D

Massive Fields as Systematics for Single Field Inflation [CEA]

http://arxiv.org/abs/1703.04477

During inflation, massive fields can contribute to the power spectrum of curvature perturbation via a dimension-5 operator. This contribution can be considered as a bias for the program of using $n_s$ and $r$ to select inflation models. Even the dimension-5 operator is suppressed by $\Lambda = M_p$, there is still a significant shift on the $n_s$-$r$ diagram if the massive fields have $m\sim H$. On the other hand, if the heavy degree of freedom appear only at the same energy scale as the suppression scale of the dimension-5 operator, then significant shift on the $n_s$-$r$ diagram takes place at $m=\Lambda \sim 70H$, which is around the inflationary time-translation symmetry breaking scale. Hence, the systematics from massive fields pose a greater challenge for future high precision experiments for inflationary model selection. This result can be thought of as the impact of UV sensitivity to inflationary observables.

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H. Jiang and Y. Wang
Tue, 14 Mar 17
11/74

Comments: 11 pages, 3 figures

Cosmological Implications of Nonlocal Gravity [CL]

http://arxiv.org/abs/1703.04205

We present extensions of the treatment contained in our recent paper on nonlocal Newtonian cosmology [C. Chicone and B. Mashhoon, J. Math. Phys. 57, 072501 (2016)]. That is, the implications of the recent nonlocal generalization of Einstein’s theory of gravitation are further investigated within the regime of Newtonian cosmology. In particular, we treat the nonlocal problem of structure formation for a spherically symmetric expanding dust model and show numerically that as the central density contrast grows, it tends to decrease slowly with radial distance as the universe expands. The nonlocal violation of Newton’s shell theorem provides a physical interpretation of our numerical results.

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C. Chicone and B. Mashhoon
Tue, 14 Mar 17
24/74

Comments: 51 pages, 6 figures. Dedicated to Friedrich W. Hehl in honor of his 80th birthday

Classical collapse to black holes and white hole quantum bounces: A review [CL]

http://arxiv.org/abs/1703.04138

In the last four decades different programs have been carried out aiming at understanding the final fate of gravitational collapse of massive bodies once some prescriptions for the behaviour of gravity in the strong field regime are provided. The general picture arising from most of these scenarios is that the classical singularity at the end of collapse is replaced by a bounce. The most striking consequence of the bounce is that the black hole horizon may live for only a finite time. The possible implications for astrophysics are important since, if these models capture the essence of the collapse of a massive star, an observable signature of quantum gravity may be hiding in astrophysical phenomena. One intriguing idea that is implied by these models is the possible existence of exotic compact objects, of high density and finite size, that may not be covered by an horizon. The present article outlines the main features of these collapse models and some of the most relevant open problems. The aim is to provide a comprehensive (as much as possible) overview of the current status of the field from the point of view of astrophysics. As a little extra, a new toy model for collapse leading to the formation of a quasi static compact object is presented.

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D. Malafarina
Tue, 14 Mar 17
25/74

Comments: 29 pages, 8 figures, comments are welcome

Synthetic streams in a Gravitational Wave inspiral search with a multi-detector network [CL]

http://arxiv.org/abs/1401.7967

Gravitational Wave Inspiral search with a global network of interferometers when carried in a phase coherent fashion would mimic an effective multi-detector network with synthetic streams constructed by the linear combination of the data from different detectors. For the first time, we demonstrate that the two synthetic data streams pertaining to the two polarizations of Gravitational Wave can be derived prior to the maximum-likelihood analysis in a most natural way using the technique of singular-value-decomposition applied to the network signal-to-noise ratio vector. We construct the network matched filters in combined network plus spectral space which capture both the synthetic streams. We further show that the network LLR is then sum of the LLR of each synthetic stream. The four extrinsic parameters are mapped to the two amplitudes and two phases. The maximization over these is a straightforward approach closely linked to the single detector approach. Towards the end, we connect all the previous works related to the multi-detector Gravitational Wave inspiral search and express in the same notation in order to bring under the same footing.

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K. Haris and A. Pai
Tue, 14 Mar 17
26/74

Comments: LIGO laboratory document number: LIGO-P1300229

Mass function of galaxy clusters in relativistic inhomogeneous cosmology [CEA]

http://arxiv.org/abs/1703.04189

The current cosmological model ($\Lambda$CDM) with the underlying FLRW metric relies on the assumption of local isotropy, hence homogeneity of the Universe. Difficulties arise when one attempts to justify this model as an average description of the Universe from first principles of general relativity, since in general, the Einstein tensor built from the averaged metric is not equal to the averaged stress–energy tensor. In this context, the discrepancy between these quantities is called “cosmological backreaction” and has been the subject of scientific debate among cosmologists and relativists for more than $20$ years. Here we present one of the methods to tackle this problem, i.e. averaging the scalar parts of the Einstein equations, together with its application, the cosmological mass function of galaxy clusters.

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J. Ostrowski, T. Buchert and B. Roukema
Tue, 14 Mar 17
27/74

Comments: 4 pages, 1 figure

The Gibbs paradox, the Landauer principle and the irreversibility associated with tilted observers [CL]

http://arxiv.org/abs/1703.03958

It is well known that, in the context of General Relativity, some spacetimes, when described by a congruence of comoving observers, may consist in a distribution of a perfect (non-dissipative) fluid, whereas the same spacetime as seen by a “tilted”‘ (Lorentz-boosted) congruence of observers, may exhibit the presence of dissipative processes. As we shall see, the appearence of entropy producing processes are related to the tight dependence of entropy on the specific congruence of observers. This fact is well illustrated by the Gibbs paradox. The appearance of such dissipative processes, as required by the Landauer principle, are necessary, in order to erase the different amount of information stored by comoving observers, with respect to tilted ones.

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L. Herrera
Tue, 14 Mar 17
28/74

Comments: 10 pages Latex. Invited contribution for the special issue “Advances in Relativistic Statistical Mechanics” published in Entropy

Is the cosmological dark sector better modeled by a generalized Chaplygin gas or by a scalar field? [CEA]

http://arxiv.org/abs/1703.04149

Both scalar fields and (generalized) Chaplygin gases have been widely used separately to characterize the dark sector of the Universe. Here we investigate the cosmological background dynamics for a mixture of both these components and quantify the fractional abundances that are admitted by observational data from supernovae of type Ia and from the evolution of the Hubble rate.

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S. Campo, J. Fabris, R. Herrera, et. al.
Tue, 14 Mar 17
30/74

Comments: 15 pages, 5 figures

Precision Cosmology from Future Time Delays of Lensed Gravitational Wave and Electromagnetic Signals [CEA]

http://arxiv.org/abs/1703.04151

The”standard-siren” approaches of gravitational wave cosmology appeal to the luminosity distance estimation from the GW observation which relies on the fine details of the waveform. We propose a new waveform independent strategy based on the systems where strongly lensed gravitational waves (GWs) and their electromagnetic (EM) counterparts can be detected simultaneously. With the images and redshifts observed in the EM domain combined with very precise measurements of time delays from lensed GW signals, we can achieve precise cosmography in the era of third-generation gravitational-wave detectors. In particular we demonstrate that the uncertainty of the Hubble constant $H_0$ determination from just $10$ such systems can decrease to $\sim0.4\%$ in a flat $\Lambda$CDM universe.

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K. Liao, X. Fan, X. Ding, et. al.
Tue, 14 Mar 17
51/74

Comments: N/A

Performance comparison of multi-detector detection statistics in targeted compact binary coalescence GW search [CL]

http://arxiv.org/abs/1606.03867

Global network of advanced Interferometric gravitational wave (GW) detectors are expected to be on-line soon. Coherent observation of GW from a distant compact binary coalescence (CBC) with a network of interferometers located in different continents give crucial information about the source such as source location and polarization information. In this paper we compare different multi-detector network detection statistics for CBC search. In maximum likelihood ratio (MLR) based detection approaches, the likelihood ratio is optimized to obtain the best model parameters and the best likelihood ratio value is used as statistic to make decision on the presence of signal. However, an alternative Bayesian approach involves marginalization of the likelihood ratio over the parameters to obtain the average likelihood ratio. We obtain an analytical expression for the Bayesian statistic using the two effective synthetic data streams for targeted search of non-spinning compact binary systems with an uninformative prior on the parameters. Simulations are carried out for testing the validity of the approximation and comparing the detection performance with the maximum likelihood ratio based statistics. We observe that the MLR {\it hybrid} statistic gives comparable or better performance with respect to the Bayesian statistic.

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K. Haris and A. Pai
Tue, 14 Mar 17
60/74

Comments: N/A

The Early Scientific Contributions of J. Robert Oppenheimer: Why Did the Scientific Community Miss the Black Hole Opportunity? [CL]

http://arxiv.org/abs/1703.04234

We aim to carry out an assessment of the scientific value of Oppenheimer’s research on black holes in order to determine and weigh possible factors to explain its neglect by the scientific community, and even by Oppenheimer himself. Dealing primarily with the science and looking closely at the scientific culture and the scientific conceptual belief system of the 1930s, the present article seeks to supplement the existent literature on the subject by enriching the explanations and possibly complicating the guiding questions. We suggest a rereading of Oppenheimer as a more intriguing, ahead-of-his-time figure.

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M. Ortega-Rodriguez, H. Solis-Sanchez, E. Boza-Oviedo, et. al.
Tue, 14 Mar 17
63/74

Comments: 19 pages, Phys. Perspect. (2017)

Order-unity argument for structure-generated "extra" expansion [CEA]

http://arxiv.org/abs/1703.04191

Self-consistent treatment of cosmological structure formation and expansion within the context of classical general relativity may lead to “extra” expansion above that expected in a structureless universe. We argue that in comparison to an early-epoch, extrapolated Einstein-de Sitter model, about 10-15% “extra” expansion is sufficient at the present to render superfluous the “dark energy” 68% contribution to the energy density budget, and that this is observationally realistic.

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B. Roukema, J. Ostrowski, T. Buchert, et. al.
Tue, 14 Mar 17
67/74

Comments: 4 pages, 1 figure

Nonlinear Resonant Oscillation of Gravitational Potential Induced by Ultralight Axion in $f(R)$ Gravity [CEA]

http://arxiv.org/abs/1703.03589

We study the ultralight axion dark matter with mass around $10^{-22}$ eV in $f(R)$ gravity which might resolve the dark energy problem. In particular, we focus on the fact that the pressure of the axion field oscillating in time produces oscillations of gravitational potentials. We show that the oscillation of the gravitational potential is sensitive to the model of gravity. Remarkably, we find that the detectability of the oscillation through the gravitational wave detectors can be significantly enhanced due to the nonlinear resonance between the ultralight axion and the scalaron.

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A. Aoki and J. Soda
Mon, 13 Mar 17
4/48

Comments: 19 pages, 4 figures

An improved algorithm for narrow-band searches of continuous gravitational waves [CL]

http://arxiv.org/abs/1703.03493

Continuous gravitational waves signals, emitted by asymmetric spinning neutron stars, are among the main targets of current detectors like Advanced LIGO and Virgo. In the case of sources, like pulsars, which rotational parameters are measured through electromagnetic observations, typical searches assume that the gravitational wave frequency is at a given known fixed ratio with respect to the star rotational frequency. For instance, for a neutron star rotating around one of its principal axis of inertia the gravitational signal frequency would be exactly two times the rotational frequency of the star. It is possible, however, that this assumption is wrong. This is why search algorithms able to take into account a possible small mismatch between the gravitational waves frequency and the frequency inferred from electromagnetic observations have been developed. In this paper we present an improved pipeline to perform such narrow-band searches for continuous gravitational waves from neutron stars, about three orders of magnitude faster than previous implementations. The algorithm that we have developed is based on the {\it 5-vectors} framework and is able to perform a fully coherent search over a frequency band of width $\mathcal{O}$(Hertz) and for hundreds of spin-down values running a few hours on a standard workstation. This new algorithm opens the possibility of long coherence time searches for objects which rotational parameters are highly uncertain.

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S. Mastrogiovanni, P. Astone, S. DAntonio, et. al.
Mon, 13 Mar 17
5/48

Comments: 19 pages, 8 figures, 6 tables, submitted to CQG

The RIT binary black hole simulations catalog [CL]

http://arxiv.org/abs/1703.03423

The RIT numerical relativity group is releasing a public catalog of black-hole-binary waveforms. The initial release of the catalog consists of 126 recent simulations that include precessing and non precessing systems with mass ratios $q=m_1/m_2$ in the range $1/6\leq q\leq1$. The catalog contains information about the initial data of the simulation, the waveforms extrapolated to infinity, as well as information about the peak luminosity and final remnant black hole properties. These waveforms can be used to independently interpret gravitational wave signals from laser interferometric detectors and

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J. Healy, C. Lousto, Y. Zlochower, et. al.
Mon, 13 Mar 17
7/48

Comments: 5 pages, 3 figures

Effective Description of Higher-Order Scalar-Tensor Theories [CL]

http://arxiv.org/abs/1703.03797

Most existing theories of dark energy and/or modified gravity, involving a scalar degree of freedom, can be conveniently described within the framework of the Effective Theory of Dark Energy, based on the unitary gauge where the scalar field is uniform. We extend this effective approach by allowing the Lagrangian in unitary gauge to depend on the time derivative of the lapse function. Although this dependence generically signals the presence of an extra scalar degree of freedom, theories that contain only one propagating scalar degree of freedom, in addition to the usual tensor modes, can be constructed by requiring the initial Lagrangian to be degenerate. Starting from a general quadratic action, we derive the dispersion relations for the linear perturbations around Minkowski and a cosmological background. Our analysis directly applies to the recently introduced Degenerate Higher-Order Scalar-Tensor (DHOST) theories. For these theories, we find that one cannot recover a Poisson-like equation in the static linear regime except for the subclass that includes the Horndeski and so-called “beyond Horndeski” theories. We also discuss Lorentz-breaking models inspired by Horava gravity.

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D. Langlois, M. Mancarella, K. Noui, et. al.
Mon, 13 Mar 17
12/48

Comments: N/A

Does a Nash theory of gravity make dark energy superfluous? [CL]

http://arxiv.org/abs/1703.02653

Recently Aadne and Gr{\o}n have argued that dark energy may follow naturally from a Nash theory of gravity. In this brief note I argue why this cannot be the case.

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K. Lake
Mon, 13 Mar 17
35/48

Comments: 2 pages comments welcome

Description of the evolution of inhomogeneities on a Dark Matter halo with the Vlasov equation [CL]

http://arxiv.org/abs/1703.03286

We use a direct numerical integration of the Vlasov equation in spherical symmetry with a background gravitational potential to determine the evolution of a collection of particles in different models of a galactic halo. Such a collection is assumed to represent a dark matter inhomogeneity which reaches a stationary state determined by the virialization of the system. We describe some features of the stationary states and, by using several halo models, obtain distinctive signatures for the evolution of the inhomogeneities in each of the models.

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P. Dominguez-Fernandez, E. Jimenez-Vazquez, M. Alcubierre, et. al.
Mon, 13 Mar 17
36/48

Comments: 26 pages, 67 figures, 1 table, submitted to MNRAS

Exotic Compact Objects and How to Quench their Ergoregion Instability [CL]

http://arxiv.org/abs/1703.03696

Gravitational-wave astronomy can give us access to the structure of black holes, potentially probing microscopic or even Planckian corrections at the horizon scale, as those predicted by some quantum-gravity models of exotic compact objects. A generic feature of these models is the replacement of the horizon by a reflective surface. Objects with these properties are prone to the so-called ergoregion instability when they spin sufficiently fast. We investigate in detail a simple model consisting of scalar perturbations of a Kerr geometry with a reflective surface near the horizon. The instability depends on the spin, on the compactness, and on the reflectivity at the surface. The instability time scale increases logarithmically in the black-hole limit but, for a perfectly reflecting object, this is not enough to prevent the instability from occurring on dynamical time scales. However, we find that an absorption rate at the surface as small as 0.4% (reflectivity coefficient as large as $|{\cal R}|^2=0.996$) is sufficient to quench the instability completely. Our results suggest that exotic compact objects are not necessarily ruled out by the ergoregion instability.

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E. Maggio, P. Pani and V. Ferrari
Mon, 13 Mar 17
37/48

Comments: 10 pages, 5 figures

Gravitational collapse to a Kerr-Newman black hole [HEAP]

http://arxiv.org/abs/1703.03223

We present the first systematic study of the gravitational collapse of rotating and magnetised neutron stars to charged and rotating (Kerr-Newman) black holes. In particular, we consider the collapse of magnetised and rotating neutron stars assuming that no pair-creation takes place and that the charge density in the magnetosphere is so low that the stellar exterior can be described as an electrovacuum. Under these assumptions, which are rather reasonable for a pulsar that has crossed the ‘death line’, we show that when the star is rotating, it acquires a net initial electrical charge, which is then trapped inside the apparent horizon of the newly formed back hole. We analyse a number of different quantities to validate that the black hole produced is indeed a Kerr-Newman one and show that, in the absence of rotation or magnetic field, the end result of the collapse is a Schwarzschild or Kerr black hole, respectively.

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A. Nathanail, E. Most and L. Rezzolla
Fri, 10 Mar 17
37/52

Comments: MNRAS accepted

Sourcing Dark Matter and Dark Energy from $α$-attractors [CL]

http://arxiv.org/abs/1703.03295

Recently, Kallosh and Linde have drawn attention to a new family of superconformal inflationary potentials, subsequently called $\alpha$-attractors. The $\alpha$-attractor family can interpolate between a large class of inflationary models. It also has an important theoretical underpinning within the framework of supergravity. We demonstrate that the $\alpha$-attractors have an even wider appeal since they may describe dark matter and perhaps even dark energy. The dark matter associated with the $\alpha$-attractors, which we call $\alpha$-dark matter ($\alpha$DM), shares many of the attractive features of fuzzy dark matter, with $V(\varphi) = \frac{1}{2}m^2\varphi^2$, while having none of its drawbacks. Like fuzzy dark matter, $\alpha$DM can have a large Jeans length which could resolve the cusp-core and substructure problems faced by standard cold dark matter. $\alpha$DM also has an appealing tracker property which enables it to converge to the late-time dark matter asymptote, $\langle w\rangle \simeq 0$, from a wide range of initial conditions. It thus avoids the enormous fine-tuning problems faced by the $m^2\varphi^2$ potential in describing dark matter.

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S. Mishra, V. Sahni and Y. Shtanov
Fri, 10 Mar 17
50/52

Comments: 40 pages, 22 figures

Global stability of self-gravitating disks in modified gravity [GA]

http://arxiv.org/abs/1703.02718

We study the global stability of a self-gravitating disk in the context of Modified Gravity (MOG) using N-body simulations. This theory is a relativistic scalar-tensor-vector theory of gravity and presented to address the dark matter problem. In the weak field limit MOG possesses two free parameters $\alpha$ and $\mu_0$ which have been already determined using rotation curve data of spiral galaxies. The evolution of a stellar self-gravitating disk and more specifically the bar instability in MOG is investigated and compared to a Newtonian case. Our models have exponential and Mestel-like surface densities as $\Sigma\propto \exp(-r/h)$ and $\Sigma\propto 1/r$. It is found out that, surprisingly, the disks are more stable against the bar mode in MOG than in Newtonian gravity. In other words, the bar growth rate is effectively slower than the Newtonian disks. Also we show that both free parameters, i.e. $\alpha$ and $\mu_0$, have stabilizing effects. In other words, increase in these parameters will decrease the bar growth rate.

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N. Ghafourian and M. Roshan
Thu, 9 Mar 17
3/54

Comments: 16 pages, to appear in MNRAS

A review of gravitational waves from cosmic domain walls [CL]

http://arxiv.org/abs/1703.02576

In this contribution, we discuss the cosmological scenario where unstable domain walls are formed in the early universe and their late-time annihilation produces a significant amount of gravitational waves. After describing cosmological constraints on long-lived domain walls, we estimate the typical amplitude and frequency of gravitational waves observed today. We also review possible extensions of the standard model of particle physics that predict the formation of unstable domain walls and can be probed by observation of relic gravitational waves. It is shown that recent results of pulser timing arrays and direct detection experiments partially exclude the relevant parameter space, and that a much wider parameter space can be covered by the next generation of gravitational wave observatories.

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K. Saikawa
Thu, 9 Mar 17
7/54

Comments: 25 pages, 4 figures, invited paper to special issue `Gravitational Waves: Prospects after the First Direct Detections’ in `Universe’

Instabilities in Mimetic Matter Perturbations [CL]

http://arxiv.org/abs/1703.02923

We study cosmological perturbations in mimetic matter scenario with a general higher derivative function. We show that the model suffers from the ghost and the gradient instabilities. We perform the analysis in both comoving and Newtonian gauges and confirm that the Hamiltonians and the associated instabilities are consistent with each other in both gauges. The existence of instabilities is independent of the specific form of higher derivative function which generates gradients for mimetic field perturbations. It is verified that these instabilities are not associated with the higher derivative instabilities such as the Ostrogradsky ghost.

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H. Firouzjahi, M. Gorji and A. Mansoori
Thu, 9 Mar 17
8/54

Comments: N/A

Effects of neutrino mass hierarchies on dynamical dark energy models [CEA]

http://arxiv.org/abs/1703.02556

We investigate how three different possibilities of neutrino mass hierarchies, namely normal, inverted, and degenerate, can affect the observational constraints on three well known dynamical dark energy models, namely the Chevallier-Polarski-Linder, logarithmic, and the Jassal-Bagla-Padmanabhan parametrizations. In order to impose the observational constraints on the models, we performed a robust analysis using Planck 2015 temperature and polarization data, Supernovae type Ia from Joint Light curve analysis, baryon acoustic oscillations distance measurements, redshift space distortion characterized by $f(z)\sigma_8(z)$ data, weak gravitational lensing data from Canada-France-Hawaii Telescope Lensing Survey, and cosmic chronometers data plus the local value of the Hubble parameter. We find that the degenerate hierarchy scheme leads to significant variations on the model parameters in compared to other two neutrino mass hierarchies. It is observed that the fixation of a hierarchy scheme can play an important role in determining some crucial properties in the dynamical dark energy models. We also discussed that these dynamical dark energy models can assuage the current tension on the local Hubble parameter $H_0$.

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W. Yang, R. Nunes, S. Pan, et. al.
Thu, 9 Mar 17
14/54

Comments: 10 pages, 6 figures

Inflationary Dynamics with a Smooth Slow-Roll to Constant-Roll Era Transition [CL]

http://arxiv.org/abs/1703.02853

In this paper we investigate the implications of having a varying second slow-roll index on the canonical scalar field inflationary dynamics. We shall be interested in cases that the second slow-roll can take small values and correspondingly large values, for limiting cases of the function that quantifies the variation of the second slow-roll index. As we demonstrate, this can naturally introduce a smooth transition between slow-roll and constant-roll eras. We discuss the theoretical implications of the mechanism we introduce and we use various illustrative examples in order to better understand the new features that the varying second slow-roll index introduces. In the examples we will present, the second slow-roll index has exponential dependence on the scalar field, and in one of these cases, the slow-roll era corresponds to a type of $\alpha$-attractor inflation. Finally, we briefly discuss how the combination of slow-roll and constant-roll may lead to non-Gaussianities in the primordial perturbations.

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S. Odintsov and V. Oikonomou
Thu, 9 Mar 17
33/54

Comments: N/A

Reconstruction of extended inflationary potentials for attractors [CL]

http://arxiv.org/abs/1703.02220

We give the procedure to reconstruct the extended inflationary potentials for general scalar-tensor theory of gravity and use the $\alpha$ attractor and the constant slow-roll model as examples to show how to reconstruct the class of extended inflationary potentials in the strong coupling limit. The class of extended inflationary potentials have the same attractor in the strong coupling limit, and the reconstructed extended inflationary potentials are consistent with the observational constraints. We also derive the condition on the coupling constant $\xi$ for satisfying the strong coupling.

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Q. Gao and Y. Gong
Thu, 9 Mar 17
41/54

Comments: 12 pages, 3 figures

Axisymmetric force-free magnetosphere in the exterior of a neutron star [HEAP]

http://arxiv.org/abs/1703.02273

Magnetar magnetospheres gradually become twisted due to shearing by footpoint motion. The axisymmetric solutions for the force-free field with a power-law current model are calculated by taking into account general relativistic effects. Here we show how the magnetic energy and helicity are accumulated along a sequence of equilibria. In a strongly twisted case, a magnetic flux rope, in which a large amount of toroidal field is confined, detaches in the vicinity of the star. It is found this kind of magnetic field line structure is easily produced due to strong relativistic effects compared with the results in flat spacetime. Although the structure is not inherent to the relativity, the confinement is possible even for a smaller power-law index. There is an upper bound on the energy and helicity stored in the magnetosphere. When the helicity is further accumulated, a catastrophic event such as a giant flare may occur.

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Y. Kojima
Wed, 8 Mar 17
3/60

Comments: 8 pages, 4 color figures

Über-Gravity and the Cosmological Constant Problem [CL]

http://arxiv.org/abs/1703.02052

Recently, the idea of taking ensemble average over gravity models has been introduced. Based on this idea, we study the ensemble average over (effectively) all the gravity models dubbing the name \”ubergravity. The \”ubergravity has interesting universal properties, independent from the choice of basis: $i)$ it mimics Einstein-Hilbert gravity for high-curvature regime, $ii)$ it predicts stronger gravitational force for an intermediate-curvature regime, $iii)$ surprisingly, for low-curvature regime, i.e. $R<R_0$ where $R$ is Ricci scalar and $R_0$ is a given scale, the Lagrangian vanishes automatically and $iiii)$ there is a sharp transition between low- and intermediate-curvature regimes at $R=R_0$. We show that the \”ubergravity response is robust to any value of the vacuum energy, $\rho_{vac}$. This means the response to any non-vanishing value of $\rho_{vac}$ gives an exact deSitter solution where $R_0$ plays the role of the cosmological constant. Consequently, $R_0$ should be fixed by the observations and there is no need to fine-tune the cosmological constant.

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N. Khosravi
Wed, 8 Mar 17
21/60

Comments: 5 pages, 4 figures, comments are welcomed

General-Relativistic Large-Eddy Simulations of Binary Neutron Star Mergers [HEAP]

http://arxiv.org/abs/1703.02046

The flow inside remnants of binary neutron star (NS) mergers is expected to be turbulent, because of magnetohydrodynamics instability activated at scales too small to be resolved in simulations. To study the large-scale impact of these instabilities, we develop a new formalism, based on the large-eddy simulation technique, for the modeling of subgrid-scale turbulent transport in general relativity. We apply it, for the first time, to the simulation of the late-inspiral and merger of two NSs. We find that turbulence can significantly affect the structure and survival time of the merger remnant, as well as its gravitational-wave (GW) and neutrino emissions. The former will be relevant for GW observation of merging neutron stars. The latter will affect the composition of the outflow driven by the merger and might influence its nucleosynthetic yields. The accretion rate after black-hole formation is also affected. Nevertheless, we find that, for the most likely values of the turbulence mixing efficiency, these effects are relatively small and the GW signal will be affected only weakly by the turbulence. Thus, our simulations provide a first validation of all existing post-merger GW models.

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D. Radice
Wed, 8 Mar 17
32/60

Comments: 6 pages, 5 figures. Accepted for publication on ApJL

Unitary NEC violation in P(X) cosmologies [CL]

http://arxiv.org/abs/1703.00025

A non-singular cosmological bounce in the Einstein frame can only take place if the Null Energy Condition (NEC) is violated. We explore situations where a single scalar field drives the NEC violation and derive the constraints imposed by demanding tree level unitarity on a cosmological background. We then focus on the explicit constraints that arise in P(X) theories and show that constraints from perturbative unitarity make it impossible for the NEC violation to occur within the region of validity of the effective field theory without also involving irrelevant operators that arise at a higher scale that would enter from integrating out more massive degrees of freedom. Within the context of P(X) theories we show that including such operators allows for a bounce that does not manifestly violate tree level unitarity, but at the price of either imposing a shift symmetry or involving technically unnatural small operator coefficients within the low-energy effective field theory.

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C. Rham and S. Melville
Wed, 8 Mar 17
51/60

Comments: 35 pages, 1 figure

Scalar and vector Galileons [CL]

http://arxiv.org/abs/1703.01884

An alternative for the construction of fundamental theories is the introduction of Galileons. These are fields whose action leads to non higher than second-order equations of motion. As this is a necessary but not sufficient condition to make the Hamiltonian bounded from below, as long as the action is not degenerate, the Galileon construction is a way to avoid pathologies both at the classical and quantum levels. Galileon actions are, therefore, of great interest in many branches of physics, specially in high energy physics and cosmology. This proceedings contribution presents the generalities of the construction of both scalar and vector Galileons following two different but complimentary routes.

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Y. Rodriguez and A. Navarro
Tue, 7 Mar 17
2/66

Comments: LaTeX file in jpconf style, 12 pages, no figures. To be published in Journal of Physics: Conference Series as the proceedings of the 70&70 Classical and Quantum Gravitation Party: Meeting with Two Latin American Masters on Theoretical Physics (Cartagena de Indias – Colombia, 28th-30th September, 2016)

Massive Galileon Positivity Bounds [CL]

http://arxiv.org/abs/1702.08577

The EFT coefficients in any gapped, scalar, Lorentz invariant field theory must satisfy positivity requirements if there is to exist a local, analytic Wilsonian UV completion. We apply these bounds to the tree level scattering amplitudes for a massive Galileon. The addition of a mass term, which does not spoil the non-renormalization theorem of the Galileon and preserves the Galileon symmetry at loop level, is necessary to satisfy the lowest order positivity bound. We further show that a careful choice of successively higher derivative corrections are necessary to satisfy the higher order positivity bounds. There is then no obstruction to a local UV completion from considerations of tree level 2-to-2 scattering alone. To demonstrate this we give an explicit example of such a UV completion.

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C. Rham, S. Melville, A. Tolley, et. al.
Tue, 7 Mar 17
26/66

Comments: 31 pages

Black hole acoustics in the minimal geometric deformation of a de Laval nozzle [CL]

http://arxiv.org/abs/1703.01528

The correspondence between sound waves, in a de Laval propelling nozzle, and quasinormal modes emitted by brane-world black holes deformed by a 5D bulk Weyl fluid are here explored and scrutinised. The analysis of sound waves patterns in a de Laval nozzle at a laboratory, reciprocally, is here shown to provide relevant data about the 5D bulk Weyl fluid and its on-brane projection, comprised by the minimal geometrically deformed compact stellar distribution on the brane. Acoustic perturbations of the gas fluid flow in the de Laval nozzle are proved to coincide to the quasinormal modes of black holes solutions deformed by the 5D Weyl fluid, in the geometric deformation procedure. Hence, in a phenomenological E\”otv\”os-Friedmann fluid brane-world model, the realistic shape of a de Laval nozzle is derived and its consequences studied.

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R. Rocha
Tue, 7 Mar 17
32/66

Comments: 7 pages, 3 figures

Higher Derivative Field Theories: Degeneracy Conditions and Classes [CL]

http://arxiv.org/abs/1703.01623

We provide a full analysis of ghost free higher derivative field theories with coupled degrees of freedom. Assuming the absence of gauge symmetries, we derive the degeneracy conditions in order to evade the Ostrogradsky ghosts, and analyze which (non)trivial classes of solutions this allows for. It is shown explicitly how Lorentz invariance avoids the propagation of “half” degrees of freedom. Moreover, for a large class of theories, we construct the field redefinitions and/or (extended) contact transformations that put the theory in a manifestly first order form. Finally, we identify which class of theories cannot be brought to first order form by such transformations.

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M. Crisostomi, R. Klein and D. Roest
Tue, 7 Mar 17
34/66

Comments: 26 pages, 1 figure

Cosmological Dynamics of D-BIonic and DBI Scalar Field and Coincidence Problem of Dark Energy [CL]

http://arxiv.org/abs/1703.01745

We study the cosmological dynamics of D-BIonic and DBI scalar field, which is coupled to matter fluid. For the exponential potential and the exponential couplings, we find a new analytic scaling solution yielding the accelerated expansion of the Universe. Since it is shown to be an attractor for some range of the coupling parameters, the density parameter of matter fluid can be the observed value, as in the coupled quintessence with a canonical scalar field. Contrary to the usual coupled quintessence, where the value of matter couple giving observed density parameter is too large to satisfy observational constraint from CMB, we show that the D-BIonic theory can give similar solution with much smaller value of matter coupling. As a result, together with the fact that the D-BIonic theory has a screening mechanism, the D-BIonic theory can solve the so-called coincidence problem as well as the dark energy problem.

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S. Panpanich, K. Maeda and S. Mizuno
Tue, 7 Mar 17
39/66

Comments: 15 pages, 7 figures

COSMOS-$e'$- soft Higgsotic attractors [CL]

http://arxiv.org/abs/1703.01750

In this work, we have developed an elegant algorithm to study the cosmological consequences from a huge class of quantum field theories (i.e. superstring theory, supergravity, extra dimensional theory, modified gravity etc.), which are equivalently described by soft attractors in the effective field theory framework. In this description we have restricted our analysis for two scalar fields – dilaton and Higgsotic fields minimally coupled with Einstein gravity, which can be generalized for any arbitrary number of scalar field contents with generalized non-canonical and non-minimal interactions. We have explicitly used $R^2$ gravity, from which we have studied the attractor and non-attractor phase by exactly computing two point, three point and four point correlation functions from scalar fluctuations using In-In (Schwinger-Keldysh) and $\delta {\cal N}$ formalism. We have also presented theoretical bounds on the amplitude, tilt and running of the primordial power spectrum, various shapes (equilateral, squeezed, folded kite or counter collinear) of the amplitude as obtained from three and four point scalar functions, which are consistent with observed data. Also the results from two point tensor fluctuations and field excursion formula are explicitly presented for attractor and non-attractor phase. Further, reheating constraints, scale dependent behaviour of the couplings and the dynamical solution for the dilaton and Higgsotic fields are also presented. New sets of consistency relations between two, three and four point observables are also presented, which shows significant deviation from canonical slow roll models. Additionally, three possible theoretical proposals have presented to overcome the tachyonic instability at the time of late time acceleration. Finally, we have also provided the bulk interpretation from the three and four point scalar correlation functions for completeness.

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S. Choudhury
Tue, 7 Mar 17
47/66

Comments: 221 pages, 37 figures, 6 tables