Latest Papers in General Relativity

General Relativity And Quantum Cosmology

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Inferring Neutron Star Properties from GW170817 with Universal Relations (1902.04557v1)

Bharat Kumar, Philippe Landry

2019-02-12

Because all neutron stars share a common equation of state, tidal deformability constraints from the compact binary coalescence GW170817 have implications for the properties of neutron stars in other systems. Using equation-of-state insensitive relations between macroscopic observables like moment of inertia (), tidal deformability () and stellar compactness, we derive constraints on these properties as a function of neutron-star mass based on the LIGO-Virgo collaboration's canonical deformability measurement, . Specific estimates of , , dimensionless spin , and stellar radius for a few systems targeted by radio or X-ray studies are extracted from the general constraints. We also infer the canonical neutron-star radius as km at 90 confidence. We further demonstrate how a gravitational-wave measurement of can be combined with independent measurements of neutron-star radii to tighten constraints on the tidal deformability as a proxy for the equation of state. We find that GW170817 and existing observations of six thermonuclear bursters in low-mass X-ray binaries jointly imply at the 90 confidence level.

An alternative way to locally measure the Hubble constant using Gravitational Waves and PTA (1902.04550v1)

Jorge Alfaro, Mauricio Gamonal

2019-02-12

Recent research show that the cosmological components of the Universe should influence on the propagation and observation of Gravitational Waves. Furthermore, it has been proposed a new way to measure the cosmological constant using Pulsar Timing Arrays (PTA). However, these results have considered very particular cases (e.g. a Universe filled only by dark energy or by a mixing with dust). In this work we propose an extension of this model: We considered a Universe filled by different perfect fluids, with arbitrary equations of state, and studied the propagation of Gravitational Waves within the context of the LCDM model. From this framework we obtained a relationship, e.g. an analytical formula that differs by 1.5 % from numerical simulations, between the value of the Hubble constant and the observables of PTA experiments, which are expected to measure time residual from local pulsars due to the pass of Gravitational Waves by the IPTA project in the near future.

Stabilization of Starobinsky-Vilenkin stochastic inflation by an environmental noise (1802.00841v2)

Z. Haba

2018-02-02

We discuss the inflaton in an interaction with an infinite number of fields treated as an environment (noise) with a friction . In a Markovian approximation we obtain a stochastic wave equation (appearing also in the warm inflation models). After the replacement of the environment by the white noise, the stochastic wave equation violates the energy conservation if . We introduce a dark energy as a compensation of the inflaton energy-momentum. We add to the classical wave equation the Starobinsky-Vilenkin noise which in the slow-roll approximation describes the quantum fluctuations in an expanding metric. We investigate the resulting consistent Einstein-Klein-Gordon system in the slow-roll regime. We obtain Fokker-Planck equation for the probability distribution of the inflaton assuming that the dependence of the scale factor and the Hubble variable on the field is known. We obtain explicit stationary solutions of the Fokker-Planck equation assuming that and can approximately be determined in a slow-roll regime with the neglect of noise. We extend the results to the multifield D-dimensional configuration space. We show that in the regime the quantum noise determines the asymptotic behaviour of the stationary distribution. If stays finite then the environmental noise ensures the integrability of the stationary probability. In such a case there is no need to introduce boundary conditions with the purpose to eliminate infinite inflation. The variation of could be interpreted as a sign of a transition from cold inflation to warm inflation.

Effective Quantum Extended Spacetime of Polymer Schwarzschild Black Hole (1902.04542v1)

Norbert Bodendorfer, Fabio M. Mele, Johannes Münch

2019-02-12

The physical interpretation and eventual fate of gravitational singularities in a theory surpassing classical general relativity are puzzling questions that have generated a great deal of interest among various quantum gravity approaches. In the context of loop quantum gravity (LQG), one of the major candidates for a non-perturbative background-independent quantisation of general relativity, considerable effort has been devoted to construct effective models in which these questions can be studied. In these models, classical singularities are replaced by a "bounce" induced by quantum geometry corrections. Undesirable features may arise however depending on the details of the model. In this paper, we focus on Schwarzschild black holes and propose a new effective quantum theory based on polymerisation of new canonical phase space variables inspired by those successful in loop quantum cosmology. The quantum corrected spacetime resulting from the solutions of the effective dynamics is characterised by infinitely many pairs of trapped and anti-trapped regions connected via a space-like transition surface replacing the central singularity. Quantum effects become relevant at a unique mass independent curvature scale, while they become negligible in the low curvature region near the horizon. The effective quantum metric describes also the exterior regions and asymptotically classical Schwarzschild geometry is recovered. We however find that physically acceptable solutions require us to select a certain subset of initial conditions, corresponding to a specific mass (de-)amplification after the bounce. We also sketch the corresponding quantum theory and explicitly compute the kernel of the Hamiltonian constraint operator.

Are quantum corrections on horizon scale physically motivated? (1902.04504v1)

Geoffrey Compère

2019-02-12

The aim of this note is to give an overview to non-specialists of recent arguments from fundamental physics in favor and disfavor of quantum corrections to black hole horizons. I will mainly discuss the black hole information paradox, its possible resolutions and shortly address its relevance or irrelevance to astronomy.

Gravitational Waves from First-Order Phase Transitions: LIGO as a Window to Unexplored Seesaw Scales (1810.12306v3)

Vedran Brdar, Alexander J. Helmboldt, Jisuke Kubo

2018-10-29

Within a recently proposed classically conformal model, in which the generation of neutrino masses is linked to spontaneous scale symmetry breaking, we investigate the associated phase transition and find it to be of strong first order with a substantial amount of supercooling. Carefully taking into account the vacuum energy of the metastable minimum, we demonstrate that a significant fraction of the model's parameter space can be excluded simply because the phase transition cannot complete. We argue this to be a powerful consistency check applicable to general theories based on classical scale invariance. Finally, we show that all remaining parameter points predict a sizable gravitational wave signal, so that the model can be fully tested by future gravitational wave observatories. In particular, most of the parameter space can already be probed by the upcoming LIGO science run starting in early 2019.

Electromagnetism and hidden vector fields in modified gravity theories: spontaneous and induced vectorization (1901.02461v2)

Lorenzo Annulli, Vitor Cardoso, Leonardo Gualtieri

2019-01-08

In general relativity, Maxwell's equations are embedded in curved spacetime through the minimal prescription, but this could change if strong-gravity modifications are present. We show that with a nonminimal coupling between gravity and a massless vector field, nonperturbative effects can arise in compact stars. We find solutions describing stars with nontrivial vector field configurations, some of which are associated to an instability, while others are not. The vector field can be interpreted either as the electromagnetic field, or as a hidden vector field weakly coupled with the standard model.

Soft gravitational radiation from ultra-relativistic collisions at sub- and sub-sub-leading order (1901.10986v2)

Andrea Addazi, Massimo Bianchi, Gabriele Veneziano

2019-01-30

Using soft-graviton theorems a well-known zero-frequency limit (ZFL) for the gravitational radiation flux is re-derived and extended to order and for arbitrary massless multi-particle collisions. The (angle-integrated, unpolarized) correction to the flux turns out to be absent in the case of two-particle elastic collisions. The correction is instead non-vanishing and takes a simple general expression which is then applied to bremsstrahlung from two-particle elastic collisions. For a tree-level process the outcome is finite and consistent with expectations. Instead, if the tree-level form of the soft theorems is used at sub-sub-leading order even when the elastic amplitude needs an all-loop (eikonal) resummation, an unphysical infrared singularity occurs. Its origin can be traced to the infinite Coulomb phase of gravitational scattering in four dimensions. We briefly discuss how to get rid, in principle, of the unwanted divergences and indicate --without carrying out-- a possible procedure to find the proper correction to the naive soft theorems. Nevertheless, if a simple recipe recently proposed for handling these divergences is adopted, we find surprisingly good agreement with results obtained independently via the eikonal approach to transplanckian-energy scattering at large (small) impact parameter (deflection angle), where such Coulomb divergences explicitly cancel out.

Observing the post-merger signal of GW170817-like events with improved gravitational-wave detectors (1811.08931v3)

Andoni Torres-Rivas, Katerina Chatziioannou, Andreas Bauswein, James Alexander Clark

2018-11-21

The recent detection of a neutron star binary through gravitational waves, GW170817, has offered another source of information about the properties of cold supranuclear matter. Information from the signal emitted before the neutron stars merged has been used to study the equation of state of these bodies, however, any complementary information included in the signal emitted after the merger has been lost in the detector noise. In this paper we investigate the prospects of studying GW170817-like post-merger signals with future gravitational-wave detectors. We first compute the expected properties of the possible GW170817 post-merger signal using information from pre-merger analyses. We then quantify the required improvement in detector sensitivity in order to extract key features of the post-merger signal. We find that if we observe a signal of similar strength to GW170817 when the aLIGO detectors have been improved by times over their design sensitivity in the kHz regime, we will be able to extract the dominant frequency component of the post-merger. With further improvements and next-generation detectors we will also be able to extract subdominant frequencies. We conclude that post-merger signals could be brought within our reach in the coming years given planned detector upgrades, such as A+, Voyager, and the next-generation detectors.

Scale-invariant in inflation with 1-loop quantum corrections (1902.04434v1)

Silvia Vicentini, Luciano Vanzo, Massimiliano Rinaldi

2019-02-12

We study quantum corrections to an inflationary model, which has the attractive feature of being classically scale-invariant. In this model, quadratic gravity plays along a scalar field in such a way that inflation begins near the unstable point of the effective potential and it ends at a stable fixed point, where the scale symmetry is broken and a fundamental mass scale naturally emerges. We compute the one loop corrections to the classical action on the curved background of the model and we report their effects on the classical dynamics with both analytical and numerical methods.

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