Latest Papers in General Relativity

General Relativity And Quantum Cosmology

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A new type of dark compact objects in massive tensor-multi-scalar theories of gravity (1901.06379v1)

Stoytcho S. Yazadjiev, Daniela D. Doneva

2019-01-18

In the present paper we consider special classes of tensor-multi-scalar theories of gravity whose target-space metric admits Killing field(s) with a periodic flow. For such tensor-multi-scalar theories we show that if the dynamics of the scalar fields is confined on the periodic orbits of the Killing field(s) then there exists a new type of compact objects - the tensor-multi-scalar solitons formed by a condensation of the gravitational scalars. The existence of the tensor-multi-scalar solitons is proven by solving the fully non-linear eigenvalue problem which follows from the dimensional reduction of the field equations of tensor-multi-scalar theories of gravity. The mass of the tensor-multi-scalar solitons can range at least from the mass of a neutron star to the mass of dark objects in the center of the galaxies in dependence of mass(es) of the gravitational scalars and which sector of massive scalars is excited. These facts show that the tensor-multi-scalar solitons could have important implications for the dark matter problem. The existence of the tensor-multi-scalar solitons points towards the possibility that the dark matter, or part of it, is made of condensed gravitational scalars.

Effective field description of the Anton-Schmidt cosmic fluid (1810.05844v2)

Salvatore Capozziello, Rocco D'Agostino, Roberto Giambò, Orlando Luongo

2018-10-13

The effective theory of the Anton-Schmidt cosmic fluid within the Debye approximation is investigated. In this picture, the universe is modeled out by means of a medium without cosmological constant. In particular, the Anton-Schmidt representation of matter describes the pressure of crystalline solids under deformations imposed by isotropic stresses. The approach scheme is related to the fact that the universe deforms under the action of the cosmic expansion itself. Thus, we frame the dark energy term as a function of scalar fields and obtain the corresponding dark energy potential . Different epochs of the universe evolution are investigated in terms of the evolution of . We show how the Anton-Schmidt equation of state is capable of describing both late and early epochs of cosmic evolution. Finally, numerical bounds on the Anton-Schmidt model with are derived through a Markov Chain Monte Carlo analysis on the combination of data coming from type Ia Supernovae, observations of Hubble parameter and baryon acoustic oscillations. Statistical comparison with the CDM model is performed by the AIC and BIC selection criteria. Results are in excellent agreement with the low-redshift data. A further generalization of the model is presented to satisfy the theoretical predictions at early-stage cosmology.

Quantum Gravity on the computer: Impressions of a workshop (1811.12264v2)

Lisa Glaser, Sebastian Steinhaus

2018-11-29

Computer simulations allow us to explore non-perturbative phenomena in physics. This has the potential to help us understand quantum gravity. Finding a theory of quantum gravity is a hard problem, but in the last decades many promising and intriguing approaches that utilize or might benefit from using numerical methods were developed. These approaches are based on very different ideas and assumptions, yet they face the common challenge to derive predictions and compare them to data. In March 2018 we held a workshop at the Nordic Institute for Theoretical Physics (NORDITA) in Stockholm gathering experts in many different approaches to quantum gravity for a workshop on "Quantum gravity on the computer". In this article we try to encapsulate some of the discussions held and talks given during this workshop and combine them with our own thoughts on why and how numerical approaches will play an important role in pushing quantum gravity forward. The last section of the article is a road map providing an outlook of the field and some intentions and goalposts that were debated in the closing session of the workshop. We hope that it will help to build a strong numerical community reaching beyond single approaches to combine our efforts in the search for quantum gravity.

On particle creation/decay in nonminimally coupled models of gravity (1901.06299v1)

R. P. L. Azevedo, P. P. Avelino

2019-01-18

In extended models of gravity a nonminimal coupling to matter has been assumed to lead to irreversible particle creation. In this paper we challenge this assumption. We argue that a non-minimal coupling of the matter and gravitational sectors results in a change in particle-momentum on a cosmological timescale, irrespective of particle creation or decay. We further argue that particle creation or decay associated with a non-minimal coupling to gravity could only happen as a result of significant deviations from a homogeneous Friedmann-Robertson-Walker geometry on microscopic scales, and provide a phenomenological description of the impact of particle creation or decay on the cosmological evolution of the density of the matter fields.

Axionic instabilities and new black hole solutions (1811.04945v2)

Mateja Boskovic, Richard Brito, Vitor Cardoso, Taishi Ikeda, Helvi Witek

2018-11-12

The coupling between scalar and vector fields has a long and interesting history. Axions are one key possibility to solve the strong CP problem and axion-like particles could be one solution to the dark matter puzzle. Given the nature of the coupling, and the universality of free fall, nontrivial important effects are expected in regions where gravity is strong. Here, we show that i. A background EM field induces an axionic instability in flat space, for large enough electric fields. Conversely, a homogeneous harmonic axion field induces an instability in the Maxwell sector. When carried over to curved spacetime, this phenomena translates into generic instabilities of charged black holes (BHs). ii. In the presence of charge, BH uniqueness results are lost. We find solutions which are small deformations of the Kerr-Newman geometry and hairy stationary solutions without angular momentum, which are `dragged' by the axion. Axion fields must exist around spinning BHs if these are immersed in external magnetic fields. The axion profile can be obtained perturbatively from the electro-vacuum solution derived by Wald. iii. Ultralight axions trigger superradiant instabilities of spinning BHs and form an axionic cloud in the exterior geometry. The superradiant growth can be interrupted or suppressed through axionic or scalar couplings to EM. These couplings lead to periodic bursts of light, which occur throughout the history of energy extraction from the BH. We provide numerical and simple analytical estimates for the rates of these processes. iv. Finally, we discuss how plasma effects can affect the evolution of superradiant instabilities.

A macronova associated with GRB 070809 (1901.06269v1)

Zhi-Ping Jin, Stefano Covino, Neng-Hui Liao, Xiang Li, Paolo D'Avanzo, Yi-Zhong Fan, Da-Ming Wei

2019-01-18

GRB 070809 is a typical short gamma-ray burst (sGRB) detected by the Neil Gehrels {\it Swift} Observatory and at the location of the burst no underlying galaxy down to th AB magnitude in F606W-band has been detected. The X-ray emission was detected quickly after the trigger of the burst and the late time spectrum is very hard. The optical component, substantially brighter than the X-ray extrapolation, is also roughly consistent with a thermal-like emission, and is inconsistent with any afterglow spectrum. Such a peculiar optical to X-ray afterglow spectrum has not been identified previously for any typical sGRBs. The optical component can be naturally interpreted as a blue macronova (also known as kilonova) powered by the lanthanide-poor/free material launched during the neutron star merger. Our finding demonstrates the possibility of revealing the neutron star merger origin with the early afterglow data of some sGRBs that take place well beyond the sensitive radius of the advanced gravitational wave detectors and hence the opportunity of organizing dedicated follow-up observations for events of interest.

General relativity experiment with frozen spin rings (1901.06217v1)

Andras Laszlo

2019-01-18

In experimental proposals published in the last two decades, a so called frozen spin storage ring concept was proposed for setting upper experimental bounds to electric dipole moment (EDM) of elementary particles. In a recent paper of ours, a fully covariant general relativistic (GR) calculation was presented on the Earth's gravitational modification effect in such mixed magnetic-electric frozen spin storage ring on the spin transport, which could contribute to such measurement. It was shown that similarly to an EDM signal, GR causes a spin precession out of the orbital plane, under the frozen spin condition. The rate of the vertical polarization buildup is predicted to be -abetagammag/c, where g is the gravitational acceleration on the surface of the Earth, c is the speed of light, betagamma is the particle momentum over mass, and 'a' is its magnetic moment anomaly. It is seen that the effect increases unboundedly with the Lorentz factor gamma. Moreover, it is proportional to the magnetic moment anomaly 'a'. This paper mainly addresses the experimental perspectives to detect this effect in a realistic frozen spin storage ring configuration. Such a measurement would provide a novel test of GR, sampling the tensorial nature of GR at a microscopic level, as acting on the spin vector of elementary particles. The conclusion is that the pertinent GR experiment seems to be realistic with large magnetic moment anomaly particles, such as tritons, helion3 or protons, whereas it is not realistic with small magnetic moment anomaly particles, such as deuterons, muons or electrons.

Thermodynamics and Phase Transition of a Nonlinear Electrodynamics Black Hole in a Cavity (1901.06216v1)

Peng Wang, Houwen Wu, Haitang Yang

2019-01-18

We discuss the thermodynamics of a general nonlinear electrodynamics (NLED) asymptotically flat black hole enclosed in a finite spherical cavity. A canonical ensemble is considered, which means that the temperature and the charge on the wall of the cavity are fixed. After the free energy is obtained by computing the Euclidean action, it shows that the first law of thermodynamics is satisfied at the locally stationary points of the free energy. Focusing on a Born-Infeld (BI) black hole in a cavity, the phase structure and transition in various regions of the parameter space are investigated. In the region where the BI electrodynamics has weak nonlinearities, Hawking-Page-like and van der Waals-like phase transitions occur, and a tricritical point appears. In the region where the BI electrodynamics has strong enough nonlinearities, only Hawking-Page-like phase transitions occur. The phase diagram of the BI black hole in a cavity can have dissimilarity from that of a BI black hole using asymptotically anti-de Sitter boundary conditions. The dissimilarity may stem from a lack of an appropriate reference state with the same charge and temperature for the BI-AdS black hole.

Following the density perturbations through a bounce with AdS/CFT Correspondence (1701.04287v2)

Lei Ming, Taifan Zheng, Yeuk-Kwan E. Cheung

2017-01-16

A bounce universe model, known as the coupled-scalar-tachyon bounce (CSTB) universe, has been shown to solve the Horizon, Flatness and Homogeneity problems as well as the Big Bang Singularity problem. Furthermore a scale invariant spectrum of primordial density perturbations generated from the phase of pre-bounce contraction is shown to be stable against time evolution. In this work we study the detailed dynamics of the bounce and its imprints on the scale invariance of the spectrum. The dynamics of the gravitational interactions near the bounce point may be strongly coupled as the spatial curvature becomes big. There is no a prior reason to expect the spectral index of the primordial perturbations of matter density can be preserved. By encoding the bounce dynamics holographically onto the dynamics of dual Yang-Mills system while the latter is weakly coupled, via the AdS/CFT correspondence, we can safely evolve the spectrum of the cosmic perturbations with full control. In this way we can compare the post-bounce spectrum with the pre-bounce one: in the CSTB model we explicitly show that the spectrum of primordial density perturbations generated in the contraction phase preserves its stability as well as scale invariance throughout the bounce process.

Orbits in a stochastic Schwarzschild geometry (1901.06206v1)

R. Casadio, A. Giusti, A. Mentrelli

2019-01-18

We study geodesics in the Schwarzschild space-time affected by an uncertainty in the mass parameter described by a Gaussian distribution. This study could serve as a first attempt at investigating possible quantum effects of black hole space-times on the motion of matter in their surroundings as well as the role of uncertainties in the measurement of the black hole parameters.

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