General Relativity And Quantum Cosmology

Within vacuum Weyl gravity, we obtain a solution by which, using different choices of the conformal factor, we derive metrics describing (i)~a bounce of the universe; (ii)~toroidal and spherical wormholes; and (iii)~a change in metric signature. It is demonstrated that singularities occurring in these systems are "masked". We give a simple explanation of the possibility of masking the singularities within Weyl gravity. It is shown that in the first and third cases the three-dimensional metrics form Ricci flows. The question of the possible applicability of conformal Weyl gravity as some phenomenological theory in an approximate description of quantum gravity is discussed.

We study massive scalar field perturbation on Kerr black holes in dynamical Chern-Simons gravity by performing a (2+1) -dimensional simulation. Object pictures of the wave dynamics in time domain are obtained. The tachyonic instability is found to always occur for any nonzero black hole spin and any scalar field mass as long as the coupling constant exceeds a critical value. The presence of the mass term suppresses or even quenches the instability. The quantitative dependence of the onset of the tachyonic instability on the coupling constant, the scalar field mass and the black hole spin is given numerically.

The spacetime of a boosted Bondi-Sachs rotating black hole is considered as a proper background to examine electromagnetic configurations connected to analytic solutions of Maxwell equations. In our analysis, we first use the Bondi-Sachs transformations in order to bring the boosted rotating black hole metric into the Kerr-Schild form, from which zero angular momentum observers (ZAMOs) are constructed via the ADM formalism. In Kerr-Schild coordinates we obtain the Killing fields as sources of Maxwell electrodynamics, and we fix a ZAMO in order to evaluate the components of the electric and magnetic fields, from which we obtain nonsingular patterns of an eventual momentum-energy emission of a boosted Kerr-Schild black hole. Distinct patterns are examined and discussed in the case of variations of the boost parameter γ . We extend our analysis by considering the nonsingular electromagnetic emission in the framework of a boosted Bondi-Sachs rotating black hole, as it moves at relativistic speeds. We also discuss possible mechanisms that may resemble magnetospheres of rotating boosted black holes and give rise to hydromagnetic flows from accretion discs and to the production of jets.

Standard methodologies for the extraction of the stochastic gravitational wave background (SGWB) from auto- or cross-correlation of interferometric signals often involve the use of a filter function. The standard optimal filter maximizes the signal-to-noise ratio (SNR) between the total SGWB and the noise. We derive expressions for the optimal filter and SNR in the presence of a target SGWB plus other unwanted components. We also generalize the methodology to the case of template-free reconstruction. The formalism allows to easily perform analyses and forecasts that marginalize over foreground signals, such as the typical Ω GW ??f 2/3 background arising from binary coalescence. We demonstrate the methodology with the LISA mission and discuss possible extensions and domains of application.

The gravitational waves emitted during the coalescence of binary black holes offer an excellent probe to test the behaviour of strong gravity at different length scales. In this paper, we propose a novel test called the `merger-ringdown consistency test' that focuses on probing horizon-scale dynamics of strong-gravity using the binary black hole ringdowns. We present a proof-of-concept study of this test using simulated binary black hole ringdowns embedded in Einstein Telescope-like noise. Furthermore, we demonstrate the feasibility of our test using a deep learning framework, setting a precedence for performing precision tests of gravity with neural networks.

Transient non-gaussian noise in gravitational wave detectors, commonly referred to as glitches, pose challenges for inference of the astrophysical properties of detected signals when the two are coincident in time. Current analyses aim towards modeling and subtracting the glitches from the data using a flexible, morphology-independent model in terms of sine-gaussian wavelets before the signal source properties are inferred using templates for the compact binary signal. We present a new analysis of gravitational wave data that contain both a signal and glitches by simultaneously modeling the compact binary signal in terms of templates and the instrumental glitches using sine-gaussian wavelets. The model for the glitches is generic and can thus be applied to a wide range of glitch morphologies without any special tuning. The simultaneous modeling of the astrophysical signal with templates allows us to efficiently separate the signal from the glitches, as we demonstrate using simulated signals injected around real O2 glitches in the two LIGO detectors. We show that our new proposed analysis can separate overlapping glitches and signals, estimate the compact binary parameters, and provide ready-to-use glitch-subtracted data for downstream inference analyses.

Modelling the end point of binary black hole mergers is a cornerstone of modern gravitational-wave astronomy. Extracting multiple quasinormal mode frequencies from the ringdown signal allows the remnant black hole to be studied in unprecedented detail. Previous studies on numerical relativity simulations of aligned-spin binaries have found that it is possible to start the ringdown analysis much earlier than previously thought if overtones (and possibly mirror modes) are included. This increases the signal-to-noise ratio in the ringdown making identification of subdominant modes easier. In this paper we study, for the first time, black hole binaries with misaligned spins and find a much greater variation in the performance of ringdown fits than in the aligned-spin case. The inclusion of mirror modes and higher harmonics, along with overtones, improves the reliability of ringdown fits with an early start time; however, there remain cases with poor performing fits. While using overtones in conjunction with an early ringdown start time is an enticing possibility, it is necessary to proceed with caution. We also consider for the first time the use of numerical relativity surrogate models in this type of quasinormal mode study and address important questions of accuracy in the underlying numerical waveforms used for the fit.

In previous work, we established theoretical results concerning the effect of matter shells surrounding a gravitational wave (GW) source, and we now apply these results to astrophysical scenarios. Firstly, it is shown that GW echoes that are claimed to be present in LIGO data of certain events, could not have been caused by a matter shell. However, it is also shown that there are scenarios in which matter shells could make modifications of order a few percent to a GW signal; these scenarios include binary black hole mergers, binary neutron star mergers, and core collapse supernovae.

A consequence of adopting a modified gravitational theory (MOG) for the aLIGO GW190521 gravitational wave detection involving binary black hole sources is to fit the aLIGO strain and chirp data with lower mass, compact coalescing binary systems such as neutron star-neutron star (NS-NS), black hole - neutron star (BH-NS), and black hole-black hole (BH-BH) systems. In MOG BH - BH component masses can be smaller than the component masses m 1 =85 M ⊙ and m 2 =66 M ⊙ inferred from the aLIGO GW190521 gravitational wave event. This reduces the mass of the final remnant mass M f =150 M ⊙ and allows the primary, secondary and final remnant masses of the black holes to be formed by conventional stellar collapse models.

We study static traversable wormholes obtained by Morris and Thorne in general relativity (GR) in the framework of a modified theory of gravity. The modified gravitational action f(R,T) is a function of the Ricci scalar ( R ) and of the trace of the energy momentum tensor ( T ). For a modified gravity f(R,T)=R+α R 2 +λT , where α and λ are constants, we obtain wormhole solutions (WH) with normal matter for a relevant shape functions. The energy conditions are checked at the throat and away from the throat of the WH. The coupling parameters α and λ in the gravitational action play an important role to accommodate the matter composition. For a given λ , WH solutions are found in the presence of exotic matter at the throat for α<0 . It is shown that WH exists in the modified gravity without exotic matter when α>0 . We consider two different shape functions to investigate the existence of WH in the presence of exotic or normal matter. A class of WH solutions exist with anisotropic fluid provided λ?��?8? . In flat asymptotic regions on both sides of the throat with anisotropic source and λ=??? , it does not permit WH as per no go theorem. As λ?? in the gravitational action, all the energy conditions are obeyed with the hybrid shape function indicating existence of WH with normal matter.