Recent Detection Of Gravitational Waves Research Articles

Dark Matter as a Non-Relativistic Bose–Einstein Condensate with Massive Gravitons

We confront a non-relativistic Bose–Einstein Condensate (BEC) model of light bosons interacting gravitationally either through a Newtonian or a Yukawa potential with the observed rotational curves of 12 dwarf galaxies. The baryonic component is modeled as an axisymmetric exponential disk and its characteristics are derived from the surface luminosity profile of the galaxies. The purely baryonic fit is unsatisfactory, hence a dark matter component is clearly needed. The rotational curves of five galaxies could be explained with high confidence level by the BEC model. For these galaxies, we derive: (i) upper limits for the allowed graviton mass; and (ii) constraints on a velocity-type and a density-type quantity characterizing the BEC, both being expressed in terms of the BEC particle mass, scattering length and chemical potential. The upper limit for the graviton mass is of the order of 10 − 26 eV / c 2 , three orders of magnitude stronger than the limit derived from recent gravitational wave detections.

Tidal Love numbers of neutron stars in f(R) gravity

The recent detection of gravitational waves from a neutron star merger was a significant step towards constraining the nuclear matter equation of state by using the tidal Love numbers (TLNs) of the merging neutron stars. Measuring or constraining the neutron star TLNs allows us in principle to exclude or constraint many equations of state. This approach, however, has the drawback that many modified theories of gravity could produce deviations from General Relativity similar to the deviations coming from the uncertainties in the equation of state. The first and the most natural step in resolving the mentioned problem is to quantify the effects on the TLNs from the modifications of General Relativity. With this motivation in mind, in the present paper we calculate the TLNs of (non-rotating) neutron stars in R^2-gravity. More precisely, by solving numerically the perturbation equations, we calculate explicitly the polar and the axial l=2 TLNs for three characteristic realistic equations of state and compare the results to General Relativity. Our results show that while the polar TLNs are slightly influenced by the R^2 modification of General Relativity, the axial TLNs can be several times larger (in terms of the absolute value) compared to the general relativistic case.

Mapping incoherent gravitational wave backgrounds

Given the recent detection of gravitational waves from individual sources it is almost a certainty that some form of background of gravitational waves will be detected in future. The most promising candidate for such a detection are backgrounds made up of incoherent superposition of the signal of unresolved astrophysical or, backgrounds sourced by earlier cosmological events. Such backgrounds will also contain anisotropies about an average value. The information contained in the background level and any anisotropies will be extremely valuable as an astrophysical and cosmological probe. As such, the ability to reconstruct sky maps of the signal will become important as the sensitivity increases. We build and test a pixel--based, maximum--likelihood Gravitational Wave Background (GWB) map-maker that uses the cross-correlation of sets of generalised baselines as input. The resulting maps are a representation of the GWB power, or strain "intensity" on the sky. We test the algorithm by reconstructing known input maps with different baseline configurations. We also apply the map-maker to a subset of the Advance LIGO data.

Analogue simulation of gravitational waves in a 3+1 -dimensional Bose-Einstein condensate

The recent detections of gravitational waves (GWs) by the LIGO and Virgo collaborations have opened the field of GW astronomy, intensifying interest in GWs and other possible detectors sensitive in different frequency ranges. Although strong GW producing events are rare and currently unpredictable, GWs can in principle be simulated in analogue systems at will in the lab. Simulation of GWs in a manifestly quantum system would allow for the study of the interaction of quantum phenomena with GWs. Such predicted interaction is exploited in a recently proposed Bose-Einstein condensate (BEC) based GW detector. In this paper, we show how to manipulate a BEC to mimic the effect of a passing GW. By simultaneously varying the external potential applied to the BEC, and an external magnetic field near a Feshbach resonance, we show that the resulting change in speed of sound can directly reproduce a GW metric. We also show how to simulate a metric used in the recently proposed BEC based GW detector, to provide an environment for testing the proposed metrology scheme of the detector. Explicit expressions for simulations of various GW sources are given. This result is also useful to generally test the interaction of quantum phenomena with GWs in a curved spacetime analogue experiment.

Black hole echology: The observer’s manual

While recent detections of gravitational waves from the mergers of binary black holes match well with the predictions of General Relativity (GR), they cannot directly confirm the existence of event horizons. Exotic compact objects (ECOs) are motivated by quantum models of black holes, and can have exotic structure (or a "wall") just outside the (would-be) horizon. ECOs produce similar ringdown waveforms to the GR black holes, but they are followed by delayed "echoes". By solving linearized Einstein equations we can model these echoes and provide analytic templates that can be used to compare to observations. For concreteness, we consider GW150914 event, detected by the LIGO/Virgo collaboration, and study the model dependence of its echo properties. We find that echoes are reasonably approximated by complex gaussians, with amplitudes that decay as a power law in time, while their width in time (frequency) grows (shrinks) over subsequent echoes. We also show that trapped modes between a perfectly reflecting wall and angular momentum barrier in Kerr metric can exhibit superradiant instability over long times, as expected.

Neutron star merger GW170817 strongly constrains doubly coupled bigravity

We study the implications of the recent detection of gravitational waves emitted by a pair of merging neutron stars and their electromagnetic counterpart, events GW170817 and GRB170817A, on the viability of the doubly coupled bimetric models of cosmic evolution, where the two metrics couple directly to matter through a composite, effective metric. We demonstrate that the bounds on the speed of gravitational waves place strong constraints on the doubly coupled models, forcing either the two metrics to be proportional at the background level or the models to become singly coupled. Proportional backgrounds are particularly interesting as they provide stable cosmological solutions with phenomenologies equivalent to that of $\Lambda$CDM at the background level as well as for linear perturbations, while nonlinearities are expected to show deviations from the standard model.

Exploring stellar evolution with gravitational-wave observations

Recent detections of gravitational waves from merging binary black holes opened new possibilities to study the evolution of massive stars and black hole formation. In particular, stellar evolution models may be constrained on the basis of the differences in the predicted distribution of black hole masses and redshifts. In this work we propose a framework that combines galaxy and stellar evolution models and use it to predict the detection rates of merging binary black holes for various stellar evolution models. We discuss the prospects of constraining the shape of the time delay distribution of merging binaries using just the observed distribution of chirp masses. Finally, we consider a generic model of primordial black hole formation and discuss the possibility of distinguishing it from stellar-origin black holes.

Searching for gamma-ray counterparts to gravitational waves from merging binary neutron stars with the Cherenkov Telescope Array

The merger of binary neutron star (BNS) systems are predicted to be progenitors of short gamma-ray bursts (GRBs); the definitive probe of this association came with the recent detection of gravitational waves (GWs) from a BNS merger by Advanced LIGO and Advanced Virgo (GW170817), in coincidence with the short GRB 170817A observed by Fermi-GBM and INTEGRAL. Short GRBs are also expected to emit very-high energy (VHE, > 10S0 GeV) photons and VHE electromagnetic (EM) upper limits have been set with observations performed by ground-based gamma-ray detectors and during the intense EM follow-up campaign associated with GW170817/GRB 170817A. In the next years, the searches for VHE EM counterparts will become more effective thanks to the Cherenkov Telescope Array (CTA): this instrument will be fundamental for the EM follow-up of transient GW events at VHE, owing to its unprecedented sensitivity, rapid response (few tens of seconds) and capability to monitor large sky areas via survey-mode operation. We present a comprehensive study on the prospects for joint GW and VHE EM observations of merging BNSs with Advanced LIGO, Advanced Virgo and CTA, based on detailed simulations of the multi-messenger emission and detection. We propose a new observational strategy optimized on the prior assumptions about the EM emission. The method can be further generalized to include other electromagnetic emission models. According to this study CTA will cover most of the region of the GW skymap for the intermediate and most energetic on-axis GRBs associated to the GW event. We estimate the expected joint GW and VHE EM detection rates and we found this rate goes from 0.08 up to 0.5 events per year for the most energetic EM sources.

The future of gravitational theories in the era of the gravitational wave astronomy

We discuss the future of gravitational theories in the framework of gravitational wave (GW) astronomy after the recent GW detections (the events GW150914, GW151226, GW170104, GW170814, GW170817 and GW170608). In particular, a calculation of the frequency and angular dependent response function that a GW detector would see if massive modes from [Formula: see text] theories or scalar–tensor gravity (STG) were present, allowing for sources incident from any direction on the sky, is shown. In addition, through separate theoretical results which do not involve the recent GW detections, we show that [Formula: see text] theories of gravity having a third massless mode are ultimately ruled out while there is still room for STG having a third (massive or massless) mode and for [Formula: see text] theories of gravity having a third massive mode.

Large-scale structure phenomenology of viable Horndeski theories

Phenomenological functions $\Sigma$ and $\mu$, also known as $G_{\rm light}/G$ and $G_{\rm matter}/G$, are commonly used to parameterize modifications of the growth of large-scale structure in alternative theories of gravity. We study the values these functions can take in Horndeski theories, i.e. the class of scalar-tensor theories with second order equations of motion. We restrict our attention to models that are in a broad agreement with tests of gravity and the observed cosmic expansion history. In particular, we require the speed of gravity to be equal to the speed of light today, as required by the recent detection of gravitational waves and electromagnetic emission from a binary neutron star merger. We examine the correlations between the values of $\Sigma$ and $\mu$ analytically within the quasi-static approximation, and numerically, by sampling the space of allowed solutions. We confirm that the conjecture made in [Pogosian:2016pwr], that $(\Sigma-1)(\mu -1) \ge 0$ in viable Horndeski theories, holds very well. Along with that, we check the validity of the quasi-static approximation within different corners of Horndeski theory. Our results show that, even with the tight bound on the present day speed of gravitational waves, there is room within Horndeski theories for non-trivial signatures of modified gravity at the level of linear perturbations.

Double Neutron Star Mergers and Short Gamma-ray Bursts: Long-lasting High-energy Signatures and Remnant Dichotomy

Abstract The recent detection of gravitational waves and electromagnetic counterparts from the double neutron star merger event GW+EM170817 supports the standard paradigm of short gamma-ray bursts (SGRBs) and kilonovae/macronovae. It is important to reveal the nature of the compact remnant left after the merger, either a black hole or neutron star, and their physical link to the origin of the long-lasting emission observed in SGRBs. The diversity of the merger remnants may also lead to different kinds of transients that can be detected in future. Here we study the high-energy emission from the long-lasting central engine left after the coalescence, under certain assumptions. In particular, we consider the X-ray emission from a remnant disk and the nonthermal nebular emission from disk-driven outflows or pulsar winds. We demonstrate that late-time X-ray and high-frequency radio emission can provide useful constraints on properties of the hidden compact remnants and their connections to long-lasting SGRB emission, and we discuss the detectability of nearby merger events through late-time observations at ∼30–100 days after the coalescence. We also investigate the GeV–TeV gamma-ray emission that occurs in the presence of long-lasting central engines and show the importance of external inverse Compton radiation due to upscattering of X-ray photons by relativistic electrons in the jet. We also search for high-energy gamma rays from GW170817 in the Fermi-LAT data and report upper limits on such long-lasting emission. Finally, we consider the implications of GW+EM170817 and discuss the constraints placed by X-ray and high-frequency radio observations.

The gravitational wave stress–energy (pseudo)-tensor in modified gravity

The recent detections of gravitational waves by the advanced LIGO and Virgo detectors open up new tests of modified gravity theories in the strong-field and dynamical, extreme gravity regime. Such tests rely sensitively on the phase evolution of the gravitational waves, which is controlled by the energy–momentum carried by such waves out of the system. We here study four different methods for finding the gravitational wave stress–energy pseudo-tensor in gravity theories with any combination of scalar, vector, or tensor degrees of freedom. These methods rely on the second variation of the action under short-wavelength averaging, the second perturbation of the field equations in the short-wavelength approximation, the construction of an energy complex leading to a Landau–Lifshitz tensor, and the use of Noether’s theorem in field theories about a flat background. We apply these methods in general relativity, Jordan–Fierz–Brans–Dicky theoy, and Einstein-Æther theory to find the gravitational wave stress–energy pseudo-tensor and calculate the rate at which energy and linear momentum is carried away from the system. The stress–energy tensor and the rate of linear momentum loss in Einstein-Æther theory are presented here for the first time. We find that all methods yield the same rate of energy loss, although the stress–energy pseudo-tensor can be functionally different. We also find that the Noether method yields a stress–energy tensor that is not symmetric or gauge-invariant, and symmetrization via the Belinfante procedure does not fix these problems because this procedure relies on Lorentz invariance, which is spontaneously broken in Einstein-Æther theory. The methods and results found here will be useful for the calculation of predictions in modified gravity theories that can then be contrasted with observations.

Comments on gravitoelectromagnetism of Ummarino and Gallerati in \u201cSuperconductor in a weak static gravitational field\u201d vs other versions

Recently reported [Eur. Phys. J. C., 77, 549 (2017). https://doi.org/10.1140/epjc/s10052-017-5116-y] gravitoelectromagnetic equations of Ummarino and Gallerati (UG) in their linearized version of general relativity (GR) are shown to match with (a) our previously reported special relativistic Maxwellian Gravity equations in the non-relativistic limit and with (b) the non-relativistic equations derived here, when the speed of gravity c_g (an undetermined parameter of the theory here) is set equal to c (the speed of light in vacuum). Seen in the light of our new results, the UG equations satisfy the Correspondence Principle (cp), while many other versions of linearized GR equations that are being (or may be) used to interpret the experimental data defy the cp. Such new findings assume significance and relevance in the contexts of recent detection of gravitational waves and the gravitomagnetic field of the spinning earth and their interpretations. Being well-founded and self-consistent, the equations may be of interest and useful to researchers exploring the phenomenology of gravitomagnetism, gravitational waves and the novel interplay of gravity with different states of matter in flat space-time like UG’s interesting work on superconductors in weak gravitational fields.

Challenges for testing the no-hair theorem with current and planned gravitational-wave detectors

General relativity's no-hair theorem states that isolated astrophysical black holes are described by only two numbers: mass and spin. As a consequence, there are strict relationships between the frequency and damping time of the different modes of a perturbed Kerr black hole. Testing the no-hair theorem has been a longstanding goal of gravitational-wave astronomy. The recent detection of gravitational waves from black hole mergers would seem to make such tests imminent. We investigate how constraints on black hole ringdown parameters scale with the loudness of the ringdown signal---subject to the constraint that the post-merger remnant must be allowed to settle into a perturbative, Kerr-like state. In particular, we require that---for a given detector---the gravitational waveform predicted by numerical relativity is indistinguishable from an exponentially damped sine after time $t^\text{cut}$. By requiring the post-merger remnant to settle into such a perturbative state, we find that confidence intervals for ringdown parameters do not necessarily shrink with louder signals. In at least some cases, more sensitive measurements probe later times without necessarily providing tighter constraints on ringdown frequencies and damping times. Preliminary investigations are unable to explain this result in terms of a numerical relativity artifact.

Quasinormal modes of modified gravity (MOG) black holes

The Quasinormal modes (QNMs) for gravitational and electromagnetic perturbations are calculated in a Scalar–Tensor–Vector (Modified Gravity) spacetime, which was initially proposed to obtain correct dynamics of galaxies and galaxy clusters without the need for dark matter. It is found that for the increasing model parameter α, both the real and imaginary parts of the QNMs decrease compared to those for a standard Schwarzschild black hole. On the other hand, when taking into account the 1/(1+α) mass re-scaling factor present in MOG, Im(ω) matches almost identically that of GR, while Re(ω) is higher. These results can be identified in the ringdown phase of massive compact object mergers, and are thus timely in light of the recent gravitational wave detections by LIGO.

Probing Galactic Black Holes with Microlensing with Gaia and OGLE

AbstractAs shown by recent gravitational wave detections, galaxies harbour an unknown population of black holes at high masses. In our Galaxy such dark objects can be found and studied solely via gravitational microlensing methods. This paper described our search for black-hole lenses both in archived OGLE data and among on-going microlensing events found by OGLE and Gaia. That combination of superb time-domain astrometry and photometry will enable us to derive masses and distances to these dark lenses uniquely, and to describe the demographics of the unseen component of the Milky Way.

Gravitational wave sources: reflections and echoes

The recent detection of gravitational waves has generated interest in alternatives to the black hole interpretation of sources. A subset of such alternatives involves a prediction of gravitational wave ‘echoes’. We consider two aspects of possible echoes: first, general features of echoes coming from spacetime reflecting conditions. We find that the detailed nature of such echoes does not bear any clear relationship to quasi-normal frequencies. Second, we point out the pitfalls in the analysis of local reflecting ‘walls’ near the horizon of rapidly rotating black holes.

Gravitational-wave observations from ground-based detectors

Recent detections of gravitational waves by the LIGO detectors herald a new era of observational astronomy. Previously invisible objects and phenomena may now be uncovered through their gravitational interaction. Observation of gravitational waves allows one to explore the extremes of the Universe and study astronomy and fundamental physics like never before. This article gives a brief overview of the detection process, from the production of the data to their physical implications.

GTR component of planetary precession

Even as the theory of relativity is more than a hundred years old, it is not within easy reach of undergraduate students. These students have an insatiable urge to learn more about it even if the full machinery of the tools required to study the same is not within their comfortable reach. The recent detection of gravitational waves has only augmented their enthusiasm about the General Theory of Relativity (GTR), developed just over a hundred years now, encapsulated in Einstein’s Field Equations. The GTR provided a consistent formulation of the theory of gravity, removed the anomalies in the Newtonian model, and predicted spectacular natural phenomena which eventual experiments have testified to. This pedagogical article retraces some of the major milestones that led to the GTR and presents a simple numerical simulation of the GTR advance of the perihelion of planetary motion about the sun.

Gravitational Waves and Spacetime

The recent detection of gravitational waves by the LIGO team has rightly been hailed as “the crowning achievemen of classical physics” (Holst et al. in Bull Am Math Soc 53:513–554, 2016). This detection, which came at the end of a decade-long quest, involved 950 investigators, and cost around one billion US dollars, was the scientific star of the year 2015. What, if any, is the philosophical impact of this scientific breakthrough, which Albert Einstein had anticipated one century earlier? To answer this question we start by examining the central equations of Einstein’s theory of gravitation, also known as general relativity. Subsequently we analyze the special case of a hollow sphere, in an attempt to answer the question of the reality and even materiality of space or, rather, spacetime. As well, the view that gravitation is a manifestation of the curvature of spacetime is discussed, and the reality of gravitational waves is regarded as the coup de grâce to that view.