Gravitational Waves

We provide an updated assessment of the fundamental physics potential of LISA. Given the very broad range of topics that might be relevant to LISA, we present here a sample of what we view as particularly promising directions, based in part on the current research interests of the LISA scientific community in the area of fundamental physics. We organize these directions through a ``science-first'' approach that allows us to classify how LISA data can inform theoretical physics in a variety of areas. For each of these theoretical physics classes, we identify the sources that are currently expected to provide the principal contribution to our knowledge, and the areas that need further development. The classification presented here should not be thought of as cast in stone, but rather as a fluid framework that is amenable to change with the flow of new insights in theoretical physics.

The present Editorial introduces the Special Issue dedicated by the journal Universe to the General Theory of Relativity, the beautiful theory of gravitation of Einstein, a century after its birth. It reviews some of its key features in a historical perspective, and, in welcoming distinguished researchers from all over the world to contribute it, some of the main topics at the forefront of the current research are outlined.

Stellar-mass black holes (BHs) are expected to segregate and form a steep density cusp around supermassive black holes (SMBHs) in galactic nuclei. We follow the evolution of a multimass system of BHs and stars by numerically integrating the Fokker-Planck energy diffusion equations for a variety of BH mass distributions. We find that the BHs 'self-segregate', and that the rarest, most massive BHs dominate the scattering rate closest to the SMBH (10 −1 pc). BH-BH binaries form out of gravitational wave emission during BH encounters. We find that the expected rate of BH coalescence events detectable by Advanced LIGO is ∼1-10 2 yr −1 , depending on the initial mass function of stars in galactic nuclei and the mass of the most massive BHs. We find that the actual merger rate is likely ∼10 times larger than this due to the intrinsic scatter of stellar densities in many different galaxies. The BH binaries that form this way in galactic nuclei have significant eccentricities as they enter the LIGO band (90 per cent with e > 0.9), and are therefore distinguishable from other binaries, which circularize before becoming detectable. We also show that eccentric mergers can be detected to larger distances and greater BH masses than circular mergers, up to ∼700 M. Future ground-based gravitational wave observatories will be able to constrain both the mass function of BHs and stars in galactic nuclei.

We make a thorough investigation of the asymptotic quasinormal modes of the four and five-dimensional Schwarzschild black hole for scalar, electromagnetic and gravitational perturbations. Our numerical results give full support to all the analytical predictions by Motl and Neitzke, for the leading term. We also compute the first order corrections analytically, by extending to higher dimensions, previous work of Musiri and Siopsis, and find excellent agreement with the numerical results. For generic spacetime dimension number D the first-order corrections go as $\frac{1}{n^{(D-3)/(D-2)}}$. This means that there is a more rapid convergence to the asymptotic value for the five dimensional case than for the four dimensional case, as we also show numerically.

In this paper, the first of a series, we study the stellar dynamical and evolutionary processes leading to the formation of compact binaries containing white dwarfs in dense globular clusters. We examine the processes leading to the creation of X-ray binaries such as cataclysmic variables and AM CVn systems. Using numerical simulations, we identify the dominant formation channels and we predict the expected numbers and characteristics of detectable systems, emphasizing how the cluster sources differ from the field population. We explore the dependence of formation rates on cluster properties and we explain in particular why the distribution of cataclysmic variables has only a weak dependence on cluster density. We also discuss the frequency of dwarf nova outbursts in globular clusters and their connection with moderately strong white dwarf magnetic fields. We examine the rate of Type Ia supernovae via both single and double degenerate channels in clusters and we argue that those rates may contribute to the total SN Ia rate in elliptical galaxies. Considering coalescing white dwarf binaries we discuss possible constraints on the common envelope evolution of their progenitors and we derive theoretical expectations for gravitational wave detection by LISA.

Supermassive black hole binaries (SMBHBs) in galactic nuclei are thought to be a common by-product of major galaxy mergers. We use simple disk models for the circumbinary gas and for the binary-disk interaction to follow the orbital decay of SMBHBs with a range of total masses (M) and mass ratios (q), through physically distinct regions of the disk, until gravitational waves (GWs) take over their evolution. Prior to the GW-driven phase, the viscous decay is in the stalled "secondary-dominated" regime. SMBHBs spend a non-negligible fraction of 10^7 years at orbital periods t_var between a day and a year. A dedicated optical or X-ray survey could identify coalescing SMBHBs statistically, as a population of periodically variable quasars, whose abundance N_var is proportional to t_var^alpha, in a range of periods t_var around tens of weeks. SMBHBs with M < 10^7 M_sun, with 0.5 < alpha < 1.5, would probe the physics of viscous orbital decay, whereas the detection of a population of higher-mass binaries, with alpha=8/3, would confirm that their decay is driven by GWs. The lowest mass SMBHBs (M < 10^{5-6} M_sun) enter the GW-driven regime at short orbital periods, in the frequency band of the Laser Interferometric Space Antenna (LISA). While viscous processes are strongly sub-dominant in the last few years of coalescence, they could reduce the amplitude of any unresolved background of near-stationary LISA sources. We discuss constraints on the SMBHB population available from existing data, and the sensitivity and sky coverage requirements for a detection in future surveys. SMBHBs may also be identified from velocity shifts in their spectra; we discuss the expected abundance of SMBHBs as a function of their orbital velocity.

In this third paper in a series on stable magnetic equilibria in stars, I look at the stability of axisymmetric field configurations and in particular at the relative strengths of the toroidal and poloidal components. Both toroidal and poloidal fields are unstable on their own, and stability is achieved by adding the two together in some ratio. I use Tayler's (1973) stability conditions for toroidal fields and other analytic tools to predict the range of stable ratios and then check these predictions by running numerical simulations. It is found that while the poloidal field can account for no more than approximately 80% of the total energy, it can account for a very small fraction of the energy, i.e. that the toroidal field can be-and is likely to be-significantly stronger than the poloidal. Furthermore, the weaker the field, the weaker the poloidal component can be in relation to the toroidal. The implications of this result are discussed in various contexts such as the emission of gravitational waves by neutron stars, free precession, and a 'hidden' energy source for magnetars.

We analyze BICEP2 and Planck data using a model that includes CMB lensing, gravity waves, and polarized dust. Planck dust polarization maps have highlighted the difficulty of estimating the dust polarization in low intensity regions, suggesting that the polarization fractions have considerable uncertainties and may be significantly higher than previous predictions. In this paper, we start by assuming nothing about the dust polarization except for the power spectrum shape, which we take to be CBBl∝l−2.42. The resulting joint BICEP2+Planck analysis favors solutions without gravity waves, and the upper limit on the tensor-to-scalar ratio is r<0.11, a slight improvement relative to the Planck analysis alone which gives r<0.13 (95% c.l.). The estimated amplitude of the dust polarization power spectrum agrees with expectations for this field based on both HI column density and Planck polarization measurements at 353 GHz in the BICEP2 field. Including the latter constraint in our analysis improves the limit further to r<0.09, placing strong constraints on inflation (e.g., models with r>0.14 are excluded with 99.5% confidence). We address the cross-correlation analysis of BICEP2 at 150 GHz with BICEP1 at 100 GHz as a test of foreground contamination. We find that the null hypothesis of dust and lensing with r=0 gives Δχ2<2 relative to the hypothesis of no dust, so the frequency analysis does not strongly favor either model over the other. We also discuss how more accurate dust polarization maps may improve our constraints. If the dust polarization is measured perfectly, the limit can reach r<0.05, but this degrades quickly to almost no improvement if the dust calibration error is 20% or larger or if the dust maps are not processed through the BICEP2 pipeline, inducing sampling variance noise. (Abridged.)

In this paper, we study the formation and dynamical evolution of black hole–black hole (BH–BH) binaries in young star clusters (YSCs), by means of N-body simulations. The simulations
include metallicity-dependent recipes for stellar evolution and stellar winds, and have been run for three different metallicities (Z = 0.01, 0.1 and 1 Z ). Following recent theoretical models of wind mass-loss and core-collapse supernovae, we assume that the mass of the
stellar remnants depends on the metallicity of the progenitor stars. We find that BH–BH binaries form efficiently because of dynamical exchanges: in our simulations, we find about
10 times more BH–BH binaries than double neutron star binaries. The simulated BH–BH binaries form earlier in metal-poor YSCs, which host more massive black holes (BHs) than in
metal-rich YSCs. The simulated BH–BH binaries have very large chirp masses (up to 80 M ), because the BH mass is assumed to depend on metallicity, and because BHs can grow in mass
due to the merger with stars. The simulated BH–BH binaries span a wide range of orbital periods (10−3 –107 yr), and only a small fraction of them (0.3 per cent) is expected to merge
within a Hubble time. We discuss the estimated merger rate from our simulations and the implications for Advanced VIRGO and LIGO.

The recent Nobel-prize-winning detections of gravitational waves from merging black holes and the subsequent observations of the collision of two neutron stars have inaugurated a new era of multimessenger astrophysics. In this article, we apply deep dilated convolutional networks with time-series inputs for classification and regression with LIGO data. We demonstrate for the first time that machine learning can detect and estimate the true parameters of real events observed by LIGO. Our results show that deep learning achieves similar accuracies compared to matched-filtering while being far more computationally efficient and more resilient to glitches, allowing real-time processing of weak time-series signals in non-stationary non-Gaussian noise with minimal resources, and also enables the detection of new classes of gravitational wave sources.

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, supergrav-ity, 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 δN 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.

Initiatives by the IndIGO (Indian Initiative in Gravitational Wave
Observations) Consortium during the past three years have materialized into concrete plans and project opportunities for instrumentation and research based on advanced interferometer detectors . With the LIGO-India opportunity, this initiative has a taken a promising path towards significant participation in gravitational wave (GW) astronomy and research, and in developing and nurturing precision fabrication
and measurement technologies in India. The proposed LIGO-India detector will foster integrated development of frontier GW research in India and will provide opportunity for substantial contributions to global GW research and astronomy. Widespread interest and enthusiasm about these developments in premier research and educational institutions in India leads to the expectation that there will be a grand surge of activity in precision metrology, instrumentation, data
handling and computation etc. in the context of LIGO-India. I discuss the scope of such research in the backdrop of the current status of the IndIGO action plan and the LIGO-India project.

In this article, our prime objective is to study the inflationary paradigm
from generalized tachyon (GTachyon) living on the world volume of a non-BPS string
theory. The tachyon action is considered here is getting modified compared to the
original action. One can quantify the amount of the modification via a power q
instead of 1/2 in the effective action. Using this set up we study inflation from
various types of tachyonic potentials, using which we constrain the index q within,
1/2 < q < 2, and a specific combination (∝ α
0M4
s
/gs) of Regge slope α
0
, string
coupling constant gs and mass scale of tachyon Ms, from the recent Planck 2015
and Planck+BICEP2/Keck Array joint data. We explicitly study the inflationary
consequences from single field, assisted field and multi-field tachyon set up. Specifi-
cally for single field and assisted field case we derive the results in the quasi-de-Sitter
background in which we will utilize the details of cosmological perturbations and
quantum fluctuations. Also we derive the expressions for all inflationary observables
using any arbitrary vacuum and Bunch-Davies vacuum. For single field and assisted
field case we derive-the inflationary flow equations, new sets of consistency relations.
Also we derive the field excursion formula for tachyon, which shows that assisted
inflation is in more safer side compared to the single field case to validate effective
field theory framework. Further we study the features of CMB Angular power spectrum
from TT, TE and EE correlations from scalar fluctuations within the allowed
range of q for each potentials from single field set-up. We also put constraints from
the temperature anisotropy and polarization spectra, which shows that our analysis
is consistent with the Planck 2015 data. Finally, using δN formalism we derive the
expressions for inflationary observables in the context of multi-field tachyons

We reexamine the stochastic gravitational wave background resulting from inflation and its effect on the cosmic microwave background radiation (CMBR). Measurement by the Cosmic Background Explorer satellite of a CMBR quadrupole anisotropy places an upper limit on the vacuum ...

We also characterize two other types of gravitational wave signals that could arise in principle from a rapidly rotating, secularly unstable neutron star: a high-frequency ($f\go 1000$ Hz) wave which increases the pattern-speed of the star, and a wave that actually increases the angular momentum of the star.

Recent numerical relativistic results demonstrate that the merger of comparable-mass spinning black holes has a maximum "recoil kick" of up to ∼ 4000 km s −1 . However the scaling of these recoil velocities with mass ratio is poorly understood. We present new runs showing that the maximum possible kick perpendicular to the orbital plane does not scale as ∼ η 2 (where η is the symmetric mass ratio), as previously proposed, but is more consistent with ∼ η 3 , at least for systems with low orbital precession. We discuss the effect of this dependence on galactic ejection scenarios and retention of intermediate-mass black holes in globular clusters.

Massive merging black holes will be the primary sources of powerful gravitational waves at low frequency, and will permit to test general relativity with candidate galaxies close to a binary black hole merger. In this paper we identify the typical mass ratio of the two black holes but then show that the distance where gravitational radiation becomes the dominant dissipative effect (over dynamical friction) does not depend on the mass ratio; however the dynamical evolution in the gravitational wave emission regime does. For the typical range of mass ratios the final stage of the merger is preceded by a rapid precession and a subsequent spin-flip of the main black hole. This already occurs in the inspiral phase, therefore can be described analytically by post-Newtonian techniques. We then identify the radio galaxies with a superdisk as those in which the rapidly precessing jet produces effectively a powerful wind, entraining the environmental gas to produce the appearance of a thick disk. These specific galaxies are thus candidates for a merger of two black holes to happen in the astronomically near future.

We study the dynamics of stellar-mass black holes (BH) in star clusters with particular attention to the formation of BH-BH binaries, which are interesting as sources of gravitational waves (GW). In the present study, we examine the properties of these BH-BH binaries through direct N-body simulations of star clusters using the NBODY6 code on Graphical Processing Unit (GPU) platforms. We perform simulations for star clusters with 10 5 low-mass stars starting from Plummer models with an initial population of BHs, varying the cluster-mass and BH-retention fraction. Additionally, we do several calculations of star clusters confined within a reflective boundary mimicking only the core of a massive star cluster which can be performed much faster than the corresponding full cluster integration. We find that stellar-mass BHs with masses ∼ 10M ⊙ segregate rapidly (∼ 100 Myr timescale) into the cluster core and form a dense sub-cluster of BHs within typically 0.2 − 0.5 pc radius. In such a subcluster, BH-BH binaries can be formed through 3-body encounters, the rate of which can become substantial in dense enough BH-cores. While most BH binaries are finally ejected from the cluster by recoils received during super-elastic encounters with the single BHs, few of them harden sufficiently so that they can merge via GW emission within the cluster. We find that for clusters with N 5 × 10 4 , typically 1 -2 BH-BH mergers occur per cluster within the first ∼ 4 Gyr of cluster evolution. Also for each of these clusters, there are a few escaping BH binaries that can merge within a Hubble time, most of the merger times being within a few Gyr. These results indicate that intermediate-age massive clusters constitute the most important class of candidates for producing dynamical BH-BH mergers. Old globular clusters cannot contribute significantly to the present-day BH-BH merger rate since most of the mergers from them would have occurred much earlier. On the other hand, young massive clusters with ages less that 50 Myr are too young to produce significant number of BH-BH mergers. We finally discuss the detection rate of BH-BH inspirals by the "LIGO" and "Advanced LIGO" GW detectors. Our results indicate that dynamical BH-BH binaries constitute the dominant channel for BH-BH merger detection.

We developed a new method for time-domain signal processing based on deep (dilated) convolutional neural networks which can rapidly identify and extract signals much weaker than the background noise. We applied this method for gravitational wave analysis and demonstrated that it significantly outperforms existing matched-filtering and conventional machine learning techniques and also extends the range of gravitational waves that can be detected by advanced LIGO, allowing real-time processing of raw big data with minimal resources. This initiates a new paradigm for research which uses massively-parallel simulations to train artificial intelligence algorithms that exploit emerging hardware architectures such as deep-learning-optimized GPUs. This approach provides a natural framework to enable coincident detection campaigns of gravitational waves sources and their electromagnetic counterparts.

Neutron star mergers are among the most promising sources of gravitational waves for advanced ground-based detectors. These mergers are also expected to power bright electromagnetic signals, in the form of short gamma-ray bursts, infrared/optical transients powered by r-process nucleosynthesis in neutron-rich material ejected by the merger, and radio emission from the interaction of that ejecta with the interstellar medium. Simulations of these mergers with fully general relativistic codes are critical to understand the merger and post-merger gravitational wave signals and their neutrinos and electromagnetic counterparts. In this paper, we employ the Spectral Einstein Code (SpEC) to simulate the merger of low-mass neutron star binaries (two 1.2M neutron stars) for a set of three nuclear-theory based, finite temperature equations of state. We show that the frequency peaks of the post-merger gravitational wave signal are in good agreement with predictions obtained from recent simulations using a simpler treatment of gravity. We find, however, that only the fundamental mode of the remnant is excited for long periods of time: emission at the secondary peaks is damped on a millisecond timescale in the simulated binaries. For such low-mass systems, the remnant is a massive neutron star which, depending on the equation of state, is either permanently stable or long-lived (i.e. rapid uniform rotation is sufficient to prevent its collapse). We observe strong excitations of l = 2, m = 2 modes, both in the massive neutron star and in the form of hot, shocked tidal arms in the surrounding accretion torus. We estimate the neutrino emission of the remnant using a neutrino leakage scheme and, in one case, compare these results with a gray two-moment neutrino transport scheme. We confirm the complex geometry of the neutrino emission, also observed in previous simulations with neutrino leakage, and show explicitly the presence of important differences in the neutrino luminosity, disk composition, and outflow properties between the neutrino leakage and transport schemes.

The cosmological background of gravitational waves can be tuned by Extended Theories of Gravity. In particular, it can be shown that assuming a generic function $f(R)$ of the Ricci scalar $R$ gives a parametric approach to control the evolution and the production mechanism of gravitational waves in the early Universe.

We consider gravitational waves emitted by various populations of compact binaries at cosmological distances. We use population synthesis models to characterize the properties of double neutron stars, double black holes and double white dwarf binaries as well as white dwarf-neutron star, white dwarf-black hole and black hole-neutron star systems. We use the observationally determined cosmic star formation history to reconstruct the redshift distribution of these sources and their merging rate evolution. The gravitational signals emitted by each source during its early-inspiral phase add randomly to produce a stochastic background in the low frequency band with spectral strain amplitude between ∼ 10 −18 Hz −1/2 and ∼ 5 × 10 −17 Hz −1/2 at frequencies in the interval [∼ 5 × 10 −6 − 5 × 10 −5 ] Hz.

We incorporate a model for black hole growth during galaxy mergers into the semi-analytical galaxy formation model based on ΛCDM proposed by Baugh et al. . Our black hole model has one free parameter, which we set by matching the observed zeropoint of the local correlation between black hole mass and bulge luminosity. We present predictions for the evolution with redshift of the relationships between black hole mass and bulge properties. Our simulations reproduce the evolution of the optical luminosity function of quasars. We study the demographics of the black hole population and address the issue of how black holes acquire their mass. We find that the direct accretion of cold gas during starbursts is an important growth mechanism for lower mass black holes and at high redshift. On the other hand, the reassembly of pre-existing black hole mass into larger units via merging dominates the growth of more massive black holes at low redshift. This prediction could be tested by future gravitational wave experiments. As redshift decreases, progressively less massive black holes have the highest fractional growth rates, in line with recent claims of "downsizing" in quasar activity.

We perform magnetohydrodynamic simulations in full general relativity of uniformly rotating stars that are marginally unstable to collapse. These simulations model the direct collapse of supermassive stars (SMSs) to seed black holes that can grow to become the supermassive black holes at the centers of quasars and active galactic nuclei. They also crudely model the collapse of massive Population III stars to black holes, which could power a fraction of distant, long gamma-ray bursts. The initial stellar models we adopt are Γ = 4/3 polytropes initially with a dynamically unimportant dipole magnetic field. We treat initial magnetic-field configurations either confined to the stellar interior or extending out from the interior into the stellar exterior. We find that the black hole formed following collapse has mass MBH 0.9M (where M is the mass of the initial star) and dimensionless spin parameter aBH/MBH 0.7. A massive, hot, magnetized torus surrounds the remnant black hole. At ∆t ∼ 400−550M ≈ 2000−2700(M/10 6 M )s following the gravitational wave peak amplitude, an incipient jet is launched. The disk lifetime is ∆t ∼ 10 5 (M/10 6 M )s, and the jet outgoing Poynting luminosity is LEM ∼ 10 51−52 ergs/s. If 1 − 10% of this power is converted into gamma-rays, Swift and Fermi could potentially detect these events out to large redshifts z ∼ 20. Thus, SMSs could be sources of ultra-long gammaray bursts (ULGRBs) and massive Population III stars could be the progenitors that power a fraction of the long GRBs observed at redshift z ∼ 5−8. Gravitational waves are copiously emitted during the collapse, and peak at ∼ 15(10 6 M /M )mHz (∼ 0.15(10 4 M /M )Hz), i.e., in the LISA (DECIGO/BBO) band; optimally oriented SMSs could be detectable by LISA (DECIGO/BBO) at z 3 (z 11). Hence 10 4 M SMSs collapsing at z ∼ 10 are promising multimessenger sources of coincident gravitational and electromagnetic waves. PACS numbers: 04.25.D-, 47.75.+f, 97.60.-s, 95.30.Qd

We present a first simulation of the post-merger evolution of a black hole-neutron star binary in full general relativity using an energy-integrated general relativistic truncated moment formalism for neutrino transport. We describe our implementation of the moment formalism and important tests of our code, before studying the formation phase of a disk after a black hole-neutron star merger. We use as initial data an existing general relativistic simulation of the merger of a neutron star of 1.4 solar mass with a black hole of 7 solar mass and dimensionless spin a/M=0.8. Comparing with a simpler leakage scheme for the treatment of the neutrinos, we find noticeable differences in the neutron to proton ratio in and around the disk, and in the neutrino luminosity. We find that the electron neutrino luminosity is much lower in the transport simulations, and that the remnant is less neutron-rich. The spatial distribution of the neutrinos is significantly affected by relativistic effects. Over the short timescale evolved, we do not observe purely neutrino-driven outflows. However, a small amount of material (3e-4Msun) is ejected in the polar region during the circularization of the disk. Most of that material is ejected early in the formation of the disk, and is fairly neutron rich. Through r-process nucleosynthesis, that material should produce high-opacity lanthanides in the polar region, and could thus affect the lightcurve of radioactively powered electromagnetic transients. We also show that by the end of the simulation, while the bulk of the disk is neutron-rich, its outer layers have a higher electron fraction. As that material would be the first to be unbound by disk outflows on longer timescales, the changes in Ye experienced during the formation of the disk could have an impact on the nucleosynthesis outputs from neutrino-driven and viscously-driven outflows.

We investigate a purely stellar dynamical solution to the Final Parsec Problem. Galactic nuclei resulting from major mergers are not spherical, but show some degree of triaxiality. With N -body simulations, we show that massive black hole binaries (MBHB) hosted by them will continuously interact with stars on centrophilic orbits and will thus inspiral-in much less than a Hubble timedown to separations at which gravitational wave (GW) emission is strong enough to drive them to coalescence. Such coalescences will be important sources of GWs for future space-borne detectors such as the Laser Interferometer Space Antenna (LISA). Based on our results, we expect that LISA will see between ∼ 10 to ∼ few × 10 2 such events every year, depending on the particular MBH seed model as obtained in recent studies of merger trees of galaxy and MBH co-evolution. Orbital eccentricities in the LISA band will be clearly distinguishable from zero with e 0.001 − 0.01.

Freeman Dyson has questioned whether any conceivable experiment in the real universe can detect a single graviton. If not, is it meaningful to talk about gravitons as physical entities? We attempt to answer Dyson’s question and find it is possible concoct an idealized thought experiment capable of detecting one graviton; however, when anything remotely resembling realistic physics is taken into account, detection becomes impossible, indicating that Dyson’s conjecture is very likely true. We also point out several mistakes in the literature dealing with graviton detection and production.

The recent measurement of the Shapiro delay in the radio pulsar PSR J1614−2230 yielded a mass of 1.97±0.04 M ⊙ , making it the most massive pulsar known to date. Its mass is high enough that, even without an accompanying measurement of the stellar radius, it has a strong impact on our understanding of nuclear matter, gamma-ray bursts, and the generation of gravitational waves from coalescing neutron stars. This single high mass value indicates that a transition to quark matter in neutron-star cores can occur at densities comparable to the nuclear saturation density only if the quarks are strongly interacting and are color superconducting. We further show that a high maximum neutron-star mass is required if short duration gamma-ray bursts are powered by coalescing neutron stars and, therefore, this mechanism becomes viable in the light of the recent measurement. Finally, we argue that the low-frequency (≤ 500 Hz) gravitational waves emitted during the final stages of neutron-star coalescence encode the properties of the equation of state because neutron stars consistent with this measurement cannot be centrally condensed. This will facilitate the measurement of the neutron star equation of state with Advanced LIGO/Virgo.

The detection of gravitational waves with LIGO and Virgo requires a detailed understanding of the response of these instruments in the presence of environmental and instrumental noise. Of particular interest is the study of anomalous non-Gaussian noise transients known as glitches, since their high occurrence rate in LIGO/Virgo data can obscure or even mimic true gravitational wave signals. Therefore, successfully identifying and excising glitches is of utmost importance to detect and characterize gravitational waves. In this article, we present the first application of Deep Learning combined with Transfer Learning for glitch classification, using real data from LIGO's first discovery campaign labeled by Gravity Spy, showing that knowledge from pre-trained models for real-world object recognition can be transferred for classifying spectrograms of glitches. We demonstrate that this method enables the optimal use of very deep convolutional neural networks for glitch classification given small unbalanced training datasets, significantly reduces the training time, and achieves state-of-the-art accuracy above 98.8%. Once trained via transfer learning, we show that the networks can be truncated and used as feature extractors for unsupervised clustering to automatically group together new classes of glitches and anomalies. This novel capability is of critical importance to identify and remove new types of glitches which will occur as the LIGO/Virgo detectors gradually attain design sensitivity.

We calculate the density fluctuations-both curvature and isocurvature-that arise due to quantum fluctuations in a simple model of extended inflation based upon the 3ordan-Brans-Dicke theory. Curvature fluctuations arise due to quantum fluctuations in the Brans-Dicke field, in general have a nonscale-invariant spectrum, and can have an amplitude that is cosmologically acceptable and interesting without having to tune any coupling constant to a very small value. The density perturbations the' arise due to the inflaton field are subdominaat. If there are other massless fidds in the theory, e.g., an axion or au ilion, then isocurvature fluctuations arise in these fields too. Production of gravitational waves and the massless particles associated with excitations of the Brans-Dicke field are also discussed. Several attempts at more realistic models of extended inflation are also analyzed. The importance of the Einstein conformal frame in calculating curvature fluctuations is emphasized. When viewed in this fra_ne, extended inflation closely resembles slow-rollover inflation with an exponential potential and the usual formula for the amplitude of curvature perturbations applies.

Collapsing supermassive stars (SMSs) with masses M 10 4−6 M have long been speculated to be the seeds that can grow and become supermassive black holes (SMBHs). We previously performed general rel-ativistic magnetohydrodynamic (GRMHD) simulations of marginally stable Γ = 4/3 polytropes uniformly rotating at the mass-shedding limit and endowed initially with a dynamically unimportant dipole magnetic field to model the direct collapse of SMSs. These configurations are supported entirely by thermal radiation pressure and reliably model SMSs with M 10 6 M. We found that around 90% of the initial stellar mass forms a spinning black hole (BH) remnant surrounded by a massive, hot, magnetized torus, which eventually launches a magnetically-driven jet. SMSs could be therefore sources of ultra-long gamma-ray bursts (ULGRBs). Here we perform GRMHD simulations of Γ 4/3, polytropes to account for the perturbative role of gas pressure in SMSs with M 10 6 M. We also consider different initial stellar rotation profiles. The stars are initially seeded with a dynamically weak dipole magnetic field that is either confined to the stellar interior or extended from its interior into the stellar exterior. We calculate the gravitational wave burst signal for the different cases. We find that the mass of the black hole remnant is 90%−99% of the initial stellar mass, depending sharply on Γ−4/3 as well as on the initial stellar rotation profile. After t ∼ 250 − 550M ≈ 1 − 2 × 10 3 (M/10 6 M)s following the appearance of the BH horizon, an incipient jet is launched and it lasts for ∼ 10 4 − 10 5 (M/10 6 M)s, consistent with the duration of long gamma-ray bursts. Our numerical results suggest that the Blandford-Znajek mechanism powers the incipient jet. They are also in rough agreement with our recently proposed universal model that estimates accretion rates and electromagnetic (Poynting) luminosities that characterize magnetized BH-disk remnant systems that launch a jet. This model helps explain why the outgoing electromagnetic luminosities computed for vastly different BH-disk formation scenarios all reside within a narrow range (∼ 10 52±1 erg s −1), roughly independent of M .

Black hole-neutron star mergers resulting in the disruption of the neutron star and the formation of an accretion disk and/or the ejection of unbound material are prime candidates for the joint detection of gravitational-wave and electromagnetic signals when the next generation of gravitational-wave detectors comes online. However, the disruption of the neutron star and the properties of the post-merger remnant are very sensitive to the parameters of the binary. In this paper, we study the impact of the radius of the neutron star and the alignment of the black hole spin for systems within the range of mass ratio currently deemed most likely for field binaries (M_BH ~ 7 M_NS) and for black hole spins large enough for the neutron star to disrupt (J/M^2=0.9). We find that: (i) In this regime, the merger is particularly sensitive to the radius of the neutron star, with remnant masses varying from 0.3M_NS to 0.1M_NS for changes of only 2 km in the NS radius; (ii) 0.01-0.05M_sun of unbound material can be ejected with kinetic energy >10^51 ergs, a significant increase compared to low mass ratio, low spin binaries. This ejecta could power detectable optical and radio afterglows. (iii) Only a small fraction (<3%) of the Advanced LIGO events in this parameter range have gravitational-wave signals which could offer constraints on the equation of state of the neutron star. (iv) A misaligned black hole spin works against disk formation, with less neutron star material remaining outside of the black hole after merger, and a larger fraction of that material remaining in the tidal tail instead of the forming accretion disk. (v) Large kicks (v>300 km/s) can be given to the final black hole as a result of a precessing BHNS merger, when the disruption of the neutron star occurs just outside or within the innermost stable spherical orbit.

Double white dwarfs (WDs) are expected to be a source of confusion-limited noise for the future gravitational wave observatory LISA. In a specific frequency range, this "foreground noise" is predicted to rise above the instrumental noise and hinder the detection of other types of signals, e.g., gravitational waves arising from stellar-mass objects inspiraling into massive black holes. In many previous studies, only detached populations of compact object binaries have been considered in estimating the LISA gravitational wave foreground signal. Here, we investigate the influence of compact object detached and Roche-Lobe overflow (RLOF) Galactic binaries on the shape and strength of the LISA signal. Since >99% of remnant binaries that have orbital periods within the LISA sensitivity range are WD binaries, we consider only these binaries when calculating the LISA signal. We find that the contribution of RLOF binaries to the foreground noise is negligible at low frequencies, but becomes significant at higher frequencies, pushing the frequency at which the foreground noise drops below the instrumental noise to >6 mHz. We find that it is important to consider the population of mass-transferring binaries in order to obtain an accurate assessment of the foreground noise on the LISA data stream. However, we estimate that there still exists a sizeable number (~11,300) of Galactic double WD binaries that will have a signal-to-noise ratio >5, and thus will be potentially resolvable with LISA. We present the LISA gravitational wave signal from the Galactic population of WD binaries, show the most important formation channels contributing to the LISA disk and bulge populations, and discuss the implications of these new findings.

Recent numerical relativistic simulations of black hole coalescence suggest that in certain alignments the emission of gravitational radiation can produce a kick of several thousand kilometers per second. This exceeds galactic escape speeds, hence unless there a mechanism to prevent this, one would expect many galaxies that had merged to be without a central black hole. Here we show that in most galactic mergers, torques from accreting gas suffice to align the orbit and spins of both black holes with the large-scale gas flow. Such a configuration has a maximum kick speed < 200 km s −1 , safely below galactic escape speeds. We predict, however, that in mergers of galaxies without much gas, the remnant will be kicked out several percent of the time. We also discuss other predictions of our scenario, including implications for jet alignment angles and X-type radio sources.

Scalar-tensor gravity theories with a nonminimal Gauss-Bonnet coupling typically lead to an anomalous propagation speed for gravitational waves, and have therefore been tightly constrained by multimessenger observations such as GW170817/GRB170817A. In this paper we show that this is not a general feature of scalar-tensor theories, but rather a consequence of assuming that spacetime torsion vanishes identically. At least for the case of a nonminimal Gauss-Bonnet coupling, removing the torsionless condition restores the canonical dispersion relation and therefore the correct propagation speed for gravitational waves. To achieve this result we develop a new approach, based on the first-order formulation of gravity, to deal with perturbations on these Riemann-Cartan geometries.

Coincident detections of electromagnetic (EM) and gravitational wave (GW) signatures from coalescence events of supermassive black holes are the next observational grand challenge. Such detections will provide the means to study cosmological evolution and accretion processes associated with these gargantuan compact objects. More generally, the observations will enable testing general relativity in the strong, nonlinear regime and will provide independent cosmological measurements to high precision. Understanding the conditions under which coincidences of EM and GW signatures arise during supermassive black hole mergers is therefore of paramount importance. As an essential step towards this goal, we present results from the first fully general relativistic, hydrodynamical study of the late inspiral and merger of equal-mass, spinning supermassive black hole binaries in a gas cloud. We find that variable EM signatures correlated with GWs can arise in merging systems as a consequence of shocks and accretion combined with the effect of relativistic beaming. The most striking EM variability is observed for systems where spins are aligned with the orbital axis and where orbiting black holes form a stable set of density wakes, but all systems exhibit some characteristic signatures that can be utilized in searches for EM counterparts. In the case of the most massive binaries observable by the Laser Interferometer Space Antenna, calculated luminosities imply that they may be identified by EM searches to z ≈ 1, while lower mass systems and binaries immersed in low density ambient gas can only be detected in the local universe.

The search for extra dimensions is a challenging endeavor to probe physics beyond the Standard Model. The joint detection of gravitational waves (GW) and electromagnetic (EM) signals from the merging of a binary system of compact objects like neutron stars can help constrain the geometry of extra dimensions beyond our 3+1 spacetime ones. A theoretically well-motivated possibility is that our observable Universe is a 3+1-dimensional hypersurface, or brane, embedded in a higher 4+1-dimensional anti–de Sitter (AdS5) spacetime, in which gravity is the only force which propagates through the infinite bulk space, while other forces are confined to the brane. In these types of brane-world models, GW and EM signals between two points on the brane would, in general, travel different paths. This would result in a time lag between the detection of GW and EM signals emitted simultaneously from the same source. We consider the recent near-simultaneous detection of the GW event GW170817 from the LIGO/Virgo collaboration, and its EM counterpart, the short gamma-ray burst GRB170817A detected by the Fermi Gamma-ray Burst Monitor and the International Gamma-Ray Astrophysics Laboratory Anti-Coincidence Shield spectrometer. Assuming the standard Λ-cold dark matter scenario and performing a likelihood analysis which takes into account astrophysical uncertainties associated to the measured time lag, we set an upper limit of ℓ≲0.535  Mpc at 68% confidence level on the AdS5 radius of curvature ℓ. Although the bound is not competitive with current Solar System constraints, it is the first time that data from a multimessenger GW-EM measurement is used to constrain extra-dimensional models. Thus, our work provides a proof of principle for the possibility of using multimessenger astronomy for probing the geometry of our space-time.

In this work, we have studied the possibility of setting up Bell's inequality violating experiment in the context of cosmology, based on the basic principles of quantum mechanics. First we start with the physical motivation of implementing the Bell's inequality violation in the context of cosmology. Then to set up the cosmological Bell violating test experiment we introduce a model independent theoretical framework using which we have studied the creation of new massive particles by implementing the WKB approximation method for the scalar fluctuations in presence of additional time dependent mass contribution in the cosmological perturbation theory. Here for completeness we compute total number density and energy density of the newly created particles in terms of Bogoliubov coefficients using WKB approximation method. Next using the background scalar fluctuation in presence of new time dependent mass contribution, we explicitly compute the expression for the one point and two point correlation functions. Furthermore, using the results for one point function we introduce a new theoretical cosmological parameter which can be expressed in terms of the other known inflationary observables and can also be treated as a future theoretical probe to break the degeneracy amongst various models of inflation. Additionally, we also fix the scale of inflation in a model independent way without any prior knowledge of primordial gravitational waves. Also using the input from newly introduced cosmological parameter, we finally give a theoretical estimate for the tensor-to-scalar ratio in a model independent way. Next, we also comment on the technicalities of measurements from isospin breaking interactions and the future prospects of newly introduced massive particles in cosmological Bell violating test experiment. Further, we cite a precise example of this set up applicable in the context of string theory motivated axion monodromy model. Then we comment on the explicit role of decoherence effect and high spin on cosmological Bell violating test experiment. In fine, we provide a theoretical bound on the heavy particle mass parameter for scalar fields, graviton and other high spin fields from our proposed setup.

Context. The gravitational wave (GW) background in the range 0.01−30 mHz has been assumed to be dominated by unresolved radiation from double white dwarf binaries (DWDs). Recent investigations indicate that, at short periods, a number of DWDs should be resolvable sources of GW. Aims. We characterize the GW signal which would be detected by LISA from DWDs in the Galaxy. Methods. We have constructed a Galactic model in which we consider distinct contributions from the bulge, thin disc, thick disc, and halo, and subsequently executed a population synthesis approach to determine the birth rates, numbers, and period distributions of DWDs within each component. Results. In the Galaxy as a whole, our model predicts the current birth rate of DWDs to be 3.21 × 10 −2 yr −1 , the local density to be 2.2 × 10 −4 pc −3 and the total number to be 2.76 × 10 8. Assuming SNIa are formed from the merger of two CO white dwarfs, the SNIa rate should be 0.0013 yr −1. The frequency spectra of DWD strain amplitude and number distribution are presented as a function of galactic component, DWD type, formation channel, and metallicity. Conclusions. We confirm that CO+He and He+He white dwarf (WD) pairs should dominate the GW signal at very high frequencies (log f Hz −1 > −2.3), while CO+CO and ONeMg WD pairs have a dominant contribution at log f Hz −1 ≤ −2.3. Formation channels involving two common-envelope (CE) phases or a stable Roche lobe overflow phase followed by a CE phase dominate the production of DWDs detectable by LISA at log f Hz −1 > −4.5. DWDs with the shortest orbital periods will come from the CE+CE channel. The Exposed Core plus CE channel is a minor channel. A number of resolved DWDs would be detected, making up 0.012% of the total number of DWDs in the Galaxy. The majority of these would be CO+He and He+He pairs formed through the CE+CE channel.

We discuss the transition from quasi-circular inspiral to plunge of a system of two nonrotating black holes of masses m 1 and m 2 in the extreme mass ratio limit m 1 m 2 ≪ (m 1 +m 2 ) 2 . In the spirit of the Effective One Body (EOB) approach to the general relativistic dynamics of binary systems, the dynamics of the two black hole system is represented in terms of an effective particle of mass µ ≡ m 1 m 2 /(m 1 + m 2 ) moving in a (quasi-)Schwarzschild background of mass M ≡ m 1 +m 2 and submitted to an O(µ) radiation reaction force defined by Padé resumming high-order Post-Newtonian results. We then complete this approach by numerically computing,à la Regge-Wheeler-Zerilli, the gravitational radiation emitted by such a particle. Several tests of the numerical procedure are presented. We focus on gravitational waveforms and the related energy and angular momentum losses. We view this work as a contribution to the matching between analytical and numerical methods within an EOB-type framework.

We analytically compute the long-term orbital variations of a test particle orbiting a central body acted upon by an incident monochromatic plane gravitational wave. We assume that the characteristic size of the perturbed two-body system is much smaller than the wavelength of the wave. Moreover, we also suppose that the wave's frequency νg is much smaller than the particle's orbital one nb. We make neither a priori assumptions about the direction of the wavevector kˆ nor on the orbital configuration of the particle. While the semi-major axis a is left unaffected, the eccentricity e, the inclination I, the longitude of the ascending node Ω, the longitude of pericenter ϖ and the mean anomaly ℳ undergo non-vanishing long-term changes of the form dΨ/dt=F(Kij;e,I,Ω,ω),Ψ=e,I,Ω,ϖ,M, where Kij, i,j=1,2,3 are the coefficients of the tidal matrix K. Thus, in addition to the variations of its orientation in space, the shape of the orbit would be altered as well. Strictly speaking, such effects are not secular trends because of the slow modulation introduced by K and by the orbital elements themselves: they exhibit peculiar long-term temporal patterns which would be potentially of help for their detection in multidecadal analyses of extended data records of planetary observations of various kinds. In particular, they could be useful in performing independent tests of the inflation-driven ultra-low gravitational waves whose imprint may have been indirectly detected in the Cosmic Microwave Background by the Earth-based experiment BICEP2. Our calculation holds, in general, for any gravitationally bound two-body system whose orbital frequency nb is much larger than the frequency νg of the external wave, like, e.g., extrasolar planets and the stars orbiting the Galactic black hole. It is also valid for a generic perturbation of tidal type with constant coefficients over timescales of the order of the orbital period of the perturbed particle.

Quasinormal modes (QNMs) of perturbed black holes have recently gained much interest because of their tight relations with the gravitational wave signals emitted during the post-merger phase of a binary black hole coalescence. One of the intriguing features of these modes is that they respect the no-hair theorem, and hence, they can be used to test black hole spacetimes and the underlying gravitational theory. In this paper, we exhibit three different aspects of how black hole QNMs could be altered in theories beyond Einstein's general relativity (GR). These aspects are (i) the direct alterations of QNM spectra as compared with those in GR, (ii) the violation of the geometric correspondence between the high-frequency QNMs and the photon geodesics around the black hole, and (iii) the breaking of the isospectrality between the axial and polar gravitational perturbations. Several examples will be provided in each individual case. The prospects and possible challenges associated with future observations will be also discussed.