A new experiment for gravitational wave detection

The new release of data from Wilkinson Microwave Anisotropy Probe improves the observational status of relic gravitational waves. The 7-year results enhance the indications of relic gravitational waves in the existing data and change to the better the prospects of confident detection of relic gravitational waves by the currently operating Planck satellite. We apply to WMAP7 data the same methods of analysis that we used earlier [W. Zhao, D. Baskaran, and L.P. Grishchuk, Phys. Rev. D 80, 083005 (2009)] with WMAP5 data. We also revised by the same methods our previous analysis of WMAP3 data. It follows from the examination of consecutive WMAP data releases that the maximum likelihood value of the quadrupole ratio $R$, which characterizes the amount of relic gravitational waves, increases up to $R=0.264$, and the interval separating this value from the point $R=0$ (the hypothesis of no gravitational waves) increases up to a $2\sigma$ level. The primordial spectra of density perturbations and gravitational waves remain blue in the relevant interval of wavelengths, but the spectral indices increase up to $n_s =1.111$ and $n_t=0.111$. Assuming that the maximum likelihood estimates of the perturbation parameters that we found from WMAP7 data are the true values of the parameters, we find that the signal-to-noise ratio $S/N$ for the detection of relic gravitational waves by the Planck experiment increases up to $S/N=4.04$, even under pessimistic assumptions with regard to residual foreground contamination and instrumental noises. We comment on theoretical frameworks that, in the case of success, will be accepted or decisively rejected by the Planck observations.

The detection of gravitational waves (GWs) from binary neutron stars (BNSs) with possible telescope follow-ups opens a window to ground-breaking discoveries in the field of multi-messenger astronomy. With the improved sensitivity of current and future GW detectors, more BNS detections are expected in the future. Therefore, enhancing low-latency GW search algorithms to achieve rapid speed, high accuracy, and low computational cost is essential. One innovative solution to reduce latency is the use of machine learning (ML) methods embedded in field-programmable gate arrays (FPGAs). In this work, we present a novel WaveNet-based method, leveraging the state-of-the-art ML model, to produce early-warning alerts for BNS systems. Using simulated GW signals embedded in Gaussian noise from the Advanced LIGO and Advanced Virgo detectors’ third observing run (O3) as a proof-of-concept dataset, we demonstrate significant performance improvements. Compared to the current leading ML-based early-warning system, our approach enhances detection accuracy from 66.81% to 76.22% at a 1% false alarm probability. Furthermore, we evaluate the time, energy, and economical cost of our model across CPU, GPU, and FPGA platforms, showcasing its potential for deployment in real-time GW detection pipelines.

The advent of gravitational wave (GW) astronomy has provided a new window through which to view and understand the Universe. To fully exploit the potential of GW astronomy, an understanding of all the potential electromagnetic counterparts to a gravitational wave detected source will help maximise the science returns. Here I present a study of the electromagnetic emission from relativistic jets that accompany the merger of binary neutron stars or black hole-neutron star systems. These counterparts provide a probe for the structure and dynamics of these relativistic outflows. Binary neutron star, or neutron star-black hole, mergers are thought to be the dominant progenitor of short gamma-ray bursts (GRBs). Here we investigate the possibility that there is a hidden population of low-Lorentz factor jets resulting in failed GRBs, on-axis orphan afterglows, and what kind of counterparts can be expected given a merger-jet population dominated by these failed-GRB jets. I find that for GW detected mergers, ∼ 80% of the population of on-axis events may result in a failed GRB afterglow. The afterglow of a failed GRB is characterised by the lack of any prompt emission; where the γ-rays are emitted within an optically thick region of the low-Lorentz factor (Γ) outflow and significant suppression via pair production and a high opacity results in the photons coupled to the pair plasma. This plasma will undergo adiabatic expansion, and the photons will decouple at the photospheric radius. The energy in the prompt photons, for a sufficiently low-Γ outflow, will have been significantly suppressed. GW detected mergers have a Malmquist bias towards on-axis events (i.e. the rotational axis of the system), where the peak of the probability distribution is an inclination ∼ 30⁰. If the jets from these mergers have an intrinsic structure out to wider angles, then the majority of mergers will be accompanied by electromagnetic counterparts from these various jet structures. By making some simple assumptions about the energetic structure of a jet outside of a bright core region, the various temporal features that result from a given jet structure can be predicted. Where the population of merger jets is dominated by a single structure model, I show the expected fractions of optical counterparts brighter than m_AB = 21. On 17 August 2017, the Light Interferometer Gravitational Wave Observatory (LIGO) in collaboration with Virgo detected the merger of a binary neutron star system. Various electromagnetic counterparts were detected: the GRB 170817A by Fermi/GBM and INTEGRAL; an optical, blue to red, macro/kilo-nova from ∼ 1/2 day post merger to ∼ 5 − 10 days; and a brightening radio, and X-ray counterpart from ∼ 10 days. Optical detection of this counterpart at a magnitude ∼ 26 was made at ∼ 100 days post-merger. Analysis of this counterpart is consistent with the afterglow of a Gaussian structured jet viewed at the system inclination, ∼ 18 ± 8⁰. If all short GRB jets have a similar jet structure, then the rates of orphan afterglows in deep drilling blind surveys e.g. the Large Synoptic Survey Telescope (LSST), will be higher than those expected from a homogeneous, or ‘top-hat’ jet, population. The rates for the various jet structures for orphan afterglows from mergers are discussed, showing that for a population of failed GRBs, or an intrinsic Gaussian structure, an excess in the orphan rate may be apparent. Understanding the dynamics and structure for the jets from black-hole systems born at the merger of a compact binary can help give clues as to the nature of jets from black holes on all scales. As an aside, I show empirically that regardless of black hole mass or system phenomenology, the relativistic jets from such systems share a universal scaling for the jet power and emitted γ-ray luminosity. This scaling could be due to the similar efficiencies of various processes, or alternatively, the scaling may be able to give insights into the emission and physical processes that are responsible for high-energy photons from these outflows. GW astronomy offers a probe of the most extreme relativistic outflows in the Universe, GRBs. The predicted electromagnetic counterparts from these outflows, in association with GW detections, provides a way to probe the Lorentz-factor distribution for merger-jets. Additionally, the phenomenological shape of the afterglows, at various inclinations, gives an indication of the intrinsic structure of these jets. An understanding of these dynamical and structural qualities can be used to constrain the parent population, merger rates, and binary evolution models for compact binary systems.

The range of expected amplitudes and spectral slopes of relic (squeezed) gravitational waves, predicted by theory and partially supported by observations, is within the reach of sensitive gravity-wave detectors. In the favorable case, the detection of relic gravitational waves can be achieved by initial laser interferometers in LIGO, VIRGO, GE0600. In more conservative cases, the sensitivity of advanced ground-based and cosmic laser interferometers will be needed. The specific statistical signature of relic gravitational waves, associated with the phenomenon of squeezing, is a potential reserve for further improvement of the signal to noise ratio.KeywordsGravitational WaveSpectral IndexDensity PerturbationLaser InterferometerSpectral SlopeThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

We investigate the use of Fourier plane nonlinear filtering for phase-encoded images. We investigate the performance of the nonlinear joint transform correlator and the nonlinearly transformed matched filter for phase-encoded images with different types of input noise. We use the peak-to-output-energy ratio, peak-to-sidelobe ratio, and discrimination ratio as the metrics for measuring the performances. We mathematically analyze the peak-to-output-energy ratio of the nonlinearly transformed matched filter for phase-encoded images with spatially nonoverlapping white noise. Computer simulations are provided to show the performance improvements of the nonlinear filtering techniques for the phase-encoded images. In comparison with linear filtering techniques, we find that the nonlinear filtering techniques substantially improve the performance metrics. From the computer-simulation results it can be seen that the nonlinear joint transform correlator performs better than the nonlinearly transformed matched filter in detecting phase-encoded targets in the presence of different types of noise, such as additive overlapping white noise, spatially nonoverlapping white background noise, spatially nonoverlapping colored background noise, and nontarget objects.

The proposed TianQin gravitational wave observatory is aiming to detect gravitational wave signals in the millihertz frequency band, through the method of space-borne laser interferometry. In total, three satellites will be launched, each following a geocentric orbit, with an orbital altitude of about one hundred thousand kilometers. They form into an equilateral triangle formation, with an orbital period of <sc>3.6 d.</sc> The orbital plane of TianQin is fixed almost constant, towards one specific gravitational wave source, RXJ0806.3+1527, so that TianQins response to this source is maximized, and this source can be used to calibrate and assess the facilities performance. It is widely anticipated that in the millihertz frequency band, there lurks a wide variety of gravitational wave sources, ranging from massive black hole binary mergers, extreme mass ratio inspirals, Galactic binary white dwarves, stellar mass binary compact objects inspirals, as well as stochastic gravitational wave background. Meanwhile, the expected source number in such frequency band would also be huge. The successful implementation of the TianQin project would pave the way towards exciting prospects of future gravitational wave research. By successfully performing gravitational wave observation, the TianQin gravitational wave observatory can open a new window for the study of gravitational wave astronomy, gravitational wave cosmology as well as gravitational wave physics. The science prospect of TianQin gravitational wave observation can be further enhanced through a combination with multi-band gravitational wave detection and multi-messenger astronomical observations, by collaboration with gravitational wave detectors operating in different frequency bands as well as with other probes focusing on different messengers like electro-magnetic signals, neutrinos or cosmic rays. The TianQin project will also greatly promote the construction of a high-precision space-time reference system, ultra-long-range laser interferometry, high-precision satellite platform, etc. The development of cutting-edge space technologies has important applications in the national economy as well as the national strategic needs. Currently, the TianQin project has made important progress in the precision gravity measurement facility, the payload research base, the laser ranging station, the simulation facility for space-borne gravitation wave detection on ground, and the series test satellite for the TianQin project. Based on the long-term accumulation of relevant technology, the TianQin project team proposed the goal of achieving gravitational wave detection in the 2030s, and a 0123 plan roadmap is formulated, aiming to perform, step by step, lunar or relay satellite laser ranging, testing key technology like space-borne laser interferometry and drag-free control, verifying high precision space-borne laser interferometry technology, and ultimately detecting gravitational wave. By the 2030s, the scientific cooperation between scientists from all over the world and the use of various means to jointly conduct exploration and research on the Universe will face great development. So far, the TianQin project has held many international seminars and received extensive international attention and active participation. Currently, the TianQin Collaboration Group has been formally established, with a collaboration group advisory committee also established, consisting of scientists with significant influence in the field of gravitational wave detection all over the world. The TianQin project will establish a collaborative research or comprehensive cooperation agreement with foreign universities or research organizations, and vigorously promote the China-led space-borne gravitational wave detection business. Through extensive international cooperation, the TianQin project will unite domestic and foreign strengths and strive to take the lead in realizing the gravitational wave detection in the window of millihertz band.

Relic gravitational waves (RGWs) from the early Universe carry crucial and fundamental cosmological information. Therefore, it is of extraordinary importance to investigate potential RGW signals in the data from observatories such as the LIGO-Virgo-KAGRA network. Here, focusing on typical RGWs from the inflation and the first-order phase transition (by sound waves and bubble collisions), effective and targeted deep learning neural networks are established to search for these RGW signals within the real LIGO data (O2, O3a and O3b). Through adjustment and adaptation processes, we develop suitable Convolutional Neural Networks (CNNs) to estimate the likelihood (characterized by quantitative values and distributions) that the focused RGW signals are present in the LIGO data. We find that if the constructed CNN properly estimates the parameters of the RGWs, it can determine with high accuracy (approximately 94% to 99%) whether the samples contain such RGW signals; otherwise, the likelihood provided by the CNN cannot be considered reliable. After testing a large amount of LIGO data, the findings show no evidence of RGWs from: 1) inflation, 2) sound waves, or 3) bubble collisions, as predicted by the focused theories. The results also provide upper limits of their GW spectral energy densities of h 2Ωgw ∼ 10−5, respectively for parameter boundaries within 1) [β ∈ (−1.87, −1.85) × α ∈ (0.005, 0.007)], 2) [β/H pt ∈ (0.02, 0.16) × α ∈ (1, 10) × T pt ∈ (5*109, 1010) Gev], and 3) [β/H pt ∈ (0.08, 0.2) × α ∈ (1, 10) × T pt ∈ (5 * 109, 8 * 1010) Gev]. In short, null results and upper limits are obtained, and the analysis suggests that our developed methods and neural networks to search for typical RGWs in the LIGO data are effective and reliable, providing a viable scheme for exploring possible RGWs from the early Universe and placing constraints on relevant cosmological theories.

The Laser Interferometer Gravitational-Wave Observatory (LIGO) and Virgo Interferometer Collaborations have now detected all three classes of compact binary mergers: binary black hole (BBH), binary neutron star (BNS), and neutron star-black hole (NSBH). For coalescences involving neutron stars, the simultaneous observation of gravitational and electromagnetic radiation produced by an event, has broader potential to enhance our understanding of these events, and also to probe the equation of state (EOS) of dense matter. However, electromagnetic follow-up to gravitational wave (GW) events requires rapid real-time detection and classification of GW signals, and conventional detection approaches are computationally prohibitive for the anticipated rate of detection of next-generation GW detectors. In this work, we present the first deep learning based results of classification of GW signals from NSBH mergers in real LIGO data. We show for the first time that a deep neural network can successfully distinguish all three classes of compact binary mergers and separate them from detector noise. Specifically, we train a convolutional neural network (CNN) on ∼500,000 data samples of real LIGO noise with injected BBH, BNS, and NSBH GW signals, and we show that our network has high sensitivity and accuracy. Most importantly, we successfully recover the two confirmed NSBH events to-date (GW191219 and GW200115) and the two confirmed BNS mergers to-date (GW170817 and GW190425), together with ∼90% of all BBH candidate events from the third Gravitational Wave Transient Catalog, GWTC-3. These results are an important step towards low-latency real-time GW detection, enabling multi-messenger astronomy.

Since the first detection of gravitational waves, they have been used to investigate various fundamental problems, including the variation of physical constants. Regarding the gravitational constant, previous works focused on the effect of the gravitational constant variation on the gravitational wave generation. In this paper, we investigate the effect of the gravitational constant variation on the gravitational wave propagation. The Maxwell-like equation that describes the propagation of gravitational waves is extended in this paper to account for situations where the gravitational constant varies. Based on this equation, we find that the amplitude of gravitational waves will be corrected. Consequently the estimated distance to the gravitational wave source without considering such a correction may be biased. Applying our correction result to the well known binary neutron star coalescence event GW170817, we get a constraint on the variation of the gravitational constant. Relating our result to the Yukawa deviation of gravity, we for the first time get the constraint of the Yukawa parameters in 10 Mpc scale. This scale corresponds to a graviton mass mg∼10−31 eV.

As gravitational wave astronomy prepares for the first detections of gravitational waves from compact-object binary inspirals, theoretical work is required on the study of (i) gravitational-wave sources, (ii) the signals emitted by those sources, and (iii) the searches for those signals in detector data. This thesis describes work on all three fronts. (i) We discuss intermediate-mass-ratio inspirals (IMRIs) of black holes or neutron stars into intermediate-mass black holes (IMBHs) that could be detected with Advanced LIGO. We analyze different mechanisms of IMRI formation and compute IMRI event rates of up to tens of events per year for Advanced LIGO. We study the spin evolution of IMBHs that grow through a series of minor mergers. We explore how a deviation of an IMRI's central body from a Kerr black hole influences geodesics, including the possibility of chaotic orbital dynamics. We also address the scientific consequences of extreme-mass-ratio inspiral (EMRI) detections by LISA for astrophysics and general relativity, and the difficulties associated with detecting and analyzing EMRI signals. (ii) We study the periodic standing-wave approximation (PSWA), which can potentially provide accurate waveforms in the last inspiral cycles of a comparable-mass black-hole binary. Using a simple model, we find that the solution to Einstein's equations for inspiraling black holes can be recovered to a high accuracy by the addition a perturbative radiation-reaction field to the standing-wave, noninspiraling solution. (iii) We demonstrate the utility of searching for and analyzing tracks in time-frequency spectrograms of a gravitational-wave signal as a means of estimating the parameters of a massive black-hole binary inspiral, as observed by LISA.

Gravitational waves are basically ripples in spacetime predicted by Einstein&#39;s General Theory of Relativity (1915), have revolutionized our understanding of the universe since their first detection in 2015 by the LIGO. These waves are typically generated by extreme cosmological events such as the mergers of two or more black holes, neutron stars, and other compact objects. Black holes, regions of spacetime where gravity is so intense that not even light can escape, are both significant sources and subjects of gravitational wave studies. The observation of gravitational waves from binary black hole mergers has provided unprecedented insights into the nature of black holes, their mass distribution, and the environments in which they form. This paper reviews the current state of research on gravitational waves and black holes, focusing on their astrophysical origins, the mechanisms of wave generation, and the information encoded in the wave signals. I have discussed how gravitational wave detections have confirmed the existence of stellar-mass black hole binaries, revealed new populations of black holes, and provided tests of general relativity in the strong-field regime. Additionally, this study highlights the potential of gravitational wave astronomy to address unresolved questions, such as the formation channels of black hole binaries, the detection of primordial black holes, and the role of black holes in galaxy evolution. By bridging observational data with theoretical models, gravitational wave observations are opening new frontiers in our understanding of both black holes and the fundamental nature of gravity.

The LIGO and Virgo gravitational-wave observatories are complex and extremely sensitive strain detectors that can be used to search for a wide variety of gravitational waves from astrophysical and cosmological sources. In this thesis, I motivate the search for the gravitational wave signals from coalescing black hole binary systems with total mass between 25 and 100 solar masses. The mechanisms for formation of such systems are not well-understood, and we do not have many observational constraints on the parameters that guide the formation scenarios. Detection of gravitational waves from such systems — or, in the absence of detection, the tightening of upper limits on the rate of such coalescences — will provide valuable information that can inform the astrophysics of the formation of these systems. I review the search for these systems and place upper limits on the rate of black hole binary coalescences with total mass between 25 and 100 solar masses. I then show how the sensitivity of this search can be improved by up to 40% by the the application of the multivariate statistical classifier known as a random forest of bagged decision trees to more effectively discriminate between signal and non-Gaussian instrumental noise. I also discuss the use of this classifier in the search for the ringdown signal from the merger of two black holes with total mass between 50 and 450 solar masses and present upper limits. I also apply multivariate statistical classifiers to the problem of quantifying the non-Gaussianity of LIGO data. Despite these improvements, no gravitational-wave signals have been detected in LIGO data so far. However, the use of multivariate statistical classification can significantly improve the sensitivity of the Advanced LIGO detectors to such signals.

The extreme sensitivity required for direct observation of gravitational waves by the Advanced LIGO detectors means that environmental noise is increasingly likely to contaminate Advanced LIGO gravitational wave signals if left unaddressed. Consequently, environmental monitoring efforts have been undertaken and novel noise mitigation techniques have been developed which have reduced environmental coupling and made it possible to analyze environmental artifacts with potential to affect the 90 gravitational wave events detected from 2015–2020 by the Advanced LIGO detectors. So far, there is no evidence for environmental contamination in gravitational wave detections. However, automated, rapid ways to monitor and assess the degree of environmental coupling between gravitational wave detectors and their surroundings are needed as the rate of detections continues to increase. We introduce a computational tool, PEMcheck, for quantifying the degree of environmental coupling present in gravitational wave signals using data from the extant collection of environmental monitoring sensors at each detector. We study its performance when applied to 79 gravitational waves detected in LIGO’s third observing run and test its performance in the case of extreme environmental contamination of gravitational wave data. We find that PEMcheck’s automated analysis identifies only a small number of gravitational waves that merit further study by environmental noise experts due to possible contamination, a substantial improvement over the manual vetting that occurred for every gravitational wave candidate in the first two observing runs. Building on a first attempt at automating environmental coupling assessments used in the third observing run, this tool represents an improvement in accuracy and interpretability of coupling assessments, reducing the time needed to validate gravitational wave candidates. With the validation provided herein; PEMcheck will play a critical role in event validation during LIGO’s fourth observing run as an integral part of the data quality report produced for each gravitational wave candidate.

We show that a passing gravitational wave may influence the spin entropy and spin negativity of a system of N massive spin-(1/2) particles, in a way that is characteristic of the radiation. We establish the specific conditions under which this effect may be nonzero. The change in spin entropy and negativity, however, is extremely small. Here, we propose and show that this effect may be amplified through entanglement swapping. Relativistic quantum information theory may have a contribution towards the detection of gravitational waves.

Boson stars may consist of a new type of light singlet scalar particles with nontrivial self-interactions, and may compose a fraction of the dark matter in the Universe. In this work, we study the dynamics of boson stars with Liouville and logarithmic scalar self-interaction potentials as benchmarks. We perform a numerical analysis as well as a semi-analytic study on how the compactness and the total mass will deviate from that of the usual boson stars formed with a quartic repulsive self-interaction. We apply the recently suggested Swampland conjecture to examine whether boson stars with such benchmark potentials belong to the Landscape of a quantum gravity. Using the mass constraint on the macroscopic compact halo object (MACHO) and the cold dark matter (CDM) isocurvature mode constraint from the cosmic microwave background (CMB), we derive the allowed mass range of scalar particles which compose the boson star. We further analyze applications of the lensing of fast radio bursts (FRBs) and the gravitational wave (GW) detection to probe the presence of such boson stars and constrain the parameter space of their corresponding models. We discuss how the two types of boson star potentials can be discriminated by the FRB and GW measurements.