Gravity Seminars

Compact objects possessing complicated multipole structure will generally cause precession of the orbital plane when present in a binary system. The most common example of this within general relativity is so-called spin precession, which is caused when the spin angular momentum of the compact object couple to each other, as well as the orbital angular momentum. The precession of the latter of these induces modulations in both the frequency and amplitude of the observed gravitational wave emission of the binary, effects which play a crucial role in parameter estimation. However, if general relativity is not the correct theory of gravity at astrophysical scales and must be modified, or the compact objects have significantly more complicated multipole structure beyond that of a simple pole-dipole, the precession dynamics of the binary will also be modified from that of standard spin precession. Such modifications will necessarily be imprinted in the waveform generated by the precessing binary, opening the door to performing tests of general relativity within the precessing sector of binary dynamics.

In this talk, I will present recent work towards understanding how to use observations of gravitational waves from precessing binaries. As examples, I will discuss two modifications of the standard spin precession paradigm. The first is spin precession in dynamical Chern-Simons gravity, a parity-violating modified theory of gravity that has proven difficult to constrain with non-precessing binaries, and the second is exotic compact objects within general relativity that possess non-axisymmetric violations of the no-hair theorems, such as multipolar boson stars. I will discuss how these two examples allow us to propose an extension of the parameterized post-Einsteinian framework to generic precessing binaries. While work on the first of these two examples is ongoing, I will present projected constraints that one can obtain from this framework on violations of the no-hair theorems with future gravitational wave observations.

Gravitational wave (GW) astronomy has opened a new window for studying gravitational phenomena in its strong-field and highly dynamical regime. In this regime, the detection of GWs originating from the mergers of compact binary systems represents the most compelling probe to explore the extreme manifestations of gravity and possible deviations from GR. In this talk, I will present the formal and practical challenges that arise in nonlinear studies of modified gravity theories. In particular, I will present the results from numerical simulations where the implementation of novel methods can help us get accurate predictions in the non-linear regime for modified gravity theories motivated by Effective Field Theory arguments.

I will discuss a field theory treatment of standard gravitational waves calculations. Using the QFT formalism we can connect the gravitational memory signal with what are called 'soft-graviton theorems' which gives an insight into the origin of the memory signal and makes the calculations easier in some cases. I will also discuss if the graviton nature of gravitational waves can be proved using the existing gravitational wave detection experiments.

Neutron stars are among the most compact objects in the Universe, and the collision of two neutron stars is among the most energetic events in our Universe. In 2017, the multimessenger detection of gravitational waves and electromagnetic signals from such a collision has been a revolution in astronomy and provided a wealth of information about fundamental physics principles. Essential for an accurate interpretation of binary neutron star mergers are reliable models describing the last stages of their coalescence.  We show how numerical-relativity simulation can be used to derive such theoretical models for the gravitational-wave and electromagnetic signatures. We employ these models together with nuclear-physics computations and experimental data to measure the equation of state of neutron stars, understand heavy element production, and to provide new constraints on the Hubble constant.

Neutron stars and white dwarfs contain densest matter known to us in the universe. This brings gravity in a tug of war with nuclear, electromagnetic and weak interactions which keep these stars from collapsing. The merging of these compact objects in binary systems generates signals detectable across various observational windows, thereby broadening and enriching the realm of multimessenger astronomy.

In this talk, I will illustrate how large-scale computational frameworks serve as valuable tools for exploring the intricate interplay of these fundamental interactions and aiding in scientific discovery. I will also present the data analysis techniques to extract information from current and future gravitational wave observations. We will focus on kilonovae and gravitational waves produced by colliding neutron stars, as well as Type Ia supernovae triggered by the mergers of binary white dwarfs. I will present our evolving understanding of these systems and some future directions that the field is going towards.

The gravitational wave memory effect that manifests itself as a permanent distortion of space after a gravitational wave burst, is a prediction of GR, whose detection is expected to be around the corner. In this talk, I will present recent work, in which the memory effect of a broad class of beyond GR metric theories of gravity is shown to be modified primarily through the presence of additional propagating modes in the theory. This strongly suggests the possibility of using memory observations as a model independent test of the number of dynamical gravitational degrees of freedom. Such a general statement rests upon a reinterpretation of the memory effect as naturally arising within the broadly applicable Isaacson approach to gravitational waves.

The LIGO-Virgo-KAGRA Collaboration has observed over 70 gravitational-wave sources to date, including mergers between black holes, neutron stars, and mixed neutron star—black holes. Focusing on the black hole mergers, I will describe some recent lessons into how, when, and where black holes are made. These questions are connected to several astrophysical puzzles, including the deaths of massive stars, the growth of black holes across cosmic time, high-redshift star formation, and properties of globular clusters.

Gravitational wave observations of neutron star mergers are one of the most exciting prospects for constraining the incredibly uncertain equation of state of cold supranuclear matter. Signatures of the stellar interior are directly imprinted in the gravitational-wave signal through characteristic tidal interactions, providing a vital observational playground in our quest to understand fundamental physics interactions deep in the cores of neutron stars. In this talk I will discuss the underlying physics behind binary neutron star mergers and how recent developments are showcasing the importance of describing the dynamical response of neutron stars when studying gravitational-wave observations. Finally, I will highlight the prospects for performing gravitational-wave asteroseismology with binary neutron star inspirals in current and future detector networks.

Gravitational memory effects are persistent deformations caused due to the passage of a gravitational wave pulse. It is a nonlinear effect in General Relativity that remains yet to be detected. In this talk, primarily from a theoretical perspective, we study memory effects in two different settings: i) Radiative geometries, ii) Lorentzian wormholes. In radiative geometries, we show how geodesic and geodesic deviation equations encode the gravitational wave memory. In the latter case, we perform a Bondi-Sachs analysis and try to show how the Bondi mass loss depends on the wormhole hair.

Gamma ray bursts (GRBs) are the most luminous electromagnetic events in the universe. Short GRBs, typically lasting less than 2 seconds, have already been associated with binary neutron star (BNS) mergers, which are also sources of gravitational waves (GWs). The ultimate fate of a BNS, after coalescence, is usually expected to be a black hole (BH) with 2-3 solar masses. However, numerical relativity simulations indicate the possible formation of a short-lived hypermassive neutron star (HMNS), lasting for tens to hundreds of milliseconds after the BNS merger and before gravitational collapse forms a BH. The HMNS is expected to emit GWs with kHz frequencies that will be detectable by third generation ground-based GW detectors in the 2030s. I will present results from a recent analysis that revealed evidence for HMNSs by looking for kHz qusiperiodic oscillations in gamma-ray observations obtained in the 1990s with the Compton Gamma Ray Observatory. 

Gravitational Faraday Rotation (GFR) is a frame-dragging effect induced by rotating massive objects, which is one of the important characteristics of lensed gravitational waves (GWs). In my previous work, we calculate the GFR angle of GWs in the weak deflection limit, assuming it is lensed by a Kerr black hole (BH). We find that the GFR effect changes the initial polarization state of the lensed GW. Compared with the Einstein deflection angle, the dominant term of the rotation angle is a second-order correction to the polarization angle, which depends on the light-of-sight component of BH angular momentum. Such a rotation is tiny and degenerates with the initial polarization angle. In some critical cases, the GFR angle is close to the detection capability of the third-generation GW detector network, although the degeneracy has to be broken.

Continuing the success of gravitational wave observations demands considerable improvements of their theoretical predictions in the next decade, in order to keep their accuracy on par with upgrades of the detectors. This urges innovations on the methods by which gravitational waves from compact binaries are calculated. In this talk, we focus on approaches to analytic, perturbative predictions for relativistic binaries inspired by high-energy physics. In this area, effective field theories are highly useful and scattering amplitudes (the primary observable) can be calculated very efficiently using novel tools. These methods can indeed be applied the classical binaries and their gravitational waves. We give a basic introduction to the ideas of these approaches and recent progress.

Perhaps the most significant astronomical discovery of our lifetimes, code-named GW170817, involved the collision of two neutron stars. The collision was detected both by gravitational wave observatories, and traditional electromagnetic telescopes. As neutron stars are made of the densest form of matter in our current Universe, this single "multimessenger" event was a watershed moment in our understanding as to how matter and gravity behave at their most extreme, far beyond what we can study in laboratories on Earth. For the most part, we compare observations against theoretical models to extract science from events like this. Unfortunately, these theoretical models are severely limited both in quality and quantity, leading to a critical need to improve them. Such improvements pose a key challenge to computational astrophysics, as our most detailed models require expensive supercomputer simulations that generate full, non-perturbative solutions of the general relativistic field equations (numerical relativity). After a gentle introduction to multimessenger astrophysics and the challenges associated with multimessenger source modeling, I will outline a new approach aimed at greatly reducing the cost of these simulations. With the reduced cost comes the potential to both perform colliding black hole simulations on the consumer-grade desktop computer, as well as add unprecedented levels of physical realism to colliding neutron star simulations on supercomputers.

Non-linear memory is one of the most intriguing predictions of general relativity which is generated by the passage of gravitational waves (GWs) leaving the spacetime permanently deformed. For example a GW signal from binary black hole (BBH) will have two parts the oscillatory part which is known as the “chirp” and a much fainter non-oscillatory (DC like) part which is non-linear memory. A non-linear memory is produced by all the sources of GWs and  has the peculiarity that even if the oscillatory part of the source lies at high frequency the non-linear memory will be available at low frequency. This property of non-linear memory makes it a valuable resource for GW astronomy. In this talk I will provide and introduction to how we can use gravitational waves memory as a resource for the current and future ground based detectors. To do this I will show examples of how one can creatively use the non-linear memory to probe seemingly inaccessible sources of GWs like ultra low mass compact binary mergers where the oscillatory part lies at outside the reach of any current detectors and only non-linear memory could be detected if these sources exist. Another example will be the matter effects from binary neutron stars and black hole neutron star binaries which are at high frequency but the non-linear memory is accessible. I will also discuss the post-merger neutron star memory and the prospects of its detection.

Since 2015, the LIGO-Virgo-KAGRA Collaboration has detected 90 signals from merging compact objects such as black holes and neutron stars. Each of these is analyzed using Bayesian inference, employing a stochastic algorithm such as Markov Chain Monte Carlo to compare data against models—thereby characterizing the source. However, this is becoming extremely costly as event rates grow with improved detector sensitivity. In this talk I will describe a powerful alternative using probabilistic deep learning to analyze each event in orders of magnitude less time while maintaining strict accuracy requirements. This uses simulated data to train a normalizing flow to model the posterior distribution over source parameters given the data—amortizing training costs over all future detections. I will also describe the use of importance sampling to establish complete confidence in these deep learning results. Finally I will describe prospects going forward for simulation-based inference to enable improved accuracy in the face of non-stationary or non-Gaussian noise.

The gravitational-wave signal emitted by the black-hole remnant resulting from a binary merger such as GW190514 consists in a superposition of damped sinu- soids known as quasinormal modes. Besides the “fundamental" mode (the one with the longest damping time), it is important to detect the so-called “over- tones" (modes with shorter damping time), because a measurement of their frequencies could allow us to identify the remnant as a Kerr black hole. We discuss theoretical and observational issues in the analysis of ringdown over- tones. We present theoretical arguments showing that the spacetime is not well described as a linearly perturbed black hole close to the peak of the waveform amplitude. Then we analyze GW150914 post-merger data to understand if recent ringdown overtone detection claims are robust. We find no evidence in favor of an overtone in the data after the waveform peak. Around the peak, the log-Bayes factor does not indicate the presence of an overtone, while the support for a non-zero amplitude is sensitive to changes in the starting time much smaller than the overtone damping time. This suggests that claims of an overtone detection are noise-dominated. We perform GW150914-like injections in neighboring segments of the real detector noise, and we show that noise can indeed induce artificial evidence for an overtone.