Gravity Seminars

TBA

TBA

It is reasonable to expect that a bound source cannot produce arbitrarily large gravitational multipole moments. By observing the behavior of spacetime metric with large multipole moments at small distances from the source, we conjecture that for a sufficiently large multipole moment, there is a minimal size below which no object in nature can support such a moment. The examples we have investigated suggest that the minimal size has a power law relation with multipole moments that is different from the analogous Newtonian scaling. We also look at spacetimes with small deviations in quadrupole moment from the Kerr black hole and discuss the shift of gravitational wave phase for a test particle. Future space-based telescope LISA has a good prospect of constraining such small deviations in multipole moments.

Gravitational wave memory is a persistent non-oscillatory shift in the gravitational wave amplitude. Such effects are ubiquitous in astrophysical and cosmological cataclysmic events involving gravitational radiation. In this talk, we turn our attention to the case of a supernova neutrino burst generating gravitational radiation. Previous studies along this line have demonstrated that a neutrino burst in such scenarios gives rise to a gravitational memory signal. Here, we specifically inquire about the alterations to the memory signal when neutrinos emitted from a supernova undergo self-interaction, presenting an avenue for indirectly detecting neutrino self-interaction.
Reference: https://arxiv.org/abs/2311.03315 

The gravitational memory effect and its electromagnetic (EM) analog are potential probes in the strong gravity regime. In the literature, this effect is derived for static observers at asymptotic infinity. While this is a physically consistent approach, it restricts us to deriving memory for an asymptotically flat spacetime. In this talk, I will discuss how we evaluate EM and gravitational memory with respect to a comoving observer (defined by timelike 4-velocity u_{a}). We split 4-D the spacetime into 1+1+2 parts, consisting of one timelike, one preferred spacelike, and one 2-D hypersurface. It allows us to obtain EM memory in an arbitrary curved background spacetime and gravitational memory in a class of spacetime with spherical symmetry often referred to as Locally Rotationally Symmetric(LRS) type II in covariant formalism. The master equation corresponding to the acceleration of the comoving observer in the 2D surface provides a physical understanding of the EM memory. The leading order contribution to the EM memory comes from the total energy density of the EM field, while the subleading contributions contain the spacetime geometry and the other components of the energy-momentum tensor of the EM field. In the case of gravitational memory, we show it is manifested in the integral of the shear tensor of the 2-D hypersurface. We obtain the master equation of gravitational memory in LRS-II spacetime and finally, we show that the memory exhibits distinct signatures between accelerated and decelerated universes, potentially enabling the identification of the transition redshift from a matter-dominated to a dark-energy-dominated universe. In the end(if time permits), I will briefly discuss the generation of detectable GW from the interaction of very high energized EM wave and stellar-mass Schwarzschild black holes in the Milky Way galaxy. 
 

Tidal Love numbers describe the linear response of compact objects to external tidal perturbations, and they are found to vanish exactly for black holes within General Relativity. We first discuss the robustness of this property when non-linearities in the theory are taken into account, and then investigate the scenario of black holes dressed with an ultralight scalar field, addressing their detectability at future gravitational wave experiments. Finally, we discuss the tidal deformability of more exotic compact objects, showing the relevance of universality relations as a way to distinguish between them.

Accurate characterization of gravitational wave signals from binary black hole (BBH) mergers require efficient models for the waveform and remnant quantities. While we have accurate models for quasi-circular BBH mergers, modelling eccentric binaries is still in its nascent stage. Using both numerical relativity (NR) and black hole perturbation theory (BHPT), we study the phenomenology of eccentric BBH waveforms. We present convincing evidence that the waveform phenomenology in eccentric BBH mergers is significantly simpler than previously thought. We find that the eccentric modulations in the amplitudes and frequencies in different spherical harmonic modes are all related and can be modeled using a single time series modulation. Using this universal eccentric modulation, we provide a model named gwNRHME to seamlessly convert a multi-modal (i.e with several spherical harmonic modes) quasi-circular waveform into multi-modal eccentric waveform if the quadrupolar eccentric waveform is known. This reduces the modelling complexity of eccentric BBH mergers drastically as we now have to model only a single eccentric modulation time-series instead of modelling the effect of eccentricity in all modes. We use gwNRHME to include eccentricity in current NR surrogate waveform models for quasi-circular mergers. Additionally, we discuss efforts in building dedicated surrogate models for eccentric BBH mergers using both NR and BHPT.

Ultralight axion-like particles, an excellent candidate for dark matter, can mediate a long-range macroscopic force with monopole-monopole and monopole-dipole interactions between the planets and the Sun. The presence of these long-range potentials affects the perihelion precession of planets, gravitational light bending and Shapiro time delay. From the precision studies of these tests of gravity, we obtain new constraints on the macroscopic forces. The bound is three orders of magnitude stronger than the Eot-Wash experiment. The ultralight scalar and vector dark matter also influence active-sterile neutrino oscillation in the supernova core which is one of the solutions to explain the longstanding problem of the pulsar kick. The signal from the asymmetric emission of neutrinos in the presence of an ultralight dark matter background can be probed by future gravitational wave detectors. The effects of ultralight dark matter in explaining pulsar kick are equivalent to the Lorentz and CPT invariance violation in the theory and we obtain an equivalent bounds on these parameters. 

With the advent of gravitational wave astronomy, our view of the universe has expanded to new and exciting frontiers. One especially promising avenue to explore in fundamental physics through gravitational waves is probing dense nuclear matter contained in neutron stars; observations of gravitational waves sourced by these extremely compact objects allow one to study matter in regimes that we could never replicate on Earth. Compact stars contain finite-size effects, such as tidal deformations, which leave imprints on the gravitational wave signal that describe the internal stellar structure, so studying such effects is crucial to expanding our understanding of matter at the most extreme scales. In this seminar, I shall describe how tidal fields in magnetar systems (neutron stars with incredibly strong magnetic fields) can lead to an interesting interplay between tidal fields and magnetic fields. This interaction is encapsulated in how the tidal field changes the magnetic properties of the star and, in turn, in how the magnetic field of the star changes its tidal deformability properties. I shall also outline an effective field theory formalism to study tidal fields alongside the conventional general relativity formalism. This effective field theory approach proves to be a natural arena to identify features of this system which can be difficult to study with the usual spacetime geometric approach alone.

With the LIGO-Virgo detectors currently undergoing their 4th observing round, gravitational-wave astronomy has matured into a fast-developing field with broad implications for astrophysics, nuclear physics, gravity and cosmology. In this talk, I will focus on recent developments in probing the physics of black holes and their mergers with gravitational waves. This includes measurements of black hole spins and merger kicks, their use as cosmological probes, and the spectroscopic study of ringing black holes. I will outline some of the theoretical and observational questions driving this field: how do black holes form? Can we leverage them as probes of new fundamental fields, dark matter or cosmic expansion? I will conclude by arguing that we are at the cusp of observationally tackling these and many other fascinating questions as we enter the era of precision gravitational-wave science, with current and future observatories in space and on the ground.

Massive black holes can grow in the presence of dark-matter environments and form dark-matter spikes with large densities. When a massive black hole within a dark-matter environment is part of an inspiral with a second compact object, the environmental effects will be imprinted on the system's dynamics. Past work studying these systems has demonstrated that gravitational effects like dynamical friction and accretion effects from the dark-matter distribution can have measurable impacts on the binary inspiral rate. The emitted gravitational waves will be affected in turn; given that they will be in the observable band for upcoming space-based detectors like LISA, the dynamics of dark matter on these scales can be understood precisely. In this talk, I discuss progress in evolving these systems on three fronts. First, I will overview a generalization of dynamical friction suitable for spherical systems, and its applications to inspirals. Second, I will present refinements on the effects of dark-matter accretion in a self-consistent framework. Finally, I will discuss the impacts of the formation history of the system on both the resulting dark-matter distribution and the gravitational waveform we would detect from such a binary.

Neutron stars are some of the most compact objects in the universe, second only to black holes.  Their interior composition remains a mystery, but studies of neutron stars and pulsars can allow scientists to probe the dense nuclear regions within.  These investigations often lead to bounds on a neutron star mass, which can be compared with a given equation of state to provide the physical characteristics of a star.  In this talk, I'll speak about some work following up one of the most massive neutron stars ever measured, J0348+0432, and provide an update into the mass estimates.  This work was carried out with the Canadian Hydrogen Intensity Mapping Experiment (CHIME) telescope as well as archival data provided by the Arecibo Observatory and the Green Bank Telescope.  We have found that new estimates place a mass considerably lower than the original estimate likely due to a mis-modeling of the white dwarf companion mass.  A discussion of how this happened as well as the consequences of this new revelation will also be discussed.

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.