Raj Patil

The observations by Galileo through his telescopes opened new horizons in our understanding of the cosmos. It allowed us to see celestial objects in ways previously unimaginable, revealing moons around Jupiter, the phases of Venus, and much more. Similarly, the first detection of gravitational waves in 2015 has started a new era of astronomical exploration. It allows us to observe cosmic phenomena, such as black hole mergers and neutron star collisions, that were once beyond our reach, fundamentally altering our understanding of the universe.

The question I aim to address is the following: When two objects such as black holes and/or neutron stars, undergo a binary coalescence, they emit gravitational waves. We detect these waves using our amazing detectors. What information can we obtain about the properties of the two colliding objects by analyzing the detected gravitational waves? Specifically, what insights can we gain about their mass, spin, equation of state, their surroundings, and so on?

What are gravitational waves? [LIGO] How are gravitational waves detected? [GW Open Data Workshop]

Image credits: [Link]

One fun excercise is to hear the pleasing melody of graviational waves after converting them into sound waves. Here [Soundofspacetime] you can listen to the sound of gravitational waveforms emitted by various binary sources.
CAUTION! gravitational waves are not sound waves and this is just a fun excercise.

Interesting articles / lectures on gravitational waves:

Recent relevant reviews / snowmass papers:

A list of seminal papers in the field of gravity can be found here [link]

The most inspiring workshop I have attended was “In Pursuit of Gravitational Waves – Solving the Two-Body Problem in General Relativity.” It explored the historical development of the two-body problem, bringing together leading researchers who played a central role in its progress, along with historians and philosophers of physics. The recordings of the talks can be found on the program page here [webpage]

Currently, I work on the studying astrophysical compact objects and gravitational waves emitted by them using the techniques of quantum field theories and scattering amplitudes. This problem is interesting because the recent direct detections of gravitational waves by LIGO and Virgo observatories have proved to be a probe for the physics of celestial objects like black holes and neutron stars. Such detections have the potential to unravel the mysteries of cosmic origins, equations of state of compact objects and will prove to be a test to the theory of general relativity. On the other hand, the framework of Quantum field theory has proven to be very successful at predicting physical behavior to high accuracy and precision and is the basic language of modern theoretical physics. Moreover, the techniques of scattering amplitudes provide us with several tools which make the calculation of the physical observables systematic and insightful. I am working on computing state-of-the-art higher-order effective potentials and fluxes. These potentials and fluxes are important as they are used to compute accurate theoretical waveform templates which are compulsory ingredients for data analysis and reliable physical interpretation of the gravitational wave signals.

I am also working on developing SEOBNR-PM [arXiv:2405.19181] which is a bound-orbit waveform model that uses the post-Minkowskian (PM) results in the effective one body (EOB) formalism. A typical time-domain waveform model uses the Hamiltonian (for conservative effects) and Fluxes (for dissipative effects) to evolve the dynamics of the binary system in the inspiral phase when the orbital radius is slowly decaying due to gravitational wave emission. Then when two compact objects collide on each other, a numerical relativity (NR) and black hole perturbation theory informed templated is used to model the merger and the ringdown. In SEOBNR-PM we use the PM scattering angle (upto 4PM for non-spinning and 5PM for spinning) to construct a PM Hamiltonian, which is a new feature of this model. Here is one example of a waveform generated using the model and its comparison to the corresponding NR waveform.

One of the most fundamental concepts linking quantum and classical physics is the "eikonal". The eikonal was originally motivated by geometric optics and later played a crucial role in the formulation of quantum mechanics. It has also been used recently to analyze the two-body problem. But, there are two distinct (even classically) and incompatible usage of the name “eikonal” in present amplitudes literature. The first is proportional to the matrix element of the operator logarithm of the S-matrix and in the classical limit becomes a function whose derivatives contain all classical scattering observables and is the scattering generator (Magnusian). The second is proportional to the logarithm of an S-matrix element and in the classical limit becomes the on-shell action of the corresponding classical theory. I have contributed to the novel analysis [arXiv:2511.05649] showing explicitly how these two quantities are related, clarifying their interpretation and connection.