Sanika S. Khadkikar

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Sat, 01 Jun 2030 13:00:00 +0000

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Astro[sound]bites Episode 119 Gravitational Vibes

Sat, 04 Apr 2026 00:00:00 +0000

Episode 119 of Astro[sound]bites explores recent results in gravitational wave astrophysics. Astro[sound]bites is a podcast dedicated to making cutting-edge astrophysics research accessible and engaging for listeners across all backgrounds.

Invited Talk at APS Global Physics Summit 2026

Wed, 18 Mar 2026 00:00:00 +0000

Delivered an invited talk titled Unveiling the Equation of State Through Gravitational Wave Observations Involving Neutron Stars at the APS Global Physics Summit 2026 in Denver, CO. The talk presented methods for constraining dense matter physics from gravitational wave signals, covering equation of state inference, the impact of prior choices, and prospects with next-generation detectors.

Elected Student Member of the APS Division of Gravitational Physics Executive Committee

Sun, 01 Mar 2026 00:00:00 +0000

Elected through a division-wide vote to serve as the student member on the Executive Committee of the American Physical Society’s Division of Gravitational Physics. The role involves representing graduate student interests in divisional governance, contributing to meeting programming, and helping shape the direction of the DGRAV community.

GW190425: Classifying BNS vs BBH Mergers and Constraining Asymmetric Dark Matter

Thu, 01 Jan 2026 00:00:00 +0000

GW190425 was detected by LIGO and Virgo in April 2019 with a total mass substantially higher than any known galactic double neutron star system. This raised immediate questions about whether it was a binary neutron star or a binary black hole merger, and whether exotic physics might explain the anomaly.

This paper develops a Bayesian framework to classify compact binary mergers using only gravitational wave data and applies it to GW190425. We also show that the anomalous mass of GW190425 is consistent with neutron stars that harbor asymmetric dark matter cores, and we place the first constraints on dark matter nucleon interactions from a gravitational wave event.

Published in Physical Review D (2026)

Invited Lecture at University of Washington

Tue, 16 Sep 2025 00:00:00 +0000

Invited seminar titled Pressing Matter at the University of Washington Department of Physics. The talk covers methods for constraining the dense matter equation of state, the impact of prior choices and waveform systematics, and prospects for precision measurements with Einstein Telescope and Cosmic Explorer.

Best Student Speaker Award at APS DGRAV EGM 2025

Thu, 01 May 2025 00:00:00 +0000

Awarded Best Student Speaker at the 2025 APS Division of Gravitational Physics Early Graduate Meeting, given in recognition of an outstanding research presentation to the division.

Rick Robinett Student Service Award at Penn State Physics

Thu, 01 May 2025 00:00:00 +0000

Awarded the Rick Robinett Student Service Award by the Penn State Department of Physics in recognition of contributions to the graduate student community through mentorship, outreach, and departmental service.

GW250114 Factsheet Featured by LIGO

Tue, 14 Jan 2025 00:00:00 +0000

GW250114 is a loud binary black hole merger detected by LIGO on January 14, 2025, marking ten years of gravitational wave astronomy. The factsheet covers its observation details, signal properties, source masses, spins, and tests of general relativity including measurements of the Hawking area law. Featured on the official LIGO detection page.

Precise and Accurate Neutron Star Radius Measurements with Next-Generation Gravitational Wave Detectors

Wed, 01 Jan 2025 00:00:00 +0000

Next-generation gravitational wave detectors, Einstein Telescope and Cosmic Explorer, will observe thousands of binary neutron star mergers and offer unprecedented precision on neutron star parameters. But precision without accuracy is a problem. Systematic biases in waveform models, prior assumptions, and pipeline choices can lead to confident but wrong measurements.

This paper gives a comprehensive assessment of how accurately next-generation detectors can measure neutron star radii, identifying and quantifying the dominant sources of systematic error. We show that achieving sub-percent accuracy on the radius, a key target for constraining the equation of state, requires careful control of waveform systematics and prior choices, and we give practical recommendations for the O5 era and beyond.

Published in Physical Review D (2025)

Eccentricities in Gravitational Wave Binaries

Sat, 01 Jun 2024 00:00:00 +0000

"The most exciting phrase to hear in science, the one that heralds new discoveries, is not 'Eureka' but 'That's funny...'"

- Isaac Asimov

Recently, I have noticed increased discussion on the importance of eccentricity in the gravitational wave (GW) compact object coalescence literature. However, I lack intuition for why eccentricity is considered significant, for the following reasons:

The orbits circularize ($e \to 0$) as the compact binary approaches coalescence due to gravitational wave emission. So why include eccentricity at all stages?
The eccentricities of binary neutron star (BNS) systems formed through isolated binary evolution channels are expected to be small ($e < 0.3$). Dynamically formed binaries contribute only a small fraction of the total BNS population.
The orbital period does not depend on eccentricity. To explore this, I begin by revisiting the basics of orbital dynamics.

The Lagrangian for a two-body system in the center-of-mass frame is:

$$\mathcal{L} = \frac{1}{2}\mu\dot{r}^2 + \frac{1}{2}\mu r^2 \dot{\theta}^2 + \frac{Gm_1m_2}{r}$$

Here, $\mu = \frac{m_1 m_2}{m_1 + m_2}$ is the reduced mass. Since $\theta$ is a cyclic coordinate, we have:

$$\mu r^2 \dot{\theta} = \ell$$

And the radial equation of motion becomes:

$$\mu \ddot{r} = \frac{\ell^2}{\mu r^3} - \frac{Gm_1m_2}{r^2}$$

Using the substitution $u = \frac{1}{r}$, the solution is a conic:

$$r(\theta) = \frac{\ell^2}{\mu^2 GM (1 + e \cos\theta)}$$

Here $M = m_1 + m_2$ and $e$ is the orbital eccentricity. The orbital radius $r$ depends on $e$, but the period $T$ is independent of it for bound orbits. By Kepler’s second law:

$$\frac{dA}{dt} = \frac{1}{2} r^2 \frac{d\theta}{dt} = \frac{\ell}{2\mu} = \text{constant}$$

Integrating over a full orbit:

$$A = \frac{\ell T}{2\mu}$$

And for an ellipse:

$$A = \pi a b = \pi a^2 \sqrt{1 - e^2}$$

From the conic solution:

$$a(1 - e^2) = \frac{\ell^2}{\mu^2 GM}$$

Combining these results:

$$T^2 = \frac{4\pi^2}{GM} a^3$$

This is Kepler’s third law, which confirms that the period $T$ is independent of eccentricity.

My skepticism about the emphasis on eccentricity is rooted in these points. However, the inclusion of gravitational radiation may alter this picture. I will explore this further in future work.

Why Gravitational Radiation Starts at the Quadrupole

Mon, 01 Jan 2024 00:00:00 +0000

"Out of intense complexities, intense simplicities emerge"

- Winston Churchill

I have always been absolutely awestruck by the fact that gravitational radiation can only be quadrupolar and never understood it quite intuitively until I took some time and found analogies and visualizations to understand it. I am documenting this for future reference for me and anyone who would find this useful. So let’s begin!

For understanding something that I do not have a grasp on at all, I generally start from what I do understand about the topic. Naively, if I were to ask my undergraduate self: “How can you possibly produce gravitational radiation?” I would have had some ideas. Let me start from there.

Mass produces curvature and this curvature is what forms our notion of gravity. So if I were to provide some momentum to this mass and change its distribution spatially, the metric which helps us define curvature should change and that should produce gravitational radiation. This idea isn’t entirely wrong but to understand the beautiful nuances of general relativity, we need to dive in deeper.

Figure 1: Newtonian shell argument and equivalent point mass

Consider a uniform spherical mass distribution of mass $m$. With some notions of Newtonian gravity, we know that measuring gravitational field at $r > R$ for this mass distribution should be exactly equivalent to measuring the gravitational field of a mass $m$ concentrated at $r = 0$. This happens because the fields of all the individual masses add up and lead up to being equivalent to a concentrated mass rather than a distribution. This ideology is extremely important in understanding gravitational radiation. As long as the total mass enclosed inside a given shell is the same and has spherical symmetry, the field outside would be the same irrespective of how it’s arranged inside. The analogue of this theorem is the Birkhoff theorem in general relativity.

Birkhoff’s theorem states that any spherically symmetric solution of the vacuum field equations must be static and asymptotically flat, meaning it should follow the Schwarzschild metric:

$$ds^2 = \left(1 - \frac{2GM}{c^2r}\right)c^2dt^2 + \left(1 - \frac{2GM}{c^2r}\right)^{-1}dr^2 + r^2d\Omega^2$$

The first time I noticed that this metric has no mention of $R_{\rm obj}$ of the object under consideration, I was so surprised. This means no matter what is the difference in the compactness of two objects in consideration, as long as they are the same mass and spherically symmetric, the metric in their exteriors would look exactly the same. This is because the Schwarzschild solution in the exterior of an object is a vacuum solution, leading to the stress-energy tensor being zero identically everywhere outside the object as long as there is no energy outflow. This is very fascinating to me that the spacetime only obeys the value of one dial, the energy content or mass.

Connecting this back to the original discussion about gravitational radiation, Birkhoff theorem and the Newtonian analogue of it can help us gain better intuition. When it comes to radiation, it can be helpful to break the radiation like a Taylor series in decreasing order of contribution from different mass distributions. The monopolar radiation (if it existed), would come from the zeroth moment of a mass distribution, which is just its total mass. An oscillation of this monopole ($\ell=0$), often termed as the breathing mode, has the mass remaining unchanged and the spherical symmetry of the object still intact, but it pulsates about a mean radius. Using Birkhoff theorem here, we can understand that even if the mass distribution is changed in the case of a monopolar oscillation, since the total mass remains the same and spherically symmetric, the external metric also stays the same and there are no tides outside the object. Thus, monopolar gravitational radiation does not exist.

Figure 2: Monopole (ℓ = 0) breathing mode

The terms following the monopole in the multipole expansion can be understood as various degrees of asymmetry added into the system. The next order term is the dipole ($\ell=1$). The dipole provides one additional trinket of information about the mass distribution: the mean or the center of mass. If the system is oscillating in a dipolar mode, we get an idea of the movement of the center of mass. We can then change our frame to match the velocity of the center of mass, so we can always take our observations in a frame where there is no motion of the center of mass. Thus, dipolar gravitational radiation also does not exist.

Figure 3: Dipole (ℓ = 1) mode

If we add another degree of asymmetry to our system, we can also measure the deviation of the mass distribution from its mean, i.e. the quadrupole moment. If the object is oscillating in the quadrupolar mode ($\ell=2$), it is neither spherically symmetric, nor can we pick a frame in which this effect would go away. A time-varying quadrupolar moment thus changes the gravitational field outside the object too, causing tides. So for the quadrupole, hexapole and other higher oscillations, information about the mass distribution inside the objects leaks into the exterior with gravitational waves. As a very contextual example, neutron stars have deformities on their surface. We can actually aim to characterize this topology of neutron stars using the continuous gravitational waves emitted by them.

Figure 4: Quadrupole (ℓ = 2) mode

So yeah, it’s all very fascinating and exciting. But to conclude, monopolar and dipolar gravitational radiation does not exist. The leading order contribution to the gravitational radiation is from the quadrupolar mode but higher modes also exist. This is also why we need two polarizations for describing the gravitational waves because they are created using an $\ell=2$ excitation. I love this field man. Cheers.

Code Used to Generate the Modes

import numpy as np
import matplotlib.pyplot as plt
from matplotlib import animation
from scipy.special import sph_harm

l, m = 2, 0
A = 0.3
omega = 2 * np.pi / 5
R0 = 1.0
frames = 20
T_total = 10

theta = np.linspace(0, np.pi, 100)
phi = np.linspace(0, 2 * np.pi, 100)
theta_g, phi_g = np.meshgrid(theta, phi)

Y = np.real(sph_harm(m, l, phi_g, theta_g))
Y /= np.max(np.abs(Y))

fig = plt.figure(figsize=(6, 6))
ax = fig.add_subplot(111, projection='3d')
ax.set_box_aspect([1, 1, 1])
ax.axis('off')

def update(frame):
 ax.cla()
 t = frame / frames * T_total
 R = R0 * (1 + A * Y * np.sin(omega * t))
 X = R * np.sin(theta_g) * np.cos(phi_g)
 Y_ = R * np.sin(theta_g) * np.sin(phi_g)
 Z = R * np.cos(theta_g)
 color = (Y + 1) / 2
 ax.plot_surface(X, Y_, Z, facecolors=plt.cm.plasma(color),
 rstride=1, cstride=1, antialiased=True)
 ax.set_xlim(-1.5, 1.5)
 ax.set_ylim(-1.5, 1.5)
 ax.set_zlim(-1.5, 1.5)
 ax.axis('off')
 ax.view_init(elev=20, azim=frame * 3)
 return []

anim = animation.FuncAnimation(fig, update, frames=frames, interval=100)
anim.save("./final_breathing_mode_20.gif", writer="pillow", fps=10)

Experience

Tue, 24 Oct 2023 00:00:00 +0000

Quasi-Stationary Sequences of Hyper-Massive Neutron Stars with Exotic Equations of State

Sat, 01 Jan 2022 00:00:00 +0000

When two neutron stars merge, the remnant can survive briefly as a rapidly rotating hyper-massive neutron star before collapsing into a black hole. How long it survives and what it looks like encodes information about the equation of state at densities far beyond anything accessible in a terrestrial experiment.

This paper constructs quasi-stationary equilibrium sequences of hyper-massive neutron stars using exotic equations of state that include strange quark matter, hyperons, and other non-nucleonic degrees of freedom. We map out how the remnant mass, angular momentum, and oscillation frequencies change with the underlying microphysics, giving a theoretical foundation for interpreting post-merger gravitational wave signals from next-generation detectors.

Published in Journal of Astrophysics and Astronomy (2022)

Charpak Lab Scholarship Programme 2021

Wed, 01 Sep 2021 00:00:00 +0000

Named among the recipients of the Charpak Lab Scholarship Programme 2021, a competitive award by Campus France and the French Embassy in India recognising outstanding academic merit and supporting research internship opportunities at French institutions.

Interview with LIGO India

Thu, 01 Apr 2021 00:00:00 +0000

An interview with the LIGO India team covering research on neutron star equation of state inference, the role of prior assumptions in gravitational wave parameter estimation, and thoughts on the future of multi-messenger astrophysics with next-generation detectors.

Maximum Mass of Compact Stars from Gravitational Wave Events with Finite-Temperature Equations of State

Fri, 01 Jan 2021 00:00:00 +0000

The maximum mass of a neutron star is one of the most fundamental quantities in nuclear astrophysics. It marks the boundary between a neutron star and a black hole, and places direct constraints on the stiffness of dense matter at extreme densities. Gravitational wave observations of binary neutron star mergers, particularly signals that show a post-merger collapse, give us a rare observational window onto this threshold.

This paper extends maximum mass constraints to finite temperature equations of state, which matter for the hot dense remnant formed immediately after a merger. Using GW170817 and its multi-messenger counterparts, we derive bounds on the maximum mass of both cold and hot neutron stars. We show that thermal effects can shift the inferred maximum mass by up to roughly ten percent and need to be accounted for in any equation of state analysis.

Published in Physical Review C (2021)

Interview with BITS Pilani Hyderabad Adastra

Fri, 04 Dec 2020 00:00:00 +0000

An interview for Adastra, the science and technology magazine of BITS Pilani Hyderabad, discussing the journey from undergraduate studies into astrophysics research, what drew me to gravitational wave science, and advice for students thinking about pursuing research careers.

Curriculum Vitae

Mon, 01 Jan 0001 00:00:00 +0000