I'm Utkarsh Mali

a PhD Candidate

Welcome

at the Department of Physics | University of Toronto

About Me

About Me

I'm Utkarsh Mali, a PhD Candidate at the University of Toronto

I'm Utkarsh, a dedicated and motivated student at CITA, the Canadian Institute for Theoretical Astrophysics. As a theoretical physicist, I have a strong interest in the physics of gravitational waves and the computational methods used to understand them. Additionally, I am a member of the Laser Interferometer Gravitational-Wave Observatory (LIGO) scientific collaboration

I am also passionate about teaching and communicating science to the public. I enjoy organizing and attending various outreach and educational initiatives.

I am originally from Singapore where I have lived most of my life. When I am not studying, I enjoy playing boardgames, competing in ultimate frisbee and generally staying active.

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Research

Research

Gravitational Wave Cosmology

Studying cosmology through the gravitational-wave signals from binary coalescence. I am currently using a flexible, realistic mass distribution to study features in the spectral sirens picture. On observed data, I show that multiple features in the mass distribution correlate with cosmology.

Neutron Stars

Studying the structure of neutron stars. I am studying the consistency between the neutron star equations of state and current observations. I have also estimated the impact of tidal phasing from fundamental mode oscillations of coalescing neutron stars.

Rates and Populations

Understanding the populations of merging compact objects in the universe. I'm part of the team analysing results from LVK's O4 run. This includes estimating selection effects (injections) as well as interpreting the results of the observed events (O4a R&P).

Astro-statistics

I'm implementing robust statistical frameworks to infer single event and population level information. I use hierarchical bayesian inference and hamiltonian monte carlo to do this. I am also interested in machine learning, AI and it's applications to astrophysics.

Publications

Publications

arXiv (PRD)

Guesswork in the gap: the impact of uncertainty in the compact binary population on source classification

Utkarsh Mali & Reed Essick
Nov 2025

The nature of the compact objects within the supposed "lower mass gap" remains uncertain. Observations of GW190814 and GW230529 highlight the challenges gravitational waves face in distinguishing neutron stars from black holes. Interpreting these systems is especially difficult because classifications depend simultaneously on measurement noise, compact binary population models, and equation of state (EOS) constraints on the maximum neutron star mass. We analyze 66 confident events from GWTC-3 to quantify how the probability of a component being a neutron star, P(NS), varies across the population. The effects are substantial, the dominant drivers of classification are the pairing preferences of neutron stars with other compact objects, and the neutron star spin distributions. The data reveals that P(NS) varies between 1% - 67% for GW230529's primary and between 51% - 100% for GW190425's primary. By contrast, P(NS) for GW190814's secondary varies by <10%, demonstrating robustness from its high signal-to-noise ratio and small mass ratio. Analysis using EOS information tends to affect P(NS) through the inferred maximum neutron star mass rather than the maximum spin. As it stands, P(NS) remains sensitive to numerous population parameters, limiting its reliability and potentially leading to ambiguous classifications of future GW events.

arXiv (ApJ)

GWTC-4.0: Population Properties of Merging Compact Binaries

LIGO-Virgo-Kagra (incl. Mali)
Oct 2025

We detail the population properties of merging compact objects using 158 mergers from the cumulative Gravitational-Wave Transient Catalog 4.0, which includes three types of binary mergers: binary neutron star, neutron star--black hole binary, and binary black hole mergers. We resolve multiple over- and under-densities in the black hole mass distribution: features persist at primary masses of 10M⊙ and 35M⊙ with a possible third feature at ∼20M⊙. These are departures from an otherwise power-law-like continuum that steepens above 35M⊙. Binary black holes with primary masses near 10M⊙ are more likely to have less massive secondaries, with a mass ratio distribution peaking at q ~ 0.74, potentially a signature of stable mass transfer during binary evolution. Black hole spins are inferred to be non-extremal, with 90% of black holes having χ < 0.57, and preferentially aligned with binary orbits, implying many merging binaries form in isolation. However, we find a significant fraction, 0.24--0.42, of binaries have negative effective inspiral spins, suggesting many could be formed dynamically in gas-free environments. We find evidence for correlation between effective inspiral spin and mass ratio, though it is unclear if this is driven by variation in the mode of the distribution or the width.

arXiv (ApJ)

Striking a Chord with Spectral Sirens: multiple features in the compact binary population correlate with H₀

Utkarsh Mali & Reed Essick
April 2025

Spectral siren measurements of the Hubble constant (H₀) rely on correlations between observed detector-frame masses and luminosity distances. Features in the source-frame mass distribution can induce these correlations. It is crucial, then, to understand (i) which features in the source-frame mass distribution are robust against model (re)parametrization, (ii) which features carry the most information about H0, and (iii) whether distinct features independently correlate with cosmological parameters. We study these questions using real gravitational-wave observations from the LIGO-Virgo-KAGRA Collaborations third observing run. Although constraints on H0 are weak, we find that current data reveals several prominent features in the mass distribution, including peaks in the binary black hole source-frame mass distribution near ∼ 9M⊙ and ∼ 32M⊙ and a roll-off at masses above ∼ 46M⊙. For the first time using real data, we show that all of these features carry cosmological information and that the peak near ∼ 32M⊙ consistently correlates with H0 most strongly. Introducing model-independent summary statistics, we show that these statistics independently correlate with H₀ exactly what is required to limit systematics within future spectral siren measurements from the (expected) astrophysical evolution of the mass distribution.

arXiv (PRD)

Astrophysical constraints on neutron star f-modes

Sailesh Mohanty, Utkarsh Mali et al.
Oct 2024

We constrain the fundamental-mode (f-mode) oscillation frequencies of nonrotating neutron stars using a phenomenological Gaussian process model for the unknown dense-matter equation of state conditioned on a suite of gravitational-wave, radio and X-ray observations. We infer the quadrupolar f-mode frequency preferred by the astronomical data as a function of neutron star mass, with error estimates that quantify the impact of equation of state uncertainty, and compare it to the contact frequency for inspiralling neutron-star binaries, finding that resonance with the orbital frequency can be achieved for a subset of the coalescences. For an optimally configured binary neutron star merger, we estimate the gravitational waveform's tidal phasing due to f-mode dynamical tides as 7^{+2}_{-3} rad at merger. We assess prospects for distinguishing f-mode dynamical tides with current and future-generation gravitational-wave observatories.

SSC21-WKII

HERON: Demonstrating a Novel Biological Platform for Small Satellite Missions

Dylan Vogel et al. (incl. Utkarsh Mali)
Jul 2021

Long-duration deep space missions pose a significant health risk for both humans and their resident microorganisms. The University of Toronto Aerospace Team (UTAT) Space Systems Division has developed the HERON CubeSat. HERON houses a payload platform which measures the effects of the low-Earth-orbit (LEO) environment on the gene expression and drug resistance of Candida albicans, a yeast commonly found in the human gut microbiome. HERON launched in Q1 2023 into a Sun-synchronous orbit via a SpaceX Falcon 9 rocket at an altitude of approximately 550 km. Our platform is open-source and can serve as a low-cost template for future biological CubeSat missions. This paper serves as a technical and scientific description of the platform, along with the lessons learned during the payload design, assembly, and validation processes.

NASA ADS
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Teaching

Teaching

Introduction to Physics I Teaching Assistant, PHY131

Anonymous Student Feedback:

"I thought practicals were extremely fun and helped stimulate my learning and understanding of physics"

"Really great, extremely helpful and encouraging"

"I think the practicals were the best part of physics for me. Both instructors are awesome people that you should cherish."

"Excellent TA, very helpful, kind and approachable. Passionate about teaching students"

"Introduction to Physics II" Teaching Assistant, PHY132

Anonymous Student Feedback:

"Probably one of the best TAs I have had in my entire university experience."

"Utkarsh is an extremely helpful and thought-provoking TA. He is awesome, and his passion for physics is apparent in every one of his explanations to students. I greatly appreciate all of his help during the PHY132 practicals this semester."

"Physics II" Teaching Assistant, PHY152

Anonymous Student Feedback:

"Utkarsh is good at explaining concepts and very kind."

"Great job especially with helping for problem sets, very good at helping me understand and not just showing the answers."

"Very helpful and approachable!"