About

My name is Luka Vujeva, and I am a PhD Fellow working with Jose Maria Ezquiaga in the Center of Gravity, Niels Bohr Institute.

I grew up in Toronto, Canada, born to Croatian parents from Bosnia and Herzegovina. I did my undergraduate studies at the University of Toronto, where I did my bachelor's thesis and susbequent research with Peter Martin and Antoine Marchal at the Canadian Institute for Theoretical Astrophysics (CITA). During my time there, I studied thermal phase transitions in the interstellar medium to understand the first cold hydrogen structure formation in the galaxy, which is the initial seed for star formation.

I moved to Copenhagen for my Masters at the Niels Bohr Institute, where I worked with Charles Steinhardt at the Cosmic Dawn Center. There, I worked on leveraging the most massive objects in the Universe - galaxy clusters - as gravitational lenses to find the most efficient survey strategy for the James Webb Space Telescope (JWST) to search for the highest redshift galaxies in the Universe.

I stayed in Copenhagen to pursue my PhD with Jose Maria Ezquiaga in the Strong Group at the Center of Gravity. Our focus is on leveraging lensed gravitational waves to learn about the nature of dark matter, black holes, and cosmology.

Aside from physics, I am a massive music nerd. I love playing music with my friends, and in the past performed Croatian folk music around Canada and America. I am also very passionate about classical literature, and writing.

Research

My research primarily focuses on using the signals created by the mergers of distant black holes or neutron stars --- called gravitational waves --- to study dark matter through a mechanism called 'Gravitational Lensing'. For my full list of publications, check out my Inspire and NASA ADS pages.

Ray-Traced Arrival-Time Surfaces in the Strong Field

The diffraction integral is the standard tool for describing wave optics effects in gravitational wave lensing, but the assumptions needed to construct it break down when the source sits close to a black hole lens, deep in the strong field regime. In this project, we developed a new formalism that leverages the fact that null geodesics obey the Hamilton-Jacobi equations to ray trace the arrival-time surface directly, without relying on the thin lens approximation, and then solve the diffraction integral on that surface. For a source just 10 gravitational radii from a black hole, we recover the peak amplification and first diffractive fringe of a full relativistic wave scattering calculation to 6.5% and 0.3% respectively, while also matching the point mass lens in the weak field. This allows us to smoothly connect the strong and weak field regimes for sources anywhere from ~6 gravitational radii out to infinity. The paper can be found here: arxiv:2610.18984

Strong Field Lensing in Full Wave Optics

In this project, we studied the gravitational waves emitted by stellar-mass binaries orbiting close to a massive black hole, which can be strongly lensed by their massive companion. This setup breaks two standard assumptions of the lensing literature (geometric optics and thin lenses), so we computed the full wave optics amplification factors using a perturbative framework, for both face-on and edge-on binaries, and for spinning (Kerr) lenses. We find that for edge-on binaries the peak amplification does not occur at perfect source-lens-observer alignment, even for a non-spinning lens, as a consequence of the anisotropic quadrupolar emission of the binary. We also show that the commonly used point mass lens fails to reproduce the features of these systems, which has strong implications for forecasts of lensing by massive black holes in the strong field. The paper can be found here: arxiv:2610.18958

Lensing by Intermediate-Mass Black Holes and Globular Clusters

Intermediate-mass black holes (IMBHs) have been suggested as efficient lenses that could distort the lensed waveforms of the binaries we currently detect, as hinted by GW231123. In this project, we computed the rate at which IMBHs living in globular clusters would affect gravitational waves that are strongly lensed by galaxy-scale lenses, exploring a range of astrophysical models and lensing criteria. We find that at most 1 in 1,000 strongly lensed events would be affected, dropping to 1 in 10,000 for our fiducial optimistic scenario, which disfavors such an interpretation for GW231123. In turn, this means that lensed gravitational waves remain a clean probe of dark matter substructure and primordial black holes. The paper can be found here: arxiv:2608.07667

Dark Matter Substructure and Higher Order Catastrophes

In this project, we investigated the impact that dark matter subhalos have on lensed gravitational wave observables. We show that you can infer the existence of subhalos from even a single lensed GW detection due to deviations from the behaviors of caustics generated by single smooth dark matter halos. More specifically, a pair of lensed gravitational wave images with short arrival time differences and a relative magnification greater than 2 can only be produced by lenses with additional structure. These deviations are caused by higher order caustics, such as swallowtails and butterfly catastrophes. The paper can be found here: arxiv:2510.14953

Structure of Galaxy Clusters

This project investigated the impact that the structure of the dark and baryonic matter has on lensed gravitational waves by using state of the art cluster models made through deep electromagnetic observations of the clusters in combination with lens reconstruction. The paper can be found here: arxiv:2501.02096

lenscat

In order to facilitate the rapid follow-up imaging of candidate lensed gravitational waves, we build 'lenscat', a lightweight comprehensive catalog of known gravitational lenses with integrated tools for rapid crossmatching with gravitationtal wave (or any other) transient skymaps. The paper can be found here: arxiv:2406.04398, and the GitHub repository can be found here. Alternatively, you can play around with the web-app version here.

Contact