Vasileios Kalaitzidis — Astrophysics & Cosmology

§ 01

About

A short introduction

My name is Vasileios Kalaitzidis and I am from Athens, Greece. I am currently a Visiting Research Student at the AstroParticle and Cosmology Laboratory (APC, Paris) and the Tsung-Dao Lee Institute, Shanghai Jiao Tong University (TDLI, SJTU), working on ultra-high-energy γ-ray astrophysics with the Large High Altitude Air Shower Observatory (LHAASO) under the mentorship of Dr. Gwenael Giacinti and Dr. Hao Zhou.

I graduated from the University of St Andrews in 2025 with a Master's in Theoretical Physics (MPhys, First-Class Honours), with a thesis supervised by Dr. Martin Dominik. I spent a semester at Purdue University in 2024 as an exchange student, was a Gruber-funded Research Student at the European Southern Observatory working with Dr. Pooneh Nazari, and have carried out Royal Astronomical Society–funded research supervised by Dr. Indranil Banik.

My broader interests span γ-ray astronomy, protostellar systems, and cosmology.

Gamma-ray astronomy Cosmology Protostellar systems Microlensing Machine learning

Portrait of Vasileios Kalaitzidis — Portrait — 2025

§ 02

Research

Selected projects & ongoing work

My research spans protostellar chemistry, the Hubble tension, the ultra-high-energy diffuse γ-ray excess, and stellar-atmosphere recovery from gravitational microlensing events — bridging numerical modelling, observation, and machine learning.

Project 01

Chemistry of Protostellar Systems

ESO · Summer 2025 · w/ Dr. Pooneh Nazari

At the European Southern Observatory with Dr. Pooneh Nazari, we investigate recent mismatches between gas- and ice-phase abundance ratios reported by the JOYS survey. Through radiative-transfer modelling we test whether variations in physical structure alone can reproduce the observed differences, or whether richer chemistry is required.

We find that structural effects alone can drive order-of-magnitude shifts in the inferred column-density ratios — geometry and disk/envelope conditions strongly bias what is interpreted as chemical difference.

Column density plots — Fig. 1 — Column-density ratios under varying envelope geometry

Project 02

The Local Void & Late-Time Solutions to the Hubble Tension

RAS-funded · 2024–2025 · w/ Dr. Indranil Banik

In a study with Indranil Banik — funded by a 2024 RAS summer student grant — we tested whether a large local underdensity (a "local void") can explain the baryon acoustic oscillation data. Void models fit a 20-year compilation of BAO measurements markedly better than a homogeneous Planck-based model, reducing the overall tension to roughly the 1σ level.

The work was featured as a Royal Astronomical Society highlighted research article. I have also co-authored analyses of modified gravity in symmetric teleparallel f(Q) gravity (paper) and contributed to the CosmoVerse White Paper.

Void model BAO fit — Fig. 2 — Void-model residuals against a 20-year BAO compilation

Project 03

The Ultra-High-Energy Diffuse γ-ray Excess

TDLI · 2026– · w/ Dr. Gwenael Giacinti & Dr. Hao Zhou

At TDLI (SJTU) in Shanghai I investigate what fraction of the ultra-high-energy diffuse γ-ray excess observed in the Galactic plane by LHAASO can be attributed to unresolved sources — pulsars, supernova remnants, and γ-ray binaries.

I model their populations and Galactic distributions alongside the propagation effects relevant for comparison to observations, then tune the results to the instrument response and masking used by LHAASO, producing HEALPix sky maps for direct simulation-to-data comparison.

Diffuse gamma-ray sky map — Fig. 3 — Diffuse γ-ray sky structure, after LHAASO-equivalent masking

Project 04

Stellar Atmospheres via Microlensing Caustic Crossings

University of St Andrews · 2024–2025 · w/ Dr. Martin Dominik

My Master's thesis focused on recovering non-parametric limb-darkening profiles from caustic crossings — never previously achieved for fold caustics. The method uses finite-element and product-integration techniques to invert the Fredholm integral connecting the underlying stellar brightness profile to the flux observed during a microlensing event.

I am now extending the work with machine-learning inversion techniques that significantly outperform the numerical baselines, with publication in preparation.

Early-stage poster

Limb darkening recovery — Fig. 4 — Recovered limb-darkening profile from fold-caustic crossing

Project 05

Signal / Noise Classification for XENONnT

Purdue University · Spring 2024 · w/ Dr. Husheng Guan

With my classmate Divij Agarwal and the Purdue XENONnT group, I built a machine-learning classifier to separate signal from background noise, training on data from a yttrium–beryllium neutron source emitting at 152 keV — a calibration proxy for WIMP nuclear recoils.

Research conference poster