
Callum Witten
Astrophysicist
I am a postdoctoral researcher at the University of Geneva (UNIGE). I use the largest ground- and space-based telescopes as well as state-of-the-art simulations to study galaxies in the first few hundred million years of the Universe. I am particularly interested in the interplay between galaxies, their environments and the reionisation process.
About me
Since November 2024 I have been a postdoctoral researcher at the University of Geneva in the Galaxy Build-up at Cosmic Dawn group. I study galaxies that existed in the first ~700 million years of the Universe in the Epoch of Reionisation. This epoch hosts some of the most extreme environments — exceptionally dense gas feeding extraordinary amounts of star formation in enormous clusters of galaxies. I try to understand how these environments impact their resident galaxies during a tumultuous time in the Universe’s history.
​
Previously, I was a postgraduate student at the University of Cambridge supervised by Debora Sijacki and Nicolas Laporte. In these three years, I had observing time on the James Webb space telescope and ESO’s Very Large Telescope. I published five first-author papers, including one in Nature Astronomy with multiple press releases (including ESO and New Scientist). I successfully defended my thesis in October 2024.
I obtained a First-class Master's degree in Physics with Astrophysics from the University of Bath in 2021.

Press Releases


Read this New Scientist article about our paper - Rising from the ashes: evidence of old stellar populations and rejuvenation events in the very early Universe​​
Read this ESO press release about our paper - Deciphering Lyman-α emission deep into the epoch of reionization​​