Reading Between the Galaxies: The Secret History Written in the Cosmic Web

Most of the ordinary matter in the universe does not reside inside stars or galaxies. Instead, it drifts in the vast, dark expanses between them, woven into a colossal, interconnected structure known as the cosmic web. To read the secret history of this gas, astrophysicists look for the "Lyman-alpha forest"—a pristine record of shadows imprinted on the light of incredibly distant, bright quasars. This background illumination transforms the intergalactic medium into a unique cosmic thermometer and cartography tool, allowing us to map the invisible universe in three dimensions.

In this colloquium, I will take you on a chronological journey through cosmic time to explore what these intergalactic spaces reveal about fundamental physics. We will trace the evolution of the cosmos from the foggy, ancient dawn of the very first stars to the to the richly structured cosmic web of today, sculpted by gravity and violent galactic winds.

Translating these delicate astronomical observations into robust physics, however, presents a formidable multi-scale challenge: we must model physical processes spanning from the tiny details of individual gas clouds to the grand scale of the entire cosmos. I will discuss how we navigate this challenge by pairing high-resolution supercomputer simulations with cutting-edge machine learning pipelines and simulation-based inference. Finally, I will share some of our most recent, puzzling discoveries and highlight how an upcoming generation of ground- and space-based surveys will help us resolve these modern cosmic mysteries.

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