The Lyman-alpha Forest

Reading the Universe and Galaxies in Shadows

A ten-minute talk for a general audience, given at the Leinweber Foundation's site visit to the University of Michigan on 29 October 2026.

It shows how the light of distant quasars, crossing the hydrogen between galaxies, maps the Universe's expansion with DESI's standard ruler and reaches down to the smallest scales, where dark matter's properties start to show. Then it follows one quasar's light through a PRIYA cosmological simulation, cloud by cloud.

Start the talk → · Light version for a weak projector

→ or space next · ← back · F full screen · S stills, for a slow machine. About 71 MB to load; best in Chrome, on a laptop or desktop with a keyboard.

A thin slice of DESI's 3D map: bright, red and star-forming galaxies near us, then quasars and the Lyman-alpha forest farther out — DESI: the largest 3D map of the Universe, with galaxies and quasars · DESI DR1

Red galaxies in one patch of DESI's map with the sound horizon's ring, beside the share of galaxy pairs at each separation — Sound waves in the early Universe left a standard ruler in the map · DESI DR1, pairs counted here

A quasar's light crossing the PRIYA simulation's gas, above the forest of absorption lines it has collected — Abundant intergalactic hydrogen forms a forest of lines · PRIYA (Bird, Fernandez, Ho et al. 2023)

Zoomed in where the light passes a galaxy in the simulation, above the spectrum it has collected — This forest is called: the Lyman alpha forest · PRIYA (Bird, Fernandez, Ho et al. 2023)