Probing the Cosmic Web
Matter in the Universe is spread in a vast 3D network of filaments and sheets known as the cosmic web. These structures are dominated by dark matter and include both galaxies and the intergalactic medium: hot, nearly fully ionized hydrogen, with only about one atom in 100,000 remaining neutral. Light emitted by distant quasars travels through this cosmic web, and whenever it encounters neutral hydrogen, it’s absorbed at a specific redshifted wavelength. By the time the light reaches an Earthly observer, the spectrum is imprinted with a dense series of absorption lines known as the Lyman-alpha forest. Since the 1990s, astronomers have used the forest to track the evolution of the intergalactic medium and to trace the enveloping dark matter. However, because the medium’s dynamics are complex on small scales, extracting data from the forest has required computationally expensive hydrodynamical simulations.
Now Roger de Belsunce of MIT and his collaborators have developed a new theoretical description of the Lyman-alpha forest [1]. Their perturbation-theory approach rests on a central idea: Although the physics is highly complex on small scales, the large-scale behavior is governed by fundamental symmetries. These symmetries constrain how the forest responds to the underlying matter distribution, leading to an emergent simplified description. The researchers’ comparisons with state-of-the-art hydrodynamical simulations show subpercent agreement. They anticipate that their framework will enable the efficient analysis of observational Lyman-alpha-forest data from spectroscopic surveys such as DESI [1]. De Belsunce is especially interested in measuring neutrino masses and shedding light on the nature of dark matter.
–Charles Day
Charles Day is a Senior Editor for Physics Magazine.
References
- R. de Belsunce et al., “Modeling the cosmological Lyman- forest at the field level,” Phys. Rev. Lett. 136, 101001 (2026).