Shining Light on Cosmic Hydrogen

Avi Loeb
5 min readApr 22, 2024
Three-dimensional mapping of reionization by the 21-centimeter line of hydrogen. (Image credit: J. Pritchard and A. Loeb 2012)

Following the Big Bang, the Universe was hot and dense, composed of a soup of elementary particles. But after 400,000 years, the cosmic soup cooled below 4000 degrees Kelvin and electrons and protons combined to make hydrogen atoms. From then onward, the Universe was filled with hydrogen atoms until the first generation of stars produced ultraviolet radiation that broke the hydrogen atoms back to their constituent electrons and protons. This process, called cosmic reionization, completed about a billion years after the Big Bang, when the abundance of stars was large enough to produce more than one ionizing photon per hydrogen atom and also when the cosmic gas was sufficiently dilute for electrons and protons not to find each other within the age of the Universe.

One way to map cosmic hydrogen is through its feeble emission of a spectral line at the radio wavelength of 21-centimeter. As a result of the cosmic expansion, this wavelength is stretched or equivalently redshifted. The earlier is the emission time, the longer is the wavelength we observe since more expansion time elapsed between emission and observation. Observing the sky at a redshifted 21-centimenter wavelength slices the Universe as if it was Swiss cheese and reveals the holes in hydrogen associated with regions where reionization had already started. By scanning multiple wavelengths, one gets a three-dimensional map of this Swiss cheese structure of cosmic hydrogen during reionization. Below redshift 6 (or a wavelength of 7 times 21 centimeter, roughly the height of a person), corresponding to a billion years after the Big Bang, there is no diffuse hydrogen (cheese) left, but only small islands of hydrogen within dense pockets of galaxies.

Two decades ago, I was one of the cosmologists who laid the foundation for this 21-centimeter cosmology, as summarized in an extended review paper and a textbook that I published based on work with my students and postdocs. Currently, there are a number of experiments pursuing the detection of this 21-centimeter signal.

However, a bit earlier, 25 years ago, I proposed in an extended paper with my colleague, George Rybicki, another method for mapping the neutral hydrogen in the early Universe. In addition to the 21-centimeter split of its ground state, hydrogen has prominent transitions from its ground state to higher excited levels. These transitions are named after their discoverer, Theodore Lyman, who served as director of the Jefferson Laboratory at the Harvard Physics department between 1908–1917.

The transition to the nearest excited level, called Lyman-alpha, shows as a prominent emission line from interstellar hydrogen in nearby galaxies (as long as it is not damped by dust absorption). However, before cosmic hydrogen was reionized, the Lyman-alpha emission from early galaxies was scattered by the fog of hydrogen along our line-of-sight. My paper with Rybicki calculated the expected halo of scattered Lyman-alpha radiation around the first generation of galaxies which were embedded inside the primordial hydrogen gas.

As Lyman-alpha photons are absorbed and reemitted by cosmic hydrogen, their wavelength gets shifted as a result of the cosmic expansion, discovered by Edwin Hubble. The scattering by a medium that follows Hubble expansion resembles the scattering of a tennis ball by a moving racket or reflection of light by a moving mirror. The scattering changes the energy of the scattered particle. Once the Lyman-alpha photons move away from resonance with the transition wavelength of hydrogen, they are able to move freely and eventually reach the observer. This diffusion process in both space and wavelength leads to the appearance of a so-called `Loeb-Rybicki halo’ around the source galaxy.

When we wrote our paper with Rybicki in 1999, the design of the James Webb Space Telescope was only in its infancy. At that time, I served on the first Science Advisory Board for the telescope which was then named the `Next Generation Space Telescope (NGST)’. (The name was changed in 2002, in order to secure robust federal funding, to that of NASA’s second administrator, James Webb.) In 1999, I used the best forecasted numbers for the telescope parameters to find that our predicted Lyman-alpha halos might potentially be detectable.

This week, I completed a new paper with the brilliant young collaborator, Hamsa Padmanabhan, where we calculate the prospects for detecting the `Loeb-Rybicki halos’ based on the latest data on high-redshift galaxies from the Webb telescope.

In our new paper, Hamsa and I show that although the parameters that Rybicki and I assumed for the capabilities of the Webb telescope were optimistic, the real Universe offers brighter galaxies at early cosmic times than expected before the launch of the telescope. The latest data from the Webb telescope reveals a population of brighter galaxies than were expected beforehand. For the brightest galaxies, Hamsa and I find that Loeb-Rybicki halos are detectable out to redshifts of 9–16. The signal can be enhanced by stacking multiple halos or by taking advantage of their high level of polarization.

Writing a textbook is paying it forward, because one never knows who will benefit from reading it. I started practicing cosmology after reading the excellent textbook “Structure Formation in the Universe”, written by Hamsa’s late father, the distinguished physicist Thanu Padmanabhan. Hamsa told me that she started working on early galaxies and reionization after reading my textbook “The First Galaxies in the Universe.” Thanu’s writings paved the way for me to write a book that his daughter would be inspired by. In retrospect, he could not have given a better gift to his gifted daughter.

Let me close with a cosmic perspective about human existence in the backdrop of stars and galaxies. The mass of our body is roughly at the geometric mean between the mass of a proton and the mass of the Sun. The mass of the Milky-Way galaxy is at the geometric mean between the mass of the Sun and the mass of the observable Universe. Suppose our intelligence is at the geometric mean between the intelligence of a single-cell organism and the most intelligent lifeform beyond Earth. What would be the implications?


Image credit: Chris Michel (October 2023)

Avi Loeb is the head of the Galileo Project, founding director of Harvard University’s — Black Hole Initiative, director of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics, and the former chair of the astronomy department at Harvard University (2011–2020). He is a former member of the President’s Council of Advisors on Science and Technology and a former chair of the Board on Physics and Astronomy of the National Academies. He is the bestselling author of “Extraterrestrial: The First Sign of Intelligent Life Beyond Earth” and a co-author of the textbook “Life in the Cosmos”, both published in 2021. His new book, titled “Interstellar”, was published in August 2023.



Avi Loeb

Avi Loeb is the Baird Professor of Science and Institute director at Harvard University and the bestselling author of “Extraterrestrial” and "Interstellar".