A 25-Year-Old Insight About the First Billion Years of the Cosmos
Four-hundred thousand years after the Big Bang, the temperature of the Universe dipped for the first time below 3,000 degrees Kelvin, allowing free electrons and protons to combine into hydrogen atoms. As the cosmic expansion continued to cool these atoms, they became the major constituent of the ordinary matter in the Universe. So far, our telescopes never imaged this diffuse gas out of which the first generation of galaxies condensed over the next billion years.
The radiation emitted by these early galaxies broke (ionized) the hydrogen atoms back to free electrons and protons about a billion years later, during the era called `reionization.’ Once the diffuse cosmic gas was broken, about a billion years after the Big Bang, the cosmic expansion left it so dilute that it remained in a state of a plasma — composed of free charges, until the present time.
Backtrack to the first billion years. Inside the first generation of galaxies, massive stars produced ultraviolet radiation which was absorbed by hydrogen clouds that gave birth to them. As a result, the bound electrons in these hydrogen atoms were excited to the first energy level above their ground state. The prompt decay of the excited state back to the ground energy level resulted in the emission of a spectral line at a wavelength of 121.6 billionths of a meter, named Lyman-alpha after the Harvard physicist Theodore Lyman IV who discovered it.
The Lyman-alpha spectral line is indeed observed from quasars and galaxies after the first billion years. However, when the diffuse gas between galaxies was still full of hydrogen, these atoms resonantly absorbed and reemitted the Lyman-alpha photons. The absorption and reemission can be considered as scattering of these photons away from the line-of-sight towards the emitting galaxy. This is because the photons emitted towards us were absorbed and remitted in other directions, just like a tennis ball hit by a racket. We indeed observe a deficit of Lyman-alpha photons from galaxies or quasars around lookback times approaching the first billion years after the Big Bang. The scattering results in a distinct absorption trough in the spectrum, called the Gunn-Peterson effect, after the astronomers Jim Gunn and Bruce Peterson who discovered it in 1965. The Lyman-alpha opacity is so large that even a tiny atomic hydrogen fraction, above a millionth, is sufficient to leave a measurable imprint on the spectrum of galaxies or quasars.
A tennis ball in a field populated by many rackets could be scattered by one of them back towards us. In analogy, the scattering of Lyman-alpha photons continues until they shift out of resonance with the atoms as a result of the cosmic expansion. At that point, some of these Lyman-alpha photons are free to stream towards us.
In 1999, I entered the office next-door to mine and asked George Rybicki whether scattering of Lyman-alpha photons could be observed as Lyman-alpha halos around the first galaxies? At that time, I was making predictions about on the first galaxies, and it occurred to me that detecting such halos would offer a new way of imaging the diffuse hydrogen left over from the Big Bang.
My question resulted in a detailed paper that I wrote with George over the subsequent months, titled “Scattered Lyman-alpha Radiation Around Sources Before Cosmological Reionization.” In a follow-up paper, we calculated that the light in these halos should be highly polarized. Just like the halo of scattered light around a lamppost embedded in dense fog, the emission by early galaxies produces distinct Lyman-alpha halos in the diffuse intergalactic medium. By now, these are called `Loeb-Rybicki halos.’
This was 25 years ago. Now, inspired by new Webb telescope data, I collaborated with the brilliant young astrophysicist Hamsa Padmanabhan to revive the utility of Loeb-Rybicki halos as cosmological probes.
The Webb telescopes discovered a new population of bright galaxies in the first billion years after the Big Bang. Hamsa and I used the observed ultraviolet luminosities of these galaxies to predict the observability of Loeb-Rybicki halos before reionization. Our new paper forecasts the sensitivity and resolution required to detect these intergalactic halos, finding that individual halos are detectable by the Webb telescope. The observational discovery of these halos will shed light on the neutral hydrogen that filled the Universe during its first billion years.
Hamsa and I are currently finishing a follow-up paper in which we show that the clustering of Loeb-Rybicki halos can also be detected statistically over large areas of the sky.
The moral of this 25-year-old insight is that writing a useful paper ahead of its time is insufficient because most scientists forget about its existence by the time it becomes relevant. One has to write follow-up papers when the observable consequences are imminent, for the community to recognize the benefits of using the insight.
A notable example that echoes this lesson is the 1952 paper by Otto Struve, where he proposed to use periodic eclipses (transits) or radial velocity shifts as methods for detecting planets outside the Solar System. More than four decades later, Michel Mayor and Didier Queloz discovered the first exoplanet to orbit a star similar to the Sun, 51 Pegasi b, using the radial velocity method proposed by Struve. Their discovery paper, for which they were awarded the Physics Nobel Prize in 2019, did not include a reference to Struve’s pioneering insights.
In the long arc of history, academic credit is not so important. Scientists should get their true reward in anonymity, just from the joy of promoting humanity’s awareness about its cosmic roots.
All of us humans arrived 13.8 billion years late to the cosmic party. We are also not at the center of action. Given the huge number of exoplanets discovered by now based on the methods suggested by Struve, there are likely many other participants like us in this cosmic party. We better seek these participants in order to gain better insights on what happened before us. Extraterrestrials might have viewed the cosmic action for a longer time than we did. In the grand scheme of the cosmos, their scientific insights might be far more memorable than the forgotten insights of earthlings like Struve, Loeb or Rybicki.
ABOUT THE AUTHOR
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.
