We finally know what brought light into the dark, formless void of the early universe.
According to data from the Hubble and James Webb Space Telescopes, the origins of free-flying photons in the early universe were small dwarf galaxies that sparked life, clearing away the cloudy hydrogen haze that filled intergalactic space. New paper The research was published in February.
“This discovery reveals the crucial role that ultra-faint galaxies played in the evolution of the early universe.” Astrophysicist Irena Chemerinska said, From the Institute of Astrophysics in Paris.
“They produce ionizing photons that convert neutral hydrogen into ionized plasma during cosmic reionization. This highlights the importance of understanding low-mass galaxies in shaping the history of the universe.”
In the early universe, minutes after the Big Bang, space was filled with a dense fog of ionized plasma. What little light there was couldn’t penetrate this fog; photons would simply scatter off the free electrons floating around, making the universe effectively dark.
As the universe cooled, after about 300,000 years, protons and electrons began to combine to form neutral hydrogen gas (and a little helium). Most wavelengths of light could penetrate this neutral medium, but there were few sources of light to produce it. But from this hydrogen and helium, the first stars were born.
Those first stars emitted radiation that was powerful enough to strip electrons from their cores and reionize the gas. But by that point, the universe had expanded so much that the gas was so diffuse that it could no longer block the light from shining. About a billion years after the Big Bang, the end of the period known as the cosmic dawn, the universe was completely reionized. Tada! The lights went on.
But because there is so much haze in the cosmic dawn, and because it is so faint and distant in time and space, we have had a hard time seeing what is there. Scientists thought that the sources responsible for most of this haze must be powerful—massive black holes whose accretion produces bright light, for example, and massive galaxies in the throes of star formation (newborn stars produce a lot of ultraviolet light).
The James Webb Telescope was designed in part to peer into the early universe and try to figure out what was hiding there. And it has been a huge success, revealing all sorts of surprises about this crucial time in the formation of our universe. Surprisingly, the telescope’s observations now suggest that dwarf galaxies are the main players in reionization.
An international team led by astrophysicist Hakim Atiq of the Institut de l’Astrophysics de Paris used data from the James Webb Space Telescope on a galaxy cluster called Abell 2744, supplemented by data from Hubble. Abell 2744 is so dense that spacetime bends around it, acting as a cosmic lens; any distant light traveling to us through this spacetime is magnified. This allowed the researchers to see small dwarf galaxies near the cosmic dawn.
They then used the James Webb Space Telescope to obtain detailed spectra of these small galaxies. Their analysis revealed that these dwarf galaxies are not only the most abundant type of galaxy in the early universe, but they are also much brighter than expected. In fact, the team’s research shows that dwarf galaxies outnumber larger galaxies by a factor of 100 to 1, and that their total output is four times the ionizing radiation typically assumed for larger galaxies.
“These cosmic forces combined provide more than enough energy to get the job done.” Atik said“Despite their small size, these low-mass galaxies produce enormous amounts of energetic radiation, and their abundance during this period is so great that their collective effect could transform the state of the entire universe.”
It’s the best evidence yet for the power behind reionization, but there’s more work to be done. The researchers looked at a small patch of sky; they need to be sure that the sample they choose isn’t just a bizarre collection of dwarf galaxies, but a representative sample of the entire population at the dawn of the universe.
Scientists plan to study more lensed regions of the sky to get a broader sample of early galaxy clusters. But the results are most exciting for this sample alone. Scientists have been chasing answers to reionization ever since we first learned about it. And we’re finally about to lift the fog.
“We are now entering uncharted territory with JWST.” Astrophysicist Thimya Nanayakkara said, From Swinburne University of Technology, Australia.
“This work opens up more exciting questions that we need to answer in our efforts to chart the evolutionary history of our beginnings.”
The research was published in nature.
The original version of this article was published in March 2024.
