We finally know what lit the lights on at the dawn of time: ScienceAlert

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 cosmic dawn were small dwarf galaxies that ignited with life, clearing away the mysterious hydrogen fog that filled intergalactic space.

“This discovery reveals the crucial role played by very faint galaxies in the evolution of the early universe.” says astrophysicist Irina Chemerinska 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.”

At the beginning of the universe, within minutes of the Big Bang, space was filled with a hot, dense fog of ionized plasma. What little light there was would not have penetrated this fog; Simply put, the photons would be scattered from the free electrons floating around, effectively making the universe dark.

As the universe cooled, after about 300,000 years, protons and electrons began to come together to form neutral hydrogen gas (and a little helium). Most wavelengths of light could penetrate this neutral medium, but there were very few light sources to produce it. But from hydrogen and helium the first stars were born.

Those first stars provided radiation that was strong enough to knock electrons away from their nuclei and reionize the gas. However, by this point, the universe had expanded so much that gas was spread out and could not prevent 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. Ta-da! The lights were on.

But because there is so much darkness in the cosmic dawn, and because it is so faint and distant across time and space, we have had difficulty seeing what is there. Scientists thought that the sources responsible for most of this emptiness must be powerful—massive black holes whose accretion produces glowing light, for example, and massive galaxies in the midst of star formation (small stars produce a lot of ultraviolet light).

The James Webb Space Telescope was designed in part to peer into the cosmic dawn and try to see what lies there. It has been very successful, revealing all sorts of surprises about this crucial time in the formation of our universe. Surprisingly, telescope observations now indicate that dwarf galaxies are the main players in reionization.

An international team led by astrophysicist Hakim Atiq of the Institute of Astrophysics in Paris turned to James Webb Space Telescope data about a galaxy cluster called Abell 2744, supplemented by data from Hubble. Abell 2744 is so dense that spacetime wraps around it, forming a cosmic lens; Any distant light traveling to us through space-time becomes magnified. This allowed researchers to see small dwarf galaxies close to 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, they are also much brighter than expected. In fact, the team's research shows that dwarf galaxies outnumber large galaxies 100 to one, and that their collective output is four times the ionizing radiation normally assumed for more massive galaxies.

“Combined, these cosmic forces emit enough energy to get the job done.” Atik says. “Despite their small size, these low-mass galaxies produce energetic radiation in abundance, and their abundance during this period is so great that their collective impact could change the entire state of the universe.”

It's the best evidence yet of the power behind reionization, but there is more work to be done. The researchers looked at a small patch of sky; They need to make sure that their sample is not just an anomalous collection of dwarf galaxies, but a representative sample of the entire population at cosmic dawn.

They intend to study more cosmic lens regions in the sky to get a broader sample of early galaxy clusters. But just in this sample, the results were incredibly dramatic. Scientists have been searching for answers about reionization for as long as we've known about it. We're on the verge of finally lifting the fog.

“We have now entered uncharted territory with the James Webb Space Telescope.” says astrophysicist Thimya Nanayakkara From Swinburne University of Technology in 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.