We Finally Know What Turned On the Lights at the Beginning of Time: ScienceAlert

We finally know what brought light to the dark and formless void of the early universe.

According to data from the Hubble and James Webb Space Telescopes, the origins of the free-flying photons in the early cosmic dawn were small dwarf galaxies that sprang to life and cleared the haze of cloudy hydrogen that filled intergalactic space. A new paper on the research was published in February.

“This discovery reveals the crucial role that ultrafaint galaxies played in the evolution of the early Universe,” said astrophysicist Iryna Chemerynska of the Institut d’Astrophysique de Paris.

“They produce ionizing photons that convert neutral hydrogen into ionized plasma during cosmic reionization. It 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; photons would have simply scattered off the free electrons floating around, effectively making the universe 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 put out radiation strong enough to strip electrons from their cores and reionize the gas. By that point, however, the universe had expanded so much that the gas was diffuse and couldn’t block light from shining out. About 1 billion years after the Big Bang, marking the end of a period known as the Cosmic Dawn, the universe was fully 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 in time and space, we have had trouble seeing what is there. Scientists thought that the sources responsible for most of the brightening must have been powerful – massive black holes whose accretion produces bright light, for example, and large galaxies in the throes of star formation (baby stars produce lots of ultraviolet light).

JWST was designed in part to peer into the cosmic dawn and try to see what lurks there. It has been very successful, 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.

A JWST deep-field image showing some of the sources that the researchers identified as drivers of reionization. (Hakim Atek/Sorbonne University/JWST)

An international team led by astrophysicist Hakim Atek of the Institut d’Astrophysique de Paris focused on JWST data of a galaxy cluster called Abell 2744, supported by data from Hubble. Abell 2744 is so dense that the space-time around it warps, forming a cosmic lens; any distant light traveling through that space-time toward us is magnified. This allowed the researchers to see small dwarf galaxies close to the cosmic dawn.

They then used JWST to obtain detailed spectra of these small galaxies. Their analysis revealed that these dwarf galaxies are not only the most common 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 are 100 times more abundant than large galaxies, and their combined output is four times the ionizing radiation typically assumed for larger galaxies.

“These cosmic powerhouses collectively emit more than enough energy to get the job done,” Atek said. “Despite their small size, these low-mass galaxies are prolific producers of energetic radiation, and their abundance during this period is so substantial that their combined influence could transform the entire state of the universe.”

It’s the best evidence yet for the force behind reionization, but more work needs to be done. The researchers looked at a small patch of sky; they need to make sure their sample isn’t just an anomalous cluster of dwarf galaxies, but a representative sample of the entire population in the cosmic dawn.

They plan to study more cosmic lens regions of the sky to get a broader sample of early galactic populations. But just from this one sample, the results are incredibly exciting. Scientists have been searching for answers to reionization for as long as we know. We are about to finally blow the fog away.

“We are now entering uncharted territory with the JWST,” said astrophysicist Themiya Nanayakkara of Swinburne University of Technology in Australia.

“This work raises even more interesting questions that we need to answer in our efforts to map the evolutionary history of our beginnings.”

The research was published in Nature.

A version of this article was originally published in March 2024.

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