Many stages in the initial development of the universe remain unexplained. One significant enigma involves the origins of the universe’s earliest light sources, which are observable to us. A new study based on observations by the James Webb Space Telescope indicates that these sources were young stars that were much brighter than scientists had previously estimated.
Why does the universe look the way it does? This is one of the open questions in astrophysics and physics in general. Over its first billion years, the universe evolved from a disordered soup of high-energy particles into a more organized collection of galaxies and stars, yet many details of this process elude us.
In a recent study, an international team of researchers analyzed observations from the James Webb Space Telescope, focusing on dwarf galaxies from the early universe and have found that these galaxies emitted light at intensities much higher than those anticipated. This research constitutes a breakthrough in our understanding of the universe's first light sources.
A brief history of heat in the universe
Immediately after the Big Bang, the universe underwent rapid expansion, reaching extremely high temperatures and energy levels: the average temperature of the particles in the universe was about 10^30 degrees Celsius.
At such immense heat, subatomic particles could not bond with each other, and it was therefore devoid of matter as we recognize it today. About one second later, the universe had cooled to about a billion degrees, and the subatomic particles - the building blocks of matter according to the standard model of particle physics - were created, bonded, and formed protons and neutrons.
By the time the universe was about twenty minutes old, it had already cooled to temperatures in the hundreds of thousands of degrees Celsius. Protons and neutrons then began bonding with hydrogen, helium, and lithium ions.
Due to the universe's high temperature, the particles retained very high energy, and thus electrons could not bond with protons to form stable atoms. Consequently, much of the universe existed in a state of plasma—a milieu of electrically charged particles swirling around each other. This plasma obstructed electromagnetic radiation, hindering the propagation of light freely throughout the universe.
Over the subsequent 370,000 years, the universe continued to cool until it reached a temperature of about 4,000 degrees. At this point, electrons were finally able to bond with plasma, leading to the formation of neutral atoms. The cosmic background radiation, which provides insights into the early universe, was released during this stage. This radiation managed to travel vast distances to us because it was unimpeded by the presence of plasma.
How light was born
At this stage, when the universe was roughly 400,000 years old, it was mostly composed of neutral hydrogen and helium atoms dispersed fairly evenly throughout space. There were no stars, galaxies, or any other complex celestial bodies familiar to us from today’s night sky. In particular, there were no sources of light and the universe was essentially cloaked in darkness.
It was not until about 20 million years later, as the universe continued to expand and cool considerably, that the earliest light sources in the universe were formed. Astrophysicists studying the history of the universe remain uncertain about the nature and origin of these light sources, when they were created, and how.
The prevailing theories regarding the earliest light sources in the universe suggest these could have been massive black holes, massive galaxies, or young stars. A comprehensive theory explaining the formation of stars and galaxies in the early universe has yet to be formulated and physicists are still trying to understand when and how the first light sources appeared in the universe.
The brilliant dwarfs: Shining four times brighter
Utilizing the James Webb Space Telescope, launched at the end of 2021, researchers embarked on observations of very distant galaxies. Since light travels at a finite speed, it takes a long time for light from distant galaxies to reach us. The researchers focused their observations on light emitted from galaxies roughly 13 billion years ago, now reaching the telescope, allowing them to glimpse into processes that occurred in the early universe.
These distant galaxies appear to us as they were billions of years ago, when they were young galaxies emitting less light compared to other cosmic bodies. Consequently, it is challenging to observe such distant galaxies using conventional methods.
To overcome this, the researchers used a sophisticated technique: based on Einstein's general theory of relativity, which suggested that heavy masses can curve space and bend the path of light passing near them.
The researchers focused on galaxies located behind the massive galaxy cluster Abell 2744, which, due to its great mass, acts as a gravitational lens, magnifying and focusing the light coming from behind it. This allowed the researchers to increase the amount of light reaching the telescope, facilitating more precise observations.
The researchers analyzed the light coming from dwarf galaxies—galaxies containing only about a billion stars. For comparison, the Milky Way galaxy in which we reside contains hundreds of billions of stars. The researchers carefully analyzed the observations and discovered that these dwarf galaxies emit radiation four times stronger than previously estimated.
Moreover, these dwarf galaxies were prevalent in the early universe, more so than larger galaxies. Consequently, the researchers speculate that the majority of the universe's early light sources were galaxies of this type.
This study is another example of a scientific breakthrough achieved with the aid of the James Webb Space Telescope. While the findings are noteworthy, the researchers emphasize the need for further studies, including observation of a broader sample of galaxies, to strengthen the validity of their conclusions.