The James Webb Space Telescope (JWST) has revolutionized our understanding of the early universe, pushing the boundaries of what we can observe and study. In just four years, it has enabled astronomers to peer back to a time when the universe's very first stars and galaxies were forming, an era known as cosmic dawn. This remarkable achievement is largely due to the efforts of observational cosmologists like Richard Ellis, a professor of astrophysics at University College London (UCL).
Ellis, who began his research of the distant universe as an undergraduate 58 years ago, has been instrumental in pushing the frontiers of our knowledge. In 1968, high redshift meant luminous quasars, which are now known as active galactic nuclei (AGN). Ellis recalls the limitations of telescopes and the use of photographic plates at the time. Despite these challenges, he and his colleagues have made significant strides in understanding the early universe.
The recent multi-spectral JWST survey has revealed a steep drop in galaxy formation at only 150 to 200 million years after the Big Bang. This is an incredibly short time frame, considering the universe is currently estimated to be 13.8 billion years old. The survey covers an area of sky only about three times that of a full moon, but it provides a unique window into the initial conditions that set the stage for the formation and evolution of chemical abundances, supermassive black holes (SMBHs), and large-scale structures we see today.
One of the most intriguing aspects of this research is the quest to understand how the first galaxies formed. These early galaxies are tiny, some thirty times smaller than our Milky Way galaxy, and only 60 to 70 light-years across. Despite their physical size, they are producing stars 20 times faster than the Milky Way, making them incredibly energetic and youthful. The holy grail of this research is to find short-lived Population III stars, which only have hydrogen and helium and do not contain heavy elements. These stars live for only 5 million years before exploding, polluting the gas with heavy elements and ending their chemically pristine state.
Ellis emphasizes the importance of studying these early times, as we are made of the material synthesized in stars, and the chemistry that led to our existence began at cosmic dawn. Without understanding the first galaxies and stars, we cannot fully grasp astrobiology. As these stars explode, they produce clouds of gas and dust that circle around the next generation of stars, potentially forming planets with conditions suitable for life. This intricate process ultimately led to the emergence of single-celled life forms, and eventually, to human existence.
The study of these early times is challenging, and there are several methods to pinpoint cosmic dawn. One approach is to look for chemically pristine galaxies that have not been polluted by supernova explosions. This requires unequivocally demonstrating the absence of oxygen emissions, which is a difficult task. Another method involves tracing the declining abundance of star-forming galaxies with increasing redshift and locating a steeper decline. Additionally, the declining chemical abundance, such as the ratio of oxygen to hydrogen, can be traced with increasing redshift, although this requires a large number of spectra.
Interestingly, there is a correlation between the Lyman alpha line of hydrogen and the 21cm radio ground-state line of hydrogen when hydrogen is at its most stable, lowest possible energy level. Ellis and his colleagues expect to see this 21cm line of hydrogen redshifted in spectral lines of absorption as it is observed against the distant Cosmic Microwave Background (CMB).
Despite the challenges and limitations, the study of these early times is crucial for our understanding of the universe and our place in it. The James Webb Space Telescope and the efforts of cosmologists like Ellis have opened a new window into the cosmos, allowing us to explore the mysteries of cosmic dawn and the very first stars and galaxies.
