The NASA Nancy Grace Roman Space Telescope will investigate the transition of the universe to its current star-filled landscape, a period known as cosmic dawn. Currently, vast regions of space are clear, but this was not always the case. In its infancy, the universe was obscured by a 'fog' that hid the first stars and galaxies.
Michelle Thaller, an astrophysicist at NASA's Goddard Space Flight Center, stated, "Something fundamental about the nature of the Universe changed during this time. Thanks to Roman's sharp infrared imaging, we will finally understand what happened during a critical cosmic turning point."
Shortly after its birth, the cosmos was a boiling sea of particles and radiation. As it expanded and cooled, positively charged protons captured negatively charged electrons, forming neutral atoms, primarily hydrogen and some helium. It likely took a long time for the gaseous hydrogen and helium to coalesce into stars, which then clustered to form the first galaxies. However, even as stars began to shine, their light could not travel far before being absorbed by neutral atoms. This period, known as the cosmic dark age, lasted from about 380,000 to 200 million years after the Big Bang.
The 'fog' gradually dissipated as more neutral atoms broke apart over the following hundreds of millions of years, marking the cosmic dawn.
Aaron Yung, a Giacconi fellow at the Space Telescope Science Institute, expressed curiosity about the process, stating, "Roman's wide and sharp view of deep space will help us weigh different explanations."
The NASA team believes that primitive galaxies may largely be responsible for the energetic light that broke apart neutral atoms. Early black holes might have played a role as well. Roman will search extensively to examine potential 'culprits.'
Takahiro Morishita, an assistant scientist at Caltech/IPAC, noted, "Roman will be an excellent tool for finding the building blocks of cosmic structures, such as galaxy clusters that form later. It will quickly identify the densest regions where the most 'fog' is cleared, making Roman a key mission for investigating the early evolution of galaxies and cosmic dawn."
The first stars were likely very different from modern ones. As gravity began to attract matter, the universe was extremely dense. These stars may have been hundreds or thousands of times more massive than the Sun, emitting vast amounts of high-energy radiation. Gravity clustered young stars to form galaxies, and their accumulated explosions may have stripped electrons from protons in the surrounding space bubbles.
"We could say it was the party of the universe's beginning," Thaller remarked, adding, "We have never seen the birth of the first stars and galaxies, but it must have been spectacular!"
However, these massive stars did not last long. Scientists believe they collapsed rapidly, leaving behind black holes, objects with such extreme gravity that not even light can escape. In the young universe, which was smaller due to limited expansion, hordes of these black holes may have merged to form larger ones, with masses up to millions or even billions of times that of the Sun.
Supermassive black holes may have helped clear the hydrogen 'fog' that permeated the early universe. The hot material spinning around black holes in the bright centers of active galaxies, called quasars, can generate extreme temperatures and emit enormous jets of intense radiation. These jets can extend hundreds of thousands of light-years, stripping electrons from any atoms in their path.
The NASA James Webb Space Telescope is also exploring cosmic dawn, using its narrower but deeper view to study the early universe. By combining Webb's observations with those of Roman, scientists will create a more comprehensive picture of this era. So far, Webb has discovered more quasars than expected, given their rarity and its limited field of view. Roman's expanded view will help astronomers understand how common quasars truly are, likely finding tens of thousands compared to the few Webb can detect.