When the James Webb Space Telescope (JWST) commenced operations, it provided the first views of the period known as Cosmic Dawn. This cosmological epoch, which took place between 50 million and one billion years after the Big Bang, was when the first stars and galaxies in the Universe formed. What Webb’s observations revealed of this period surprised and intrigued scientists. In addition to spotting numerous “Little Red Dots” (LRDs), particularly bright and red galaxies, it also observed the “seeds” of Supermassive Black Holes (SMBHs).
These findings were in tension with current models of cosmology, as the BRDs and black holes appeared too large to have formed less than 1 billion years after the Big Bang. This prompted scientists to develop new and interesting theories regarding stellar and black hole formation during the Cosmic Dawn period. In a new theoretical study, University of Virginia astrophysicist Jonathan Tan proposes a comprehensive framework for the birth of SMBHs that suggests that they formed as the remnants of the earliest stars in the Universe.
Tan is a research professor with the College and Graduate School of Arts & Sciences’ Department of Astronomy and a professor in the Department of Space, Earth and Environment at Chalmers University of Technology in Gothenburg, Sweden. The paper that describes his “Pop III.1” theory was recently published in Astrophysical Journal Letters. In addition to offering an explanation for the formation of SMBH seeds, his proposed theory could revolutionize our understanding of the cosmological period known as the “Epoch of Reionization.”
According to the most widely accepted theory of cosmology, known as the Lambda Cold Dark Matter (LCDM) model, the Universe was enshrouded in darkness shortly after the Big Bang. This period is known as the “Cosmic Dark Ages,” where the Universe was permeated by neutral hydrogen. The only sources of light were photons from the “relic radiation” left over by the Big Bang – the Cosmic Microwave Background (CMB) – and those released by the reionization of neutral hydrogen. This latter source of photons was created by all the ultraviolet radiation released by the first stars in the Universe, Population III, which were particularly massive, bright, and short-lived.
Schematic representation of the view into cosmic history provided by the bright light of distant quasars.Credit: Carnegie Institution for Science/MPIA
Hence why astronomers also refer to this cosmological epoch, which took place from roughly 380,000 to 1 billion years after the Big Bang, as the “Epoch of Reionization.” In addition, scientists have known for decades that Supermassive Black Holes reside at the center of massive galaxies and play a vital role in their evolution. For a long time, it was theorized that SMBHs existed during the early Universe and were part of the first galaxies, which was confirmed by Webb’s observations. However, the black hole seeds were larger than the predominant cosmological model allowed for. This discovery injected new life into the debate surrounding how these gravitational behemoths formed.
On the one side, some scientists argued that SMBHs formed from smaller black holes, or the stellar remnants of Population III stars that underwent gravitational collapse at the end of their lives. On the other side, some astronomers theorized that SMBHs formed from giant clouds of dust that collapsed without first becoming stars – aka. Direct Collapse Black Holes (DCBHs). Tan’s proposed Pop. III.1 model endorses the direct collapse school of thought. As he explained in a University of Virginia press release, this theory could have implications for several aspects of cosmology:
Our model requires that the supermassive star progenitors of the black holes rapidly ionized the hydrogen gas in the Universe, announcing their birth with bright flashes that filled all of space. Intriguingly, this extra phase of ionization, occurring much earlier than that powered by normal galaxies, may help resolve some recent conundrums and tensions that have arisen in cosmology, including the “Hubble Tension”, “Dynamic Dark Energy,” and preference for “Negative Neutrino Masses,” all of which challenge the standard model of the Universe. It’s a connection we didn’t anticipate when developing the Pop III.1 model, but it may prove profoundly important.
Evidence supporting the existence of DCBHs has existed for some time, as indicated by strange quasars that existed as early as 500 million years after the Big Bang. But thanks to Webb, astronomers are seeing the first direct evidence of these early black holes. Said Richard Ellis, one of the world’s leading observational cosmologists and a professor of astrophysics at University College London:
Professor Tan has developed an elegant model that could explain a two-stage process of stellar birth and ionization in the early Universe. It’s possible the very first stars formed in a brief, brilliant flash, then vanished — meaning what we now see with the James Webb Telescope may be just the second wave. The Universe, it seems, still holds surprises.
Further Reading: University of Virginia