Scientists May Have Solved a Baffling Mystery at the Dawn of the Universe

Scientists have presented a compelling explanation for a mysterious population of radiant galaxies that existed in an era known as “cosmic dawn,” which elapsed between 100 million years to one billion years after the Big Bang, reports a new study. 

These luminous galaxies have baffled astronomers since they were first spotted over the past year by the James Webb Space Telescope (JWST); nobody knows how such luminous behemoths developed at this early period in the universe, contrary to all expectations. Some researchers have even suggested that the galaxies could expose exotic new physics beyond the standard model of cosmology, also known as ΛCDM, which is a well-tested framework that describes many phenomena in the universe.

Now, scientists led by Guochao Sun, a CIERA postdoctoral fellow at Northwestern University, have discovered new evidence that a process called “bursty star formation” could naturally explain the existence of the galaxies without invoking any new physics. Bursty star formation occurs when galaxies experience a baby boom of stars that is unusually intense, making these objects extremely bright even at a great distance.

The brightness of a galaxy normally correlates to its mass, but bursty star formation might make these ancient galaxies appear much more massive than they actually are, a finding that could solve this perplexing mystery. Sun and his colleagues used sophisticated simulations to demonstrate that “bursty star formation naturally explained the abundance of bright galaxies at cosmic dawn,” according to a study published on Tuesday in The Astrophysical Journal Letters.

“Now that we have actual data, we can go back to simulations produced before the launch of JWST to draw an apples-to-apples comparison and see how they agree,” Sun told Motherboard in a call. “It turns out that our simulations agree with the observations pretty well, and we can show that burstiness actually plays an important role, because if we remove the burstiness from our simulations, we won't be able to match the observations.”

Scientists have seen traces of these bright galaxies at cosmic dawn for many years, but JWST, which is the most powerful telescope ever launched to space, threw them into sharper focus and revealed that they are common at cosmic dawn. The telescope’s observations have sparked a huge debate among astronomers and cosmologists about the unknown mechanisms that could fuel the formation of such apparently massive objects over a short timescale in which they really shouldn’t exist.

Sun and his colleagues are not the first to suggest that bursty star formation could be a critical piece of this puzzle, and the team noted that many other factors might play a role in the brightness of the galaxies. However, the new study’s state-of-the-art simulations, which were developed by a project called Feedback of Relativistic Environments (FIRE), show that episodes of bursty star formation can dramatically brighten galaxies in ways that fit well with JWST’s discoveries.

“The role played by bursty star formation is something we're not so surprised about because our simulations predict bursty star formation in these early epochs,” Sun explained. “What we didn’t entirely expand is the absolute extent to which the predictions and observations agree.” 

“We know it's important but we don't know if that's the full story,” he added. “There are still many moving parts and things that need to be ironed out, and those will be natural next steps for us to to look into.”

To that end, Sun and his colleagues plan to continue running their FIRE simulations to pinpoint how much bursty star formation contributes to the brightness of these early galaxies, and to investigate other mechanisms that might also be in the mix. 

Bursty star formation has been observed in many galaxies across the universe’s lifespan, but the underlying dynamics of the brilliant episodes is still poorly understood, which raises other open questions that the team hopes to pursue. For now, though, Sun said that the results suggest that the bright galaxies do not necessarily pose a challenge to the standard model, or ΛCDM.

“It is too early to say anything conclusive about ΛCDM, especially if we want to break it,” Sun said. “The astrophysics of those very distant galaxies are very much like the tip of the iceberg that we see. But in order to say something about the big picture, we need to be super careful or cautious about any potential pitfalls or unaccounted physics that might bias our assessment and analysis.”