2024 A Mystery about the Universe’s First Black Holes May Be Solved at Last-- USA

As astronomers delve into the earliest chapters of the universe's history, they have uncovered a multitude of gigantic black holes that matured far quicker than previously thought possible. Priyamvada Natarajan, who has often been likened to a cosmic biologist, studies these early and unusually massive black holes. During her time as an astronomy graduate student, Natarajan was among the pioneers in treating black holes as populations rather than isolated objects, studying their general taxonomy and evolution akin to observing species in a rainforest. Currently, an astrophysicist at Yale University, she continues to explore their behavior, focusing on understanding their origins. Traditionally, black holes are formed following the explosive death of massive stars, gradually increasing in mass by consuming surrounding gas. However, a series of observations of supermassive black holes in the very early universe suggests a more complex picture.

In 2006, Natarajan and her colleagues proposed a groundbreaking theory: disks of gas could collapse directly into massive black holes without forming stars first. Last year, observations from the James Webb Space Telescope (JWST) and the Chandra X-ray Observatory revealed a distant, radiant black hole that appears to confirm Natarajan’s hypothesis.

“It’s definitely a very strong case in favor of these heavy black hole seeds,” says Raffaella Schneider, an astrophysicist at Sapienza University of Rome. “[Natarajan] having proposed this idea really helped the community to enlarge our view on the different possibilities that can occur.”

Natarajan shared her insights with Scientific American, discussing how the recent observations align with her proposal for "direct-collapse black holes" and what these findings reveal about the ancestry of these enigmatic entities.

What sparked your interest in studying black holes and their origins?

I've always been fascinated by the universe's invisible entities. My primary focus has been on understanding dark matter, dark energy, and black holes on a fundamental level. These objects are incredibly seductive and enigmatic, serving as a reminder of the limits of our knowledge and the points where the known laws of physics break down. Over recent decades, black holes have transitioned from theoretical constructs to observable phenomena, taking center stage in our comprehension of galaxy formation. With black holes of various sizes scattered throughout the cosmos, understanding their origins is a fundamental question.

What do we still need to understand about black hole formation? Typically, black holes are born from the gravitational collapse of massive stars, leaving behind a dense core. However, about two decades ago, as we looked further back into the universe with missions like the Sloan Digital Sky Survey, we discovered several extremely massive black holes—up to a billion times the mass of the sun—when the universe was just one to two billion years old. Given the known feeding rate of black holes, there simply wasn't enough time for these tiny seeds from the first stars to grow into such colossal entities. This conundrum suggested that there might be an entire population of supermassive black holes in the early universe. Some researchers explored whether black holes could feed faster than previously thought possible, but we lacked convincing observational evidence.

My team proposed starting with larger seeds. We theorized that if a gas disk, irradiated by nearby stars, could bypass the star-formation stage and collapse directly into a black hole, it would result in a much larger initial mass—ranging from 1,000 to 100,000 times the mass of the sun. This direct-collapse black hole could then merge with a nearby galaxy and grow rapidly.

How did the scientific community react to this proposal? There was significant skepticism. Critics argued that while the physics made sense, the process might not be efficient enough to occur in the universe. At that time, these early epochs of the universe were beyond our observational reach. We needed to observe the first billion years after the universe's formation to see these initial seeds. The potential of JWST motivated us to continue our work. We hypothesized that direct-collapse black holes would exhibit distinct signatures: for a brief period, the black hole's mass could be comparable to the mass of the stars in its galaxy, resulting in an extremely bright, actively feeding black hole outshining its galaxy's stars. This signature would be detectable in x-ray and infrared light.

Even with JWST and Chandra, witnessing early black hole seeds directly is challenging. However, I speculated that one of these galaxies might be magnified by a galaxy cluster rich in dark matter, acting as a gravitational lens. Focusing on such a cluster, Abell 2744, paid off spectacularly.

How did this discovery come about? Last year, my colleague Akos Bogdan informed me of a Chandra observation of galaxies behind the Abell 2744 lens. To our amazement, one galaxy's spectrum matched our 2017 predictions for a hypothetical detection, confirming the existence of direct-collapse black holes. This evidence is compelling, but there might be other pathways to form black hole seeds. My next goal is to uncover these alternative mechanisms and their unique observational signatures.

How does it feel to see your theory validated? This is what makes astrophysics so thrilling—seeing theoretical ideas confronted with observational data. We're in a golden age of cosmology where predictions can be validated or invalidated within a scientist's lifetime. I am profoundly grateful for this journey.

These insights are crucial in understanding when the first black holes formed, marking a significant advancement in our comprehension of the universe’s formative years and the first black holes that emerged.