Astronomers have detected the oldest black hole ever observed, dating back more than 13bn years to the dawn of the universe.
The observations, by the James Webb space telescope (JWST), reveal it to be at the heart of a galaxy 440m years after the big bang. At around a million times the mass of the sun, it is surprisingly big for a baby black hole, raising the question of how it grew so big so quickly.
Prof Roberto Maiolino, an astrophysicist at the University of Cambridge, who led the observations, said: “The surprise is in it being so very massive. That was the most unexpected thing.”
The observations, published on the preprint website Arxiv, do not take a direct image, which is unseeable because no light can escape its grip. But astronomers detected telltale signatures of its accretion disk, the halo of gas and dust that swirls rapidly around the cosmic sinkhole.
Astronomers believe the earliest black holes could help unlock a puzzle of how their gargantuan counterparts at the centre of galaxies such as the Milky Way grew to billions the times the mass of the sun. Until recently, they were assumed to have simply snowballed over nearly 14bn years, steadily growing through mergers and by gobbling up stars and other objects. But this snowball scenario cannot fully account for the epic proportions of present-day supermassive black holes.
The latest observations, of the galaxy called GN-z11, push the origins of this mystery back to black holes’ infancy and suggest that they were either born big or ballooned extremely rapidly early on.
“Understanding where the black holes came from in the first place has always been a puzzle, but now that puzzle seems to be deepening,” said Prof Andrew Pontzen, a cosmologist at University College London, who was not involved in the research. “These results, using the power of JWST to peer back through time, suggest that some black holes instead grew at a tremendous rate in the young universe, far faster than we expected.”
One explanation, known as the heavy seeds scenario, is that an early generation of black holes was born from the direct collapse of vast clouds of gas, rather than from burnt-out stars that collapsed under their own gravity at the end of their life. Another possibility is that compact clusters of stars and black holes merged very rapidly in the early universe.
A third, more speculative, hypothesis is the existence of so-called primordial black holes that came into existence during cosmic inflation, the period of faster-than-light expansion of the universe that occurred a fraction of a second after the big bang.
This would flip on its head the presumed order of play, in which galaxies came first and then black holes start growing within them. Primordial black holes would be effectively woven into the fabric of the cosmos from the outset.
“If that were true, it would have deep implications for the opening fraction of a second of our universe,” said Pontzen. “Either way, the story of how black holes and galaxies grew up together is a riveting one that we are only just starting to piece together.”
The findings are the latest in a series of stunning discoveries by Nasa’s space observatory just two years after its launch. JWST is about 100 times more sensitive than previous telescopes, such as Hubble, at detecting infrared light, the part of the spectrum used to see the most distant objects. “It is essentially equivalent to upgrading Galileo’s telescope to modern telescope. It’s 400 years of discoveries potentially compressed in the time span of JWST operations,” said Maiolino.
He said that before the telescope’s launch there had been a possibility that a new window would open up on to “a boring extension of what we know”. “That’s not what we’re seeing,” said Maiolino. “The universe has been quite generous. We’re really finding things that we were not expecting.”
What is a black hole?
Black holes are among the universe’s weirdest and most ominous objects. They have such intense gravity that neither matter nor light can escape their grip. A black hole’s threshold is traced out by its event horizon, the point of no return. Anything that strays across this boundary is gone for good.
They are challenging to study because they are fundamentally unseeable, but applying the laws of physics offers some bizarre insights. On the approach to a black hole, the gravitational gradient can be so extreme that objects would be stretched out in a process known as spaghettification. At the event horizon, gravity is so fierce that light is bent in a perfect loop around the black hole, meaning that if you stood there you would be able to see the back of your own head.
What lies beyond the event horizon is unknown. Einstein’s theory of general relativity suggests that at the centre of a black hole density would become infinite, creating a gravitational singularity. This rupture in space-time would have no “where” or “when” and would sit beyond the realm of the conventional laws of physics. But it is not clear whether such singularities actually exist.
Black holes come in a range of sizes. Stellar black holes, formed from the remnants of massive stars, can be up to 20 times more massive than our sun. Supermassive black holes, like Sagittarius A* at the centre of the Milky Way, can have masses equivalent to millions or billions of suns and play a crucial role in galactic evolution.
Astronomers have made significant advances in observations of black holes in the past decade, with the first image of one’s halo captured by the Event Horizon telescope in 2019, and observations of cataclysmic black hole mergers through the detection of gravitational waves sent rippling across space-time. The latest observations, and even more distant James Webb targets, will start to piece together the origins of these enigmatic objects.
