In the middle of the path that separates the small constellations of Delphinus the Dolphin and the hind hoof of Pegasus the flying horse, an immaculate pinwheel moves through space.
For billions of years, the woolly spiral arms of the galaxy UCG 11700 have spun peacefully undisturbed by collisions and mergers that have warped other galaxies.
However, as the UCG 11700 rotates harmoniously in space, something monstrous lurks at its center.
At the heart of this beautiful cosmic wheel is one of the most mysterious objects in the universe: a supermassive black hole.
While the mass of standard black holes is about four times that of our Sun, their huge relatives are millions and sometimes billions of times more massive.
Scientists believe that almost all large galaxies have a supermassive black hole at their heart, even though no one knows how they got there.
This is where the galaxy UCG 11700 could come in handy.
“The ideal galaxies for my study are the most beautiful and perfect spirals you can imagine,” says Oxford University junior researcher Becky Smethurst, who studies supermassive black holes.
“The most beautiful galaxies are the ones that could help us solve the mystery of how these black holes grow,” she adds.
Studying something that by its nature is so dense that not even light can escape from its center makes learning difficult.
But new techniques that look for the effects that supermassive black holes have on the interstellar objects that surround them – and even the ripples they create in the fabric of space and time? they are giving new clues.
How a black hole appears
There’s a little secret to how conventional, if you can call it that, the way a black hole appears and grows.
A dying star runs out of fuel, explodes in a supernova, collapses in on itself, and becomes so dense that not even light can escape its intense gravity.
The idea of black holes has been around for a century and was predicted by Albert Einstein’s Theory of General Relativity.
In popular culture, black holes are perfectly dark and endlessly hungry.
They traverse the universe absorbing everything in their path, growing larger and more voracious as they do so.
Mystery solved, one might think: supermassive black holes are simply the hungriest and oldest of their kind.
However, black holes do not live up to their monstrous reputation.
They are surprisingly ineffective at accreting (the scientific term for “absorbing”) surrounding material, even in a dense galactic nucleus.
In fact, collapsed stars grow so slowly that they couldn’t become supermassive simply by absorbing new material.
“Suppose the first stars formed black holes around 200 million years after the Big Bang,” says Smethurst.
“After they collapse, you have about 13.5 billion years to grow your black hole to billions of times the mass of the Sun. It’s too short a time to make it that big just by absorbing material,” he adds.
Even more puzzling is learning that supermassive black holes already existed when the universe was still in its relative infancy.
Distant quasars, some of the brightest objects in the night sky, are actually ancient supermassive black holes that have set the cores of dying galaxies ablaze.
Some of these giants have been around at least since the universe was just 670 million years old, at a time when some of the oldest known galaxies were forming.
The reality about these energetic engines
While the heart of a black hole remains unknown to outside observers, supermassive black holes can glow brighter than an entire galaxy of stars, and can even “burp” ultraviolet radiation as they consume material around them.
Black holes have a spherical boundary known as the “event horizon.” Within this sphere, light, energy and matter are inescapably trapped.
Space and time fold in on themselves and the physical laws that describe how most of our universe works are broken.
But, just outside the event horizon, a spinning black hole can turn nearby material into a superheated spinning disk.
Reaching temperatures above 10 million degrees Celsius, the accretion disk in a quasar releases blindingly bright radiation across the entire electromagnetic spectrum.
“Black holes are the most effective and efficient engines in the universe,” says Marta Volonteri, a black hole researcher at the Institut d’Astrophysique de Paris.
“They transform mass into energy with an efficiency of up to 40%. If you think about anything that we burn with carbon or chemical energy or even what happens in stars, it is only a small fraction of what produces a black hole “.
Supermassive black holes interest scientists for more than just their energy efficiency. Their formation and evolution are clearly connected with the development of galaxies and with the even greater theme of the history and structure of our entire universe.
Solving the mystery of these cosmic giants would represent a significant step in scientists’ ongoing effort to understand why things are the way they are.
Gravitational waves and their role in hole size
The release of energy is one of the many ways that black holes divulge their secrets.
When black holes merge or collide with slightly less dense objects like neutron stars, the events create ripples in space-time called gravitational waves.
These waves move through the cosmos at the speed of light and were first detected on Earth in 2015.
Since then, large centers such as the Laser Interferometric Gravitational Wave Observatory (LIGO) in the United States and the Virgo facility near Pisa, Italy, have been collecting the waves created by these collisions.
But although these observatories use instruments that measure several kilometers in length, they can only detect waves from relatively modestly sized black holes.
“LIGO has detected mergers of only about 150 solar masses,” says Nadine Neumayer, who heads the Galactic Nuclei research group at the Max Planck Institute for Astronomy.
“There is a gap in the data about what people call ‘intermediate mass black holes’ of about 10,000 solar masses or so. And those could actually be the seeds of supermassive black holes.”
The expert points out that intermediate mass black holes could have formed in the universe very early from the collapse of giant gas clouds or uncontrolled collisions of stars.
In the tight environment of the young universe, successive collisions between these medium-sized black holes, combined with a rapid accumulation of surrounding material, could have accelerated their growth to supermassive scales.
Still, the intermediate-mass black hole seed theory has problems. The little fledgling universe was also very hot.
The gas clouds would have been bathed in radiation, possibly giving them too much energy to collapse in on themselves.
Even in a dense cosmos, the laws of physics also limit the maximum speed at which black holes can absorb matter.
Volonteri says that any current explanation for supermassive black holes has “bottlenecks and drawbacks” that prevent scientists from converging on a definitive answer.
“Theories that involve what we call ‘dynamic processes,’ meaning that a black hole forms from many, many stars instead of just one, are possible, but these processes must happen under very specific conditions,” he says he.
“There are also theories about ‘primordial black holes’, which could have existed and started growing before there were stars. But this is completely unknown territory. We don’t have any observational evidence to prove this principle,” he adds.
Volonteri says he loves the physics of dynamic processes, but acknowledges that it is very difficult for theory to credibly predict something that grows more than about 1,000 solar masses.
“When you consider quasars that were already 1 billion solar masses when the universe was 1 billion years old, it’s very difficult to get to those numbers,” he says.
This expert believes that the true story of how supermassive black holes arose has yet to be told.
“The more we investigate, the more we discover that there are problems that we think we understood. We are missing something fundamental,” she says.
The current generation of observing instruments has begun to fill in the gaps. Virgo, LIGO and similar observatories are providing ever deeper “demographic information” about the size, age and location of the Universe’s black hole population.
But to complete this kind of data on supermassive black holes, researchers will need even larger detectors.
Efforts to measure holes and “adaptive optics”
In the 2030s, NASA and the European Space Agency will launch the ambitious Laser Interferometer Space Antenna (LISA), comprising three satellites flying in a triangle with sides 2.5 million kilometers apart. long.
This array will work on principles similar to those of Ligo and Virgo, but its massive scale will allow it to detect gravitational waves from very large black holes beyond the reach of existing technology.
But there are other, more direct ways of looking at black holes.
The Event Horizons telescope recently captured the first photographic images of black holes, pulling these mysterious objects out of the shadows and revealing more about their nature and the effects of their gravity and magnetism on the galaxies they inhabit.
Astrophysicists can also track the motion of stars in close orbits around black holes in the galactic core, extrapolating information about massive objects at their center.
Most observations of this type are based on ground-based telescopes that use a technology called “adaptive optics.”
Observers analyze a bright star (or a human-generated laser beam) to measure atmospheric distortions that would otherwise reduce image quality.
Computer-controlled signals correct for these distortions by making small adjustments to the physical shape of the telescope’s mirror.
The result is accurate observations of the hearts of galaxies billions of light years away and a wealth of data on their supermassive black holes.
Neumayer was one of the first scientists to use adaptive optics to study galactic nuclei.
“It was just amazing that you could have better resolution from Earth than from the Hubble Space Telescope,” he says.
“I worked on measuring specific masses of black holes. There is a close correlation: the more mass a galaxy has, the more massive its central supermassive black hole,” she says.
Despite this correlation, there is no clear evidence that massive galaxies create massive black holes, or vice versa. They are connected, but the nature of that connection remains a mystery.
Part of the explanation could involve collisions between galaxies.
Most of the two trillion observable galaxies in the universe are moving away from each other, but many collisions occur, creating opportunities for two very large central black holes to merge into something even larger.
Some scientists believe this could be the way truly monstrous supermassive black holes form.
When comparatively small stellar black holes collide, they release enormous amounts of energy for a fraction of a second, producing a flash so bright that it briefly outshines everything else in the sky.
If we were to see a similar event involving supermassive black holes, it would be one of the most cataclysmic events that could be detected in the night sky.
However, while scientists suspect that supermassive black hole mergers do occur, they may be less common due to another troublesome aspect of the dynamics of
Black holes going on a collision path spin around each other faster as they get closer. But very large black holes reach a point about a parsec (3.26 light years) away where their orbital velocity begins to even out the gravitational pull.
The degradation of their orbits would occur so slowly that the current merger would not happen within the current age of the universe.
However, physicists do believe that these mergers do occur, requiring new theories of how to overcome the so-called “final parsec problem.”
Some kind of additional force is needed to bring the orbiting black holes back together.
The universe is teeming with galaxies believed to have been formed by mergers, including our own Milky Way, suggesting that they do occur.
When galaxies collide, their original spiral structure is destroyed as stars, gas clouds, dark matter, and black holes interact. Even a friction between galaxies can destabilize their structures, making them easy to detect.
The loneliness of some galaxies
But that means supermassive black holes at the center of spotless pinwheel galaxies like UCG11700 cannot be explained in terms of collisions. Their structures suggest that they have never approached another galaxy.
“I select very rare galaxies that have been alone their entire existence, that have been very, very isolated in the universe,” says Becky Smethurst. “With those we are sure that the black hole in the center has never grown by merging with something else.”
That means they must have been formed differently.
Smethurst works retroactively to determine how large these black holes would have to be at first to reach their current size.
The best models of it indicate that a black hole that formed early in the universe of between 1,000 and 10,000 solar masses might be sufficient – figures that square with Neymayer’s theories of intermediate-size “seed” black holes.
But those black holes probably don’t come from collapsed stars.
Astrophysicists are also exploring the possibility that supermassive black holes form directly from dark matter, the mysterious material that holds galaxies together.
But dark matter, which is a theoretical type of particle that interacts with gravity but is invisible to light and electromagnetism, is itself very poorly understood.
The combination of the mysteries of black holes and dark matter only makes physics more complex.
“There’s still a lot we don’t know about,” says Smethurst.
“I think it would be arrogant of us to conclude that the only way to form a black hole is through a supernova, because we don’t know that for sure.
Perhaps the explanation is something completely unthinkable until now. I look forward to the day the universe surprises us with the answer. I think it will be a great day for science. “
NASA and its James Webb Space Telescope
More advanced observation instruments are on the way.
This year, NASA plans to launch the James Webb Space Telescope (although there is currently a campaign to change the name of the instrument due to homophobic policies imposed by the eponymous director of NASA), whose unprecedented size and sensory capacity will make it a Valuable tool in supermassive black hole research.
The Lisa mission, when launched, will also equip scientists with new ways to observe supermassive black holes through their gravitational waves.
Other scientists are creating increasingly detailed maps of the places, motions, shapes, and sizes of millions of galaxies, fueling research for both observers and theoreticians.
“The pace of work is just phenomenal,” says Smethurst.
“We have the equivalent of 100 years of research on black holes. But compared to the 14,000 million years that the universe has been, that is not enough to solve all the mysteries. I intend to answer one question and I end up with five more. And that’s fine. by my side”.
Neumayer agrees with Smethurst that the most fascinating discoveries about black holes will probably have to do with questions no one has asked.