Picture a black hole so massive it contains billions of times the mass of our Sun, surrounded by a swirling disk of matter so hot it outshines entire galaxies. For nearly 50 years, astronomers thought they understood how these cosmic monsters behave. They were wrong.

In 2025, an international team led by Maria Chira at the National Observatory of Athens discovered something unexpected: the relationship between ultraviolet and X-ray light from supermassive black holes has changed over the past 6.5 billion years. This finding challenges fundamental assumptions about how black holes grow and radiate energy across cosmic time.

The Brightest Objects in the Universe

Quasars—short for "quasi-stellar objects"—were first identified in the 1960s. They looked like stars through telescopes, but something was clearly off. These objects emitted far too much energy to be ordinary stars.

We now know that quasars are supermassive black holes actively feeding on surrounding matter. They rank among the most luminous objects in the universe. A single quasar can emit 100 to 1,000 times more light than an entire galaxy of 100 billion stars.

The source of this incredible brightness is the accretion disk. Matter spirals inward under immense gravity, forming a rotating disk before falling into the black hole. Friction within this disk heats matter to extreme temperatures, producing enormous amounts of ultraviolet light.

But there's another component to this system: the corona. This region of highly energized particles sits very close to the black hole, with temperatures around 100,000 electron volts. When ultraviolet light from the disk interacts with these particles, it gains energy and transforms into intense X-ray radiation.

A Relationship Written in Light

For decades, astronomers observed a predictable correlation between the ultraviolet and X-ray light from quasars. More UV light meant more X-rays, following a consistent mathematical relationship. This correlation became a cornerstone of black hole physics.

Scientists used this relationship to understand how black holes grow. They employed it as a tool to measure cosmic distances, using quasars as "standard candles" to map the geometry of the universe. It helped probe the nature of dark matter and dark energy.

The assumption was simple: this relationship was universal. It worked the same way everywhere, at all times in cosmic history.

When Assumptions Crumble

The breakthrough came from combining data from two powerful X-ray telescopes. The eROSITA telescope provided unprecedented wide coverage of the sky, detecting thousands of quasars. ESA's XMM-Newton observatory contributed deeper archival observations.

The challenge was that eROSITA's broad survey meant many quasars were detected with only a few X-ray photons each. Traditional analysis methods wouldn't work. Chira and her team developed a robust Bayesian statistical framework to combine these sparse measurements and uncover subtle patterns.

What they found was startling. The UV-to-X-ray relationship wasn't constant over time. Quasars from 6.5 billion years ago—when the universe was roughly half its present age—showed a different relationship than quasars today.

"Confirming a non-universal X-ray-to-ultraviolet relation with cosmic time is quite surprising," said Dr. Antonis Georgakakis, a co-author on the study. "It challenges our understanding of how supermassive black holes grow and radiate."

What Changed and Why It Matters

The discovery suggests that the structure and behavior of matter around supermassive black holes has fundamentally shifted over cosmic history. Either the accretion disks themselves evolved, or the coronas changed, or both.

Several possibilities exist. Perhaps the physical properties of accretion disks were different in the younger universe. The density, temperature, or composition of infalling matter might have varied. The corona's structure or temperature could have evolved as galaxies and black holes matured.

This has profound implications. If quasars behaved differently in the past, we need to reconsider how supermassive black holes grew to their current sizes. The timeline of black hole growth might need revision.

The finding also affects cosmological measurements. Using quasars as standard candles requires understanding their intrinsic properties. If those properties changed over time, distance measurements based on the old assumptions could be systematically wrong.

The Bigger Picture of Black Hole Evolution

This discovery fits into a broader pattern of surprises about supermassive black hole evolution. Recent observations from the James Webb Space Telescope have revealed dozens of active black holes in the very early universe, at redshifts between 4 and 11.

Many of these early black holes appear "overmassive"—the ratio of black hole mass to galaxy stellar mass is much higher than in the local universe. This suggests black holes grew faster relative to their host galaxies in the early cosmos.

JWST also discovered peculiar objects called "Little Red Dots." These compact, red sources appear to be moderately obscured black holes surrounded by dust. Surprisingly, most of these newly discovered high-redshift black holes are X-ray weak compared to their lower-redshift counterparts.

The pattern emerging is one of genuine evolution. Black holes and their accretion systems weren't simply smaller versions of what we see today. They operated differently, with different physical conditions governing their behavior.

Downsizing Across Cosmic Time

Astronomers have identified a phenomenon called "downsizing" in black hole activity. More luminous black holes reached their peak activity earlier in cosmic history, at higher redshifts. Less luminous ones peaked more recently.

This challenges simple models of galaxy and black hole formation. If everything formed hierarchically—small structures merging into larger ones—we'd expect the opposite pattern. The most massive systems should form last, not first.

The changing UV-X-ray relationship might be connected to this downsizing. Perhaps the physical conditions that produced the brightest quasars in the early universe also altered the relationship between disk emission and corona emission.

The Role of Environment

Supermassive black holes don't exist in isolation. They sit at the centers of galaxies, surrounded by gas, dust, and stars. A structure called the "torus"—a doughnut-shaped ring of gas and dust—surrounds the accretion disk at larger scales.

The properties of this environment likely influence the accretion process. In the younger universe, galaxies were more gas-rich. Mergers and interactions were more common. The fuel supply to black holes was more abundant and perhaps chemically different.

These environmental factors could explain why accretion physics evolved. A black hole feeding on pristine, metal-poor gas in the early universe might produce a different disk-corona system than one feeding on enriched material today.

What Comes Next

The 2025 discovery opens more questions than it answers. Is this evolution gradual or did it happen in distinct phases? Does it affect all black holes equally, or do mass and luminosity matter?

Future observations from eROSITA will help. As the telescope continues its all-sky scans, it will detect fainter and more distant quasars. This will extend the timeline further back, potentially revealing when and how the UV-X-ray relationship began to change.

The key question is whether this represents genuine physical evolution or selection effects. Are we seeing a true change in black hole physics, or are we simply detecting different populations of black holes at different cosmic epochs?

Chira noted the methodological innovation that made the discovery possible: "The eROSITA survey is vast but relatively shallow. By combining these data in a robust Bayesian statistical framework, we could uncover subtle trends that would otherwise remain hidden."

This approach—combining wide but shallow surveys with deep archival data—will likely reveal more surprises. The universe's early history remains poorly understood, and each new observation challenges existing models.

Rethinking Black Hole Growth

The implications extend to fundamental questions about cosmic structure formation. Supermassive black holes and galaxies grow together, with strong correlations between black hole mass and galactic properties like bulge mass and stellar velocity dispersion.

If the accretion process evolved, our understanding of this co-evolution needs updating. The efficiency with which black holes convert infalling matter to radiation—typically assumed to be about 10%—might have been different in the past.

This affects calculations of how much matter black holes consumed over cosmic time. It influences models of how black hole feedback—the energy and momentum they inject into their surroundings—shaped galaxy evolution.

Active galactic nuclei can reach luminosities 10^15 times that of the Sun. This energy output profoundly affects the gas in and around galaxies, regulating star formation and potentially driving galactic winds. If this process operated differently in the past, it could explain puzzles in galaxy evolution.

A Universe Still Full of Surprises

The discovery that supermassive black hole accretion disks evolved over cosmic time reminds us how much we still don't know. For half a century, a fundamental relationship was assumed to be universal. It wasn't.

This humility is healthy in science. The universe is under no obligation to conform to our assumptions. Each generation of telescopes reveals phenomena that challenge existing theories.

The next decade promises more revelations. JWST continues to peer into the earliest epochs of cosmic history. Next-generation X-ray telescopes are in development. Ground-based observatories are growing more powerful.

These tools will map the evolution of black holes and their accretion systems in unprecedented detail. They'll reveal whether the changing UV-X-ray relationship is part of a broader pattern of evolution in the extreme physics near black holes.

For now, we know that the cosmic monsters at the centers of galaxies behaved differently when the universe was young. Understanding why and how they changed will reshape our picture of cosmic history—and perhaps reveal new physics in the most extreme environments nature can create.

The black holes themselves remain unchanged, eternal and patient. But the disks of matter swirling around them tell a story of evolution, adaptation, and change across billions of years of cosmic time.