An international group of astronomers has uncovered strong evidence suggesting that the material surrounding supermassive black holes has not remained the same throughout the history of the universe. The findings indicate that the structure and behavior of this matter may have shifted over billions of years.
Led by researchers at the National Observatory of Athens, the study was published in Monthly Notices of the Royal Astronomical Society. If confirmed, the results would challenge a foundational idea in astronomy that has guided research for nearly 50 years.
What Makes Quasars So Bright
Quasars, first identified in the 1960s, rank among the most luminous objects known. They shine so intensely because they are powered by supermassive black holes pulling in surrounding matter. As this material spirals inward under immense gravity, it forms a rotating, disk-shaped structure before falling into the black hole.
Friction within this disk heats the matter to extreme temperatures. As a result, it can emit 100 to 1,000 times more light than an entire galaxy made up of about 100 billion stars. This overwhelming brightness allows quasars to outshine their host galaxies and makes them visible to telescopes across vast cosmic distances.
From Ultraviolet Light to Powerful X-Rays
The glowing disk around a black hole produces enormous amounts of ultraviolet light. Scientists believe this light plays a key role in generating the even more energetic X-rays emitted by quasars. As ultraviolet rays travel outward, they pass through clouds of highly energized particles located very close to the black hole, a region known as the “corona.”
When ultraviolet light interacts with these particles, it gains energy and transforms into intense X-ray radiation. These X-rays can then be detected by space-based observatories.
A Long-Standing Cosmic Relationship Under Question
Because both types of light originate near the same black hole, astronomers have long known that ultraviolet and X-ray emissions from quasars are closely linked. Typically, brighter ultraviolet light goes hand in hand with stronger X-ray output. This relationship, identified almost five decades ago, has offered critical clues about the physical conditions near supermassive black holes.
The new study challenges the assumption that this connection is universal. That assumption suggests that the structure of matter around black holes is essentially the same everywhere and at all times in the universe.
Instead, the researchers found that when the universe was younger (about half its present age), the relationship between ultraviolet and X-ray light looked noticeably different from what astronomers see in nearby quasars today. This points to changes in how the accretion disk and corona interact over roughly the last 6.5 billions of years.
“Confirming a non-universal X-ray-to-ultraviolet relation with cosmic time is quite surprising and challenges our understanding of how supermassive black holes grow and radiate,” said Dr. Antonis Georgakakis, one of the study’s authors.
“We tested the result using different approaches, but it appears to be persistent.”
How the Researchers Made the Discovery
To reach their conclusions, the team combined fresh X-ray observations from the eROSITA X-ray telescope with archival data from the European Space Agency’s XMM-Newton X-ray observatory. Together, these datasets allowed scientists to analyze the X-ray and ultraviolet emissions of an exceptionally large sample of quasars.
The broad and consistent sky coverage provided by eROSITA proved especially important. It enabled the team to examine quasar populations on a scale that was not possible before.
Why the Findings Matter for Cosmology
The idea that the ultraviolet and X-ray relationship in quasars is universal underlies some methods that use quasars as (standard candles) to map the shape of the universe and study dark matter and dark energy. The new results suggest scientists need to be cautious, since the assumption of an unchanging black hole environment over cosmic time may not hold.
“The key advance here is methodological,” said postdoctoral researcher Maria Chira of the National Observatory of Athens, who led the study.
“The eROSITA survey is vast but relatively shallow — many quasars are detected with only a few X-ray photons. By combining these data in a robust Bayesian statistical framework, we could uncover subtle trends that would otherwise remain hidden.”
What Comes Next
Upcoming eROSITA all-sky scans will allow astronomers to observe even fainter and more distant quasars. By combining these future observations with next-generation X-ray and multiwavelength surveys, researchers hope to determine whether the observed changes reflect real physical evolution or are influenced by how the data were collected.
These efforts promise deeper insight into how supermassive black holes power the brightest objects in the universe and how their behavior has transformed over cosmic time.
