Astronomers have uncovered surprising evidence that challenges a long-standing rule about quasars, some of the brightest objects in the universe.
The discovery suggests that the environment around supermassive black holes may not have stayed the same throughout cosmic history, as scientists have assumed for nearly 50 years.
Quasars are powered by supermassive black holes sitting at the centres of galaxies. As matter falls toward these black holes, it forms a spinning disc of extremely hot gas.
This disc shines intensely in ultraviolet light, often outshining the entire galaxy around it.
Very close to the black hole, some of this ultraviolet light is transformed into even more energetic X-rays by a cloud of high-energy particles known as the corona.
For decades, astronomers have known that ultraviolet and X-ray light from quasars are closely linked. In general, brighter ultraviolet light means stronger X-ray emission.
This relationship was discovered in the 1970s and has been considered universal, meaning it was thought to apply equally to all quasars, no matter where or when they existed in the universe.
That assumption has played an important role in studies of black holes and even in methods that use quasars to measure the size and shape of the universe.
Now, an international team led by researchers at the National Observatory of Athens has found evidence that this relationship changes with time.
Using data from the eROSITA X-ray telescope and the European Space Agency’s XMM-Newton observatory, the team studied an unusually large sample of quasars.
They discovered that when the universe was about half its current age, the link between ultraviolet and X-ray light was noticeably different from what is seen in nearby, more recent quasars.
This result suggests that the physical processes near supermassive black holes may have evolved over the last 6.5 billion years.
In other words, the structure of the disc and corona around black holes in the early universe may not have been the same as it is today. According to the researchers, this finding is unexpected and challenges current ideas about how black holes grow and release energy.
One reason this discovery was possible is the unique strength of the eROSITA survey, which observes the entire sky in X-rays. Although many of the detected quasars are faint and produce only a few X-ray photons, the team used advanced statistical methods to combine the data and reveal subtle trends that would otherwise remain hidden.
The researchers stress that more work is needed to confirm whether this change truly reflects a physical evolution or is partly due to how quasars are selected in surveys. Future observations of even fainter and more distant quasars will help settle the question.
If confirmed, the finding could reshape our understanding of supermassive black holes and force scientists to rethink how quasars are used to study the universe itself.
