Astronomers may have uncovered evidence that challenges one of the longest-standing ideas about quasars, the brilliant beacons powered by supermassive black holes.
A new international study suggests that the structure of matter surrounding these black holes has changed over billions of years, calling into question a rule that has guided quasar research for nearly 50 years.
Quasars were first identified in the 1960s and quickly stood out as some of the brightest objects in the universe.
At their heart lies a supermassive black hole, surrounded by a swirling disk of gas and dust.
As matter spirals inward, intense friction heats the disk to extreme temperatures, causing it to shine fiercely in ultraviolet light.
In fact, a single quasar can produce hundreds of times more light than an entire galaxy filled with billions of stars.
This ultraviolet light does not tell the whole story. Close to the black hole, clouds of extremely energetic particles form a region known as the corona.
When ultraviolet light from the disk passes through this region, it is boosted to much higher energies, producing powerful X-rays. Because these two forms of light are linked, astronomers discovered nearly five decades ago that brighter ultraviolet emission usually goes hand in hand with stronger X-ray emission.
This relationship became a cornerstone of quasar science. It suggested that the basic structure around black holes—the disk and the corona—was the same everywhere and at all times in the universe.
The new study, led by researchers at the National Observatory of Athens, now challenges that assumption.
By examining a huge sample of quasars at different distances, the team found that the link between ultraviolet and X-ray light changes when looking far back in time. When the universe was about half its current age, quasars showed a noticeably different relationship between these two types of emission compared with nearby quasars.
This finding suggests that the physical processes near supermassive black holes may have evolved over the last 6.5 billion years. In other words, quasars in the younger universe may not have worked in quite the same way as those we see today.
To reach this conclusion, the team combined new data from the eROSITA X-ray telescope with older observations from the European Space Agency’s XMM-Newton mission. The strength of eROSITA lies in its ability to scan the entire sky in a uniform way, giving scientists access to an unprecedented number of quasars. Although many of these objects are faint in X-rays, advanced statistical techniques allowed the researchers to detect subtle patterns that would otherwise be missed.
The results have important consequences beyond black hole physics. Some astronomers use quasars as “standard candles” to measure the size and shape of the universe and to study dark matter and dark energy. If the ultraviolet-to-X-ray relationship is not constant over time, those methods may need to be carefully re-evaluated.
Future observations with eROSITA and next-generation telescopes will help clarify whether this change reflects a real shift in black hole behavior or is influenced by observational limits.
Either way, the study opens a new window on how the universe’s most powerful objects have evolved—and reminds us that even long-trusted cosmic rules may not be set in stone.
Source: Royal Astronomical Society.
