Astronomers have an answer for a long-running mystery in astrophysics: why is the growth of supermassive black holes so much lower today than in the past?

A new study using NASA’s Chandra X-ray Observatory and other X-ray telescopes found that supermassive black holes are unable to consume material as rapidly as they did in the distant past.

Ten billion years ago, there was a period that astronomers call “cosmic noon,” when the growth of supermassive black holes (those with millions to billions of times the mass of the Sun) was at its peak across the entire history of the universe. Between cosmic noon and now, however, astronomers have seen a major slowdown in how rapidly black holes are growing.

“A longstanding mystery has been the cause of this big slowdown,” said Zhibo Yu of Penn State University, lead author of the new study. “With these X-ray data and supporting observations at other wavelengths, we can test different ideas and narrow down the answer.”

When gas falls into a supermassive black hole, it heats up and produces large amounts of radiation including X-rays. For decades, Chandra and other X-ray telescopes have shown a decline in black hole growth by looking at these black holes at different distances across the universe. Crucially, black holes that are growing more quickly produce more X-rays.

By analyzing observations of about 1.3 million galaxies and 8,000 growing supermassive black holes from Chandra, ESA’s XMM-Newton and eROSITA (the extended ROentgen Survey with an Imaging Telescope Array, a German and Russian mission), the team was able to isolate the ‘why’ behind this black hole slowdown.

“It appears that black holes’ consumption of material has greatly slowed down as the universe has aged,” said co-author Niel Brandt, also of Penn State University. “This is probably because the amount of cold gas available for them to ingest has decreased since cosmic noon.”

In this study, researchers determined the brightness and mass of the black holes, and how many galaxies in the survey have X-ray sources, implying that they contain growing supermassive black holes. The team used a combination of surveys ranging from shallow surveys of large regions of the sky to extremely long studies of small fields. This ensemble is often visualized in tiered layers that form a “wedding cake” design.

In the observations, XMM-Newton and eROSITA provided the middle and bottom tiers with wider but shallower observations. Meanwhile, Chandra contributed the top tier with deep observations covering a relatively small area that allowed the detection of fainter and more distant growing black holes.

“By combining these data from different X-ray telescopes, we can construct a better picture of how these black holes are growing than any one telescope could do alone,” said co-author Fan Zou of the University of Michigan. “We can find out why over ten billion years the growth of supermassive black holes has gone from hectic to leisurely to glacial.”

The team ran tests of the three main possible scenarios currently being considered for the slowdown of black hole growth. These options were: could the decline in black hole growth be caused by less efficient rates of consumption, or by smaller typical black hole masses, or by fewer actively growing black holes?

Their analysis of the data, extending over billions of years of cosmic history, led them to the conclusion that black holes are indeed consuming material less rapidly the later they are found after the big bang. The researchers expect this trend of slower-growing black holes to continue into the future.

A key challenge in this study is that both more massive black holes and faster-growing black holes produce brighter X-ray emission. Observations at other wavelengths, including optical and infrared data were used to estimate black hole masses and disentangle these two factors.

A paper describing these results appeared in the December 2025 issue of the Astrophysical Journal.

NASA's Marshall Space Flight Center manages the Chandra program. The Smithsonian Astrophysical Observatory's Chandra X-ray Center controls science operations from Cambridge, Massachusetts, and flight operations from Burlington, Massachusetts.