Astronomers have long wondered why the universe’s largest black hole has grown so much more slowly over the past 10 billion years. Now, new research offers a potential solution to this astrophysical puzzle. That means they’re hungry for gas.
Supermassive black holes (SMBHs) have enormous gravitational appetites and were able to grow at an astonishing rate to millions or billions of times the mass of the Sun during the first billions of years after the Big Bang. But since a period known as “cosmic noon,” when the universe was less than a quarter of its current age, small businesses have grown slower and slower.
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The actual reason may be that they simply have less material to munch on, scientists suggested in a paper published Dec. 17 in The Astrophysical Journal.
“We knew that black holes were growing slowly, but we didn’t know why. And we found that individual black holes consumed matter much more slowly than simply that there were fewer or smaller black holes growing,” study co-author Fan Zou, an astronomer at the University of Michigan, told Live Science in an email.
A massive black hole while on a diet
Because SMBHs and their host galaxies evolve in concert, studying black hole growth is critical to understanding galaxy evolution and star birth. The size of the SMBH also correlates with the total mass of stars and the chaotic motion of stars in the galaxy’s bulge, the soccer-shaped central region where stars are densely packed.
To measure how the black hole’s growth has changed over cosmic time, the researchers used data from nine extragalactic surveys collected in a “wedding cake” design. These incremental layers include shallow surveys of large regions relatively close to the sky, as well as much deeper “pencil beam” observations of smaller regions collected from the world’s major X-ray-based space telescopes, including NASA’s Chandra X-ray Observatory, the European Space Agency’s XMM Newton, and Germany and Russia’s eROSITA.
“X-ray light is probably the best way to track black hole growth,” lead author Zhibo Yu, an astronomer at Pennsylvania State University, told Live Science via email. “They are ubiquitously produced by growing supermassive black holes and have high contrast compared to background starlight. They are also commonly used in medical imaging because they have high penetrating power, so they are less affected by obscuring gas and dust in galaxies.”
In all, the researchers analyzed multi-wavelength observations of 1.3 million galaxies and 8,000 actively feeding, X-ray emitting SMBHs to understand why the black hole’s growth rate suddenly slowed down.
Did black holes ever have a heyday?
The study tested three main ideas. For example, are black holes in the modern universe swallowing less material? Or are they simply smaller and therefore have less of a gravitational appetite than their ancient predecessors? Or are there fewer actively growing black holes overall?
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The researchers concluded that since cosmic noon some 10 billion years ago, black holes’ consumption has slowed because they swallowed less cold gas. “What surprised me the most was that we were actually able to isolate the main causes, and there was actually a dominant reason instead of a messy combination,” Zou explained.
This decline in growth rate is considered significant. “Our best estimate is that the reduction is 22-fold,” study co-author Neil Brandt, an astrophysicist at Pennsylvania State University, told Live Science in an email. Although the study does not address the inexplicably fast growth of black holes in the early universe, it is “the best result so far.” [addressing] The last 75% of cosmic time, the bulk of it!”
Future research may focus on additional datasets, such as wide-field X-ray surveys from Chandra and XMM-Newton, and multiwavelength data from other observatories. As a result, astronomers will be able to discover larger SMBH populations, including even older examples and those hidden by dense dust and gas.
Finally, this study further confirms that the era of small business prevalence is over. “We don’t think that many small and medium-sized enterprises will emerge and grow significantly in the future,” Zou said. “In fact, we found that in 2024, the number of small and medium-sized enterprises is almost as settled as it was 7 billion years ago, and that this trend is likely to continue.”
Nao Z. Chi. Y., Brandt, W. N., Zou, F., Luo, B., Ni, Q., Schneider, D. P., and Vito, F. (2025). Factors contributing to the reduced growth of supermassive black holes at z < 2. The Astrophysical Journal, 995(2), 205. https://doi.org/10.3847/1538-4357/ae173d
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