For billions of years, the most massive entities in the cosmos have operated like gravitational vacuum cleaners, sweeping up vast clouds of gas and dust to reach sizes millions or even billions of times that of our sun. But a new analysis suggests that the hungriest black holes in the universe are running out of food, leading to a significant slowdown in their growth over the last 10 billion years.
The findings, published in The Astrophysical Journal, address a long-standing astrophysical enigma: why supermassive black holes (SMBHs) have transitioned from a period of rapid, rampant expansion to a state of relative stagnation.
By surveying 8,000 actively feeding “cosmic monsters” across 1.3 million galaxies, researchers have determined that the decline isn’t due to a shortage of black holes or a change in their size. Instead, the universe is simply running out of the cold gas required to fuel their appetite.
This discovery provides a clearer picture of the lifecycle of the universe, marking the end of an era of extreme growth and the beginning of a quieter, more stable cosmic epoch.
The peak of cosmic appetite
In the early history of the universe, supermassive black holes grew at an astonishing rate. This period of intensity reached its zenith during what astronomers call “cosmic noon,” roughly 10 billion years ago, when the universe was less than a quarter of its current age. During this window, the abundance of available matter allowed SMBHs to swell rapidly, influencing the birth of stars and the structure of the galaxies that housed them.
However, since that peak, the growth rate has plummeted. Researchers found that the decrease in consumption is staggering, estimating that the growth rate has dropped by a factor of 22. This suggests that the “golden age” of black hole expansion is firmly in the past.
The relationship between a black hole and its host galaxy is deeply symbiotic. The size of an SMBH typically correlates with the total mass of stars and their movements within the galactic bulge—the densely packed, football-shaped central region of a galaxy. When the black hole stops growing, it signals a broader shift in how the entire galaxy evolves.
Mapping the void with X-rays
To solve the mystery of why these giants stopped growing, astronomers needed a way to “see” black hole growth across vast distances and time. Since black holes themselves emit no light, scientists gaze for the X-ray signatures produced by the material swirling into the event horizon.
Lead author Zhibo Yu, an astronomer at Penn State, noted that X-ray light is the most effective tracer for this process because it provides high contrast against the light of background stars. X-rays possess high penetrating power, allowing them to pass through the dense clouds of gas and dust that often obscure the centers of galaxies.
The team employed a “wedding cake” survey design to gather their data. This method involves layering different types of observations: shallow surveys that cover large, nearby regions of the sky, and extremely deep “pencil-beam” looks at much smaller, distant fields. To achieve this, the team utilized data from three of the world’s premier X-ray space telescopes:
- NASA’s Chandra X-ray Observatory: Known for its high resolution and ability to pinpoint X-ray sources.
- ESA’s XMM-Newton: A European Space Agency telescope designed to collect large amounts of X-ray photons.
- eROSITA: A German-Russian mission that has provided an all-sky X-ray survey.
Isolating the cause of the slowdown
Before this study, astronomers debated several theories regarding the decline of SMBH growth. Some hypothesized that there were simply fewer active black holes in the modern universe. Others wondered if modern black holes were smaller and therefore had weaker gravitational pulls, making them less “gluttonous” than their ancestors.
By analyzing the multiwavelength observations of 8,000 actively feeding black holes, the researchers were able to test these competing ideas. They discovered that the number of growing black holes and the size of the black holes themselves were not the primary drivers of the decline.
Instead, the evidence pointed to a lack of fuel. The cold gas that black holes “munch” on has depleted significantly since cosmic noon. Fan Zou, an astronomer at the University of Michigan and study co-author, explained that individual black holes are simply consuming material much less rapidly than they used to.
Zou noted that the ability to isolate a single dominating reason, rather than a complex mix of factors, was one of the most surprising aspects of the research. This clarity helps astronomers refine their models of how galaxies and their central black holes evolve in tandem.
A settled cosmic population
While this study explains the decline over the last 10 billion years, it does not account for the “rule-breaking” black holes found in the very early universe, which grew far faster than current physics suggests should be possible. However, for the vast majority of cosmic history—approximately 75%—this gas-starvation model provides the most comprehensive explanation to date.
The research suggests that the era of rampant growth is effectively over. Data from 2024 indicates that the total number of supermassive black holes was largely settled by 7 billion years ago. Because the fuel sources are dwindling, astronomers do not expect many new SMBHs to emerge or grow significantly in the foreseeable future.
Moving forward, the scientific community will look toward wider X-ray surveys and multiwavelength data from other observatories to discover older, more obscured black hole populations. These efforts aim to uncover the “hidden” monsters that are buried under dense layers of dust and gas, potentially offering further clues about the early universe’s appetite.
For those following the evolution of the cosmos, the next critical updates will likely approach from expanded datasets provided by the continued operations of the Chandra and XMM-Newton observatories.
Do you think the discovery of “starving” black holes changes how we view the eventual fate of our own galaxy? Share your thoughts in the comments below.
