In a recent study published in The Astrophysical Journal, researchers revealed a new explanation for a phenomenon that has puzzled scientists for years—the slowdown in the growth of massive black holes over cosmic time.
The findings show that around 8,000 black holes are no longer growing at the same rate as before, not because they are rare, but due to a decline in their ability to “feed” on surrounding matter.
The study analyzed a massive dataset covering more than 1.3 million galaxies, collected through multiple astronomical surveys using advanced observatories such as Chandra X-ray Observatory, XMM-Newton, and eROSITA. X-rays are considered the most effective way to track black hole growth, as they are emitted when matter is consumed.
The data revealed that the primary cause of this slowdown is the decline in cold gas, the essential fuel for black hole growth. Since the period known as the “cosmic noon”—when star formation peaked—this resource has steadily diminished.
As a result, black hole growth rates have dropped by nearly 22 times over the past 10 billion years, suggesting that most of these cosmic giants reached their final stages long ago.
The study also indicates that the number of supermassive black holes has remained nearly constant for about 7 billion years, with little to no significant growth expected in the future.
These findings paint a new picture of the universe—one that has moved past its peak activity into a quieter phase, where once “voracious” black holes now exist in a calmer, resource-limited environment.
