A survey of about 8,000 supermassive black holes has tracked their growth across roughly the last 10 billion years and found that accretion rates have been declining broadly across that population over that period, according to research published in The Astrophysical Journal, led by Zhibo Yu of Pennsylvania State University. The work drew on X-ray observations from NASA’s Chandra X-ray Observatory and other telescopes, covering data from nine well-characterised extragalactic fields. The slowdown, on the team’s reading of the published account, is not driven by any new force acting on the black holes. It is driven by the universe itself running low on the cold gas that feeds them.
The result is worth taking carefully. The headline framing is striking. The underlying claim is also reasonably well grounded in the established picture of how the universe has evolved since its star-forming peak, which is dated to roughly 10 billion years ago. Star formation across the universe has been declining since then. Supermassive black hole accretion rates, by this and several earlier studies, appear to be declining alongside it. The new work strengthens the link by tracking a large sample across a long time window.
What the survey actually measured
The work draws on a sample of around 8,000 active galactic nuclei — the bright X-ray signatures of supermassive black holes that are currently accreting matter. The team measured how much mass each black hole was pulling in relative to its theoretical maximum rate, and compared that figure against the age of the universe at the time the light was emitted.
The pattern, according to the published findings, is that the typical accretion rate across the population peaked several billion years ago and has been declining since. Crucially, the study found that this decline is driven primarily by falling accretion rates themselves — not by a shift toward less-massive black holes doing the accreting. The decline shows up broadly across the sample. That breadth is what makes the gas-supply reading the natural one.
Why the explanation is about gas, not gravity
Supermassive black holes grow by accreting matter, and the matter that matters, in practice, is cold gas. Hot gas does not fall in efficiently. It has too much thermal pressure to sink to the centre of a galaxy on a useful timescale. Cold molecular gas does. It cools, it loses angular momentum, it spirals in, and some fraction of it ends up on the black hole at the centre.
The same cold gas is what stars form out of. That is why the star formation history of the universe and the black hole accretion history of the universe track each other reasonably well. Both peaked at roughly the same time, around 10 billion years ago, in what is sometimes called cosmic noon. Both have been declining since. The dominant explanation, in the existing literature on galaxy evolution, is that the universe’s reservoir of cold gas has been steadily converted into stars, ejected by stellar feedback, or heated by active galactic nuclei themselves, and not replenished at the rate it is consumed.
The new survey, in our reading, does not propose a new mechanism. It tightens the picture by giving the black hole side of the story a larger and more uniform sample, and by identifying decreasing accretion rates — rather than changes in black hole mass or population — as the dominant driver of the observed decline.
What the result does not say
It is worth being precise about what is and is not being claimed. The work does not say that supermassive black holes have stopped growing. It says the average accretion rate across a large population has been falling. Individual black holes can still feed vigorously when a fresh supply of gas reaches them, typically through a galaxy merger or a disturbance that drives gas inward.
The result also does not say anything direct about the very early universe. Some of the most active accretion in the universe’s history happened in the first billion or two billion years, when quasars were at their most luminous. That period sits outside the 10-billion-year window the survey is focused on. The question of how the earliest supermassive black holes grew so large so quickly is a separate and currently unsettled question in the literature.
And the result does not, on the available evidence, settle the question of which feedback mechanism dominates the cold gas decline. Stellar winds, supernovae, and the black holes’ own outflows are all candidates. The relative contributions are still being worked out.
The pattern that does seem to be holding
What the survey does add, on our reading, is statistical weight to a picture that has been forming in the literature for some time. The universe is past its star-forming peak. It is past its supermassive black hole feeding peak. Both declines appear to be driven by the same underlying scarcity. The matter that powers both processes is finite and is, at the largest scales, being used up faster than it is being replaced.
This is not a claim about the end of anything. The decline is slow on human timescales and slow even on galactic ones. The Milky Way will continue to form stars for a long time. Sagittarius A* will continue to accrete, episodically, for a long time. The shape of the curve is what the survey is reading, not its endpoint.
What to watch next
The natural next steps, by the team’s read of the field, are higher-resolution measurements of cold gas content in the host galaxies of these accreting black holes, and continued work with radio surveys that can pick up faint accretion that optical surveys miss. The Square Kilometre Array, currently under construction in Australia and South Africa, is the instrument most often pointed to for that work. First science from the SKA is still some years away.
The accretion history, on the available evidence, will keep getting better resolved. The story it tells is unlikely to change shape. The food is running low, slowly, and has been for a long time.
