When we imagine black holes, we often think of giant cosmic vacuum cleaners that suck in everything nearby — gas, stars, even light itself. But the truth is much more fascinating. Black holes don’t just pull things in; they can also lose energy in mysterious ways. And sometimes, short bursts of “feeding” — when a black hole suddenly starts swallowing more gas than usual — can completely change their long-term behavior.
A new study by Nandakumar, Beckmann, and Irsic (2025) explores how these bursts of feeding, called accretion boosts, affect the life story of rapidly spinning black holes. The researchers looked at how these events interact with a strange quantum process called superradiance, which allows certain types of particles — known as axions — to steal energy from a spinning black hole.
By running detailed computer simulations, the team found that the timing of these feeding bursts is crucial. If the accretion boost happens before the black hole starts losing spin energy through superradiance, it can dramatically delay or even reshape that process. But if it happens after, the effect is much smaller and temporary.
Their findings shed new light on how black holes grow, how they lose spin, and even how we might detect hidden dark matter particles that surround them.
The Spinning Power of Black Holes
Most black holes don’t just sit still — they spin, sometimes incredibly fast, close to the speed of light. The spin of a black hole is important because it affects how it interacts with everything around it: how it swallows gas, how it powers bright jets, and how it evolves over millions of years.
Black holes grow by pulling in gas and dust from their surroundings — a process called accretion. As this matter spirals inward, it forms a bright, hot disk. This disk doesn’t just feed the black hole with mass; it also gives it angular momentum, making it spin faster, much like an ice skater pulling in their arms.
Normally, this would mean that the more a black hole eats, the faster it spins. But nature has another trick up its sleeve — a mysterious process that can actually slow a spinning black hole down, even without anything physically touching it.
The Strange Process of Superradiance
Superradiance is one of the most fascinating predictions of modern physics. It happens when a black hole interacts with a special kind of lightweight particle field. In this case, scientists are especially interested in axions, which are hypothetical particles that could make up dark matter — the invisible substance that fills most of the universe.
Here’s how it works: when a black hole spins, it drags the space around it. If a field of axions surrounds it, the black hole can actually transfer some of its spin energy to that field. Instead of falling in, the axions form a huge, cloud-like structure around the black hole. This is called an axion cloud.
Over time, the cloud grows larger as it steals more and more energy from the black hole’s spin. This process continues until the black hole slows down enough that the energy transfer stops. The result is a sudden drop in spin, while the mass of the black hole stays nearly the same. Scientists call this dramatic moment the superradiance drop.
The Regge Plane: A Map of Black Hole Behavior
To study how black holes evolve, scientists use a kind of map called the Regge plane, which shows black holes based on their mass and spin. When superradiance occurs, certain combinations of mass and spin become unstable — black holes quickly lose spin and move out of these regions. These unstable areas are known as exclusion regions because very few black holes are expected to exist there.
By comparing real black hole data with this map, researchers can figure out which kinds of axion particles might exist. For example:
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Smaller, stellar black holes can rule out heavier axions.
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Bigger, supermassive black holes can rule out lighter axions.
So, studying how black holes move through this mass–spin space doesn’t just tell us about the black holes themselves — it can also give us clues about dark matter.
Real Black Holes Don’t Feed Steadily
Many previous studies of superradiance assumed that black holes eat at a steady rate. But in the real universe, that’s rarely the case. The environment around a black hole is chaotic. Gas clouds, stars, and entire galaxies can collide, changing how much material falls into the black hole at any given time.
Sometimes, black holes go through quiet phases where they barely eat anything. Other times, they experience feeding frenzies — sudden bursts of accretion that can last millions of years. These bursts can be triggered by:
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Galaxy mergers, when two galaxies collide and funnel gas into the center.
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Starbursts, when new stars form rapidly and stir up surrounding gas.
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Clumpy gas flows, which naturally vary over short timescales.
In some cases, these feeding events can go above the so-called Eddington limit, the maximum steady rate at which a black hole can accrete without blowing material away with radiation pressure. Such super-Eddington accretion events are rare but powerful.
The question Nandakumar and colleagues asked was simple: what happens to the delicate dance between accretion and superradiance when the black hole’s feeding rate suddenly jumps?
Simulating the Dance Between Feeding and Spin Loss
To answer this, the researchers simulated how a fast-spinning black hole behaves when it is surrounded by an axion cloud and then suddenly starts accreting more mass.
They ran several models, varying how strong the accretion burst was, how long it lasted, and when it happened relative to the superradiance drop.
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Some boosts were mild, close to normal accretion rates.
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Others were strong, up to five times the Eddington rate.
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The boosts lasted from a few million to tens of millions of years.
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Crucially, some boosts occurred before the superradiance drop and some after.
The goal was to see how these changes affected the black hole’s spin and how its trajectory on the Regge plane evolved over time.
Key Discovery: Timing Is Everything
The main result of the study is surprisingly simple but powerful: the timing of the accretion burst determines how much it changes the black hole’s long-term spin evolution.
1. Early Feeding Changes Everything
If the accretion boost happens before the black hole’s spin starts dropping due to superradiance, it can have a big impact. The sudden increase in accretion gives the black hole a lot of extra spin and mass. This delays the superradiance drop or even reshapes it entirely.
In some cases, an early boost can make the superradiance exclusion region — the area where rapidly spinning black holes are rare — much smaller. For example, a black hole that accretes at five times the Eddington rate for four million years, starting thirty million years before the superradiance drop, can reduce the exclusion region by up to 40%.
That’s a major change. It means black holes could stay spinning fast for much longer, and the regions of the Regge plane thought to be “forbidden” might actually host more black holes than we expected.
2. Late Feeding Has Little Lasting Impact
On the other hand, if the accretion burst happens after the superradiance drop, the effect is only temporary. The black hole spins up a bit while it’s feeding, but once the burst ends, it quickly settles back to its slow, stable state. The exclusion region remains mostly the same.
3. Lighter Axion Fields Are More Sensitive
The researchers also found that the effect of accretion boosts depends on the type of axion field involved. Lighter axions, which create larger, more spread-out clouds, make black holes more sensitive to early boosts. These systems can show noticeable changes in spin that last tens to hundreds of millions of years — long enough to be seen in future observations.
In contrast, heavier axion fields are more stable. It takes much stronger or longer accretion bursts to produce the same effects.
A Battle of Timescales
The study also identified two key “clocks” that control the evolution of a spinning black hole under superradiance:
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The accretion timescale (τ_acc) — how quickly the black hole gains angular momentum from incoming gas.
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The superradiance timescale (τ_s) — how quickly the axion cloud extracts angular momentum from the black hole.
When the superradiance timescale becomes shorter than the accretion timescale, the black hole can’t keep up — and the superradiance drop begins. The timing of this drop depends on how these two processes compete.
An early accretion boost speeds up τ_acc, letting the black hole delay the spin-down. But once the drop starts, superradiance dominates. That’s why later boosts don’t make much difference.
What Does This Mean for Real Black Holes?
In simple terms, this study shows that black holes remember their feeding history. Short-lived bursts of heavy accretion can have effects that last millions or even hundreds of millions of years.
For astronomers, this is an exciting result because it means that the current distribution of black hole spins across the universe could carry hidden clues about ancient feeding events.
It also helps refine our understanding of dark matter. By including variable accretion in superradiance models, scientists can make more realistic predictions about which regions of the Regge plane should be empty. This, in turn, helps narrow down which axion masses are still possible.
Possible Gravitational Wave Signals
Another fascinating consequence involves gravitational waves — ripples in spacetime caused by moving mass and energy. The black hole–axion cloud system is expected to emit faint gravitational waves as it evolves. If early accretion boosts make these clouds larger or longer-lived, the gravitational wave signals could be stronger or last longer than expected.
Future detectors like LISA (Laser Interferometer Space Antenna) could potentially pick up these signals, helping confirm the existence of axions and providing new insight into black hole physics.
Limitations and Next Steps
Of course, like all theoretical studies, this one makes a few simplifying assumptions. The black holes in the simulations all started with maximum spin, while in reality, black holes can have a range of starting spins. Lower-spin black holes might behave differently, possibly delaying the superradiance drop.
The study also ignored gravitational wave losses from the axion cloud, which could change the timing of the process. And while the researchers tested single bursts of accretion, real black holes probably experience many smaller bursts throughout their lives.
The next step will be to use cosmological simulations that track black holes over billions of years and include realistic, variable accretion histories. Combining this with superradiance physics will allow scientists to predict what the actual spin distribution of supermassive black holes should look like if axions exist.
Why It Matters
At first glance, this might sound like a niche study about some details of black hole behavior. But its implications reach far beyond astrophysics. It connects three of the biggest mysteries in modern science:
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How black holes grow and evolve.
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How matter behaves under extreme gravity.
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What dark matter — which makes up most of the universe — is made of.
By studying how black holes spin and slow down, we’re not just learning about these cosmic giants — we’re also testing the laws of physics in the most extreme conditions possible.
The Cosmic Story of Balance
If there’s one lesson from this research, it’s that the universe is full of delicate balances. A black hole’s fate depends on a tug-of-war between feeding and energy loss, between matter and quantum fields, between chaos and stability.
A short burst of gas falling into a black hole — lasting just a few million years — can set off changes that echo for hundreds of millions of years. These changes can decide whether a black hole stays a rapid spinner or slows down to a quiet giant.
Even the invisible axion fields, if they exist, play a role in shaping this dance, quietly siphoning away energy and leaving their fingerprint on the cosmic landscape.
Conclusion
The study by Nandakumar, Beckmann, and Irsic reveals a simple but profound truth: black holes are not just passive eaters of matter, but dynamic systems shaped by the rhythm of their environment.
Their simulations show that:
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Early bursts of accretion — before the superradiance drop — can strongly reshape how a black hole’s spin evolves.
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Late bursts have only short-term effects.
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Lighter axion clouds make black holes more sensitive to early feeding events.
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Even with these variations, the exclusion regions caused by superradiance still exist, meaning the overall picture remains stable.
In short, a black hole’s past feeding habits determine its future. By studying these patterns, we may one day not only understand how black holes grow but also unlock the secrets of the dark universe that surrounds them.
Reference: Adithya Nandakumar, Ricarda S. Beckmann, Vid Irsic, "The impact of superradiance on the spin evolution of variably accreting massive black holes", Arxiv, 2025. https://arxiv.org/abs/2510.19443
Technical Terms
🌀 1. Black Hole Spin
Every black hole can rotate, just like Earth spins on its axis.
This rotation is called its spin.
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A fast-spinning black hole means it’s rotating very quickly, close to the speed of light.
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A slow-spinning black hole means it has lost some of its rotational energy.
Spin is very important because it affects how much energy a black hole can release and how it interacts with matter around it.
🌌 2. Accretion
Accretion means “gathering” or “collecting.”
In space, it refers to the process where a black hole pulls in nearby gas, dust, or stars.
This material forms a swirling, hot accretion disk around the black hole before falling in.
As the material spirals closer, it transfers some of its spin and energy to the black hole — making the black hole grow in both mass and speed of spin.
So, accretion = feeding for a black hole.
⚡ 3. Accretion Rate
This tells us how fast a black hole is eating.
It’s usually compared to a limit called the Eddington rate.
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If a black hole eats below the Eddington rate, the feeding is calm and steady.
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If it eats at or above the Eddington rate, it’s feeding very fast — sometimes explosively fast.
Such cases are called super-Eddington accretion, like a black hole binge-eating gas.
💥 4. Super-Eddington Accretion
The Eddington limit is the balance point between two forces:
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Gravity, which pulls gas in.
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Radiation pressure, which pushes gas out.
If the black hole accretes (feeds) faster than this limit, the incoming gas should, in theory, be blown away by radiation.
But sometimes, under special conditions, black holes can temporarily exceed this limit, pulling in matter faster than expected.
That’s called super-Eddington accretion — an extreme feeding event.
🌫️ 5. Axion
An axion is a hypothetical particle — something scientists believe might exist but haven’t yet proven.
Axions are thought to be:
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Extremely light (many billions of times lighter than electrons).
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Very weakly interacting (they almost never collide with normal matter).
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Candidates for dark matter — the invisible substance that makes up most of the mass in the universe.
In short:
If axions exist, they could fill space around black holes and play a key role in cosmic physics.
☁️ 6. Axion Cloud
If axions are present around a spinning black hole, they can gather into a structure that looks like a cloud around the black hole — not a normal gas cloud, but a cloud of energy and axion particles.
This axion cloud can pull energy from the black hole’s spin through a process called superradiance (explained next).
It behaves a bit like an atom, with the black hole at the center and the axion field forming “orbits” around it — similar to how electrons orbit the nucleus in a hydrogen atom.
🌟 7. Superradiance
This is one of the most fascinating ideas in modern physics.
Superradiance happens when a spinning black hole transfers some of its rotation energy to the axion cloud.
It’s like the black hole “rubbing off” some of its spin energy onto the surrounding field.
As a result:
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The axion cloud grows (it gains energy).
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The black hole slows down (it loses spin).
This continues until the black hole slows enough that energy transfer stops.
At that point, the axion cloud stops growing.
This process doesn’t destroy the black hole — it just drains some of its rotational energy.
📉 8. Superradiance Drop
The superradiance drop is the moment when the black hole’s spin suddenly decreases due to superradiance.
During this drop:
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The black hole loses spin energy very quickly.
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Its mass stays almost constant (since no matter is falling in at that exact moment).
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The surrounding axion cloud becomes large and energetic.
Think of it like a spinning top suddenly slowing down because it’s transferring energy to the air around it — that’s the “drop.”
📊 9. Regge Plane (or Mass–Spin Plane)
The Regge plane is a graph scientists use to track black holes.
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The x-axis shows the mass of the black hole.
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The y-axis shows its spin.
Each black hole can be represented as a point on this graph.
Some areas of the Regge plane are called “exclusion regions” because black holes can’t stay there for long — superradiance forces them to move away by spinning down.
By studying which parts of the plane are “empty,” scientists can learn about how superradiance shapes real black holes and what types of axions might exist.
⏳ 10. Timescales (τ_acc and τ_s)
In the study, two “clocks” or timescales describe how things change over time:
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τ_acc (accretion timescale):
How quickly the black hole gains spin energy from incoming matter.
(Shorter τ_acc means faster feeding.) -
τ_s (superradiance timescale):
How quickly the axion cloud removes spin energy from the black hole.
(Shorter τ_s means faster spin loss.)
The balance between these two determines what happens next:
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If accretion dominates (τ_acc < τ_s): the black hole spins up.
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If superradiance dominates (τ_s < τ_acc): the black hole spins down.
When the two become equal, the superradiance drop begins.
💨 11. Angular Momentum
Angular momentum is just a fancy term for “rotational energy.”
It describes how much spin something has.
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A figure skater spinning with arms out has less angular momentum than when they pull their arms in and spin faster.
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A black hole’s spin energy works the same way — it’s its angular momentum.
Accretion adds angular momentum (makes it spin faster), while superradiance removes it (slows it down).
🌠 12. Dark Matter
Dark matter is invisible material that makes up about 85% of the matter in the universe.
We can’t see it because it doesn’t emit or absorb light, but we can tell it’s there because its gravity affects stars and galaxies.
Axions are one of the main candidates for what dark matter could be made of.
If axions exist, studying superradiance around black holes might help us finally detect them.
🌊 13. Gravitational Waves
Gravitational waves are ripples in spacetime created when massive objects move or collide.
Einstein predicted them over 100 years ago, and they were first detected in 2015.
When a black hole and an axion cloud interact, the system can produce faint gravitational waves as the cloud grows or decays.
Detecting these signals could reveal both black hole properties and evidence of axions.
🔲 14. Exclusion Region
In the Regge plane (mass vs. spin plot), the exclusion region is the area where black holes should not remain for long if superradiance is active.
This region becomes “empty” because:
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Superradiance quickly removes energy from any black hole that enters it.
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Those black holes lose spin and move out of that area.
By studying which parts of the Regge plane are empty, astronomers can test whether superradiance — and possibly axions — are real.
💡 15. Eddington Limit
The Eddington limit is like a speed limit for how fast a black hole can accrete (feed).
If a black hole pulls in gas too fast, the gas heats up and produces strong radiation that pushes back outward.
At the Eddington limit, this inward pull and outward push balance each other.
If the black hole accretes faster than this limit (super-Eddington), the balance is broken — the black hole is feeding extremely fast.
🧠 16. Simulation
In this study, the scientists didn’t observe real black holes directly.
Instead, they used computer simulations — detailed models that use physics equations to show how black holes and axion clouds evolve over time.
Simulations let scientists test “what-if” situations — like what happens if the black hole suddenly gets an extra burst of gas — which would take millions of years to see in real life.
🧩 17. Boson / Light Bosonic Field
A boson is a type of fundamental particle (like photons, the particles of light).
A light bosonic field refers to a field made up of very lightweight bosons — such as axions.
These particles can interact with black holes in unique ways, causing superradiance.
⚙️ 18. Kerr Metric
The Kerr metric is a solution to Einstein’s equations of general relativity.
It describes the spacetime geometry (the “shape” of space and time) around a rotating black hole.
When physicists say “the Kerr metric still describes the black hole,” they mean that even after complex effects like superradiance, the black hole’s structure still follows the same general relativistic rules.
🧭 19. Gravitational Orbitals
Just like electrons orbit around the nucleus of an atom, particles like axions can form gravitational orbitals around a black hole — bound states where they circle the black hole repeatedly.
That’s why scientists sometimes describe the black hole–axion system as a “gravitational atom.”
🪐 20. Regge Trajectory / Critical Spin
A Regge trajectory (or critical spin line) is a boundary on the Regge plane that shows where superradiance starts or stops.
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Above this line → the black hole spins fast enough for superradiance to happen.
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On or below this line → superradiance stops, and the black hole stabilizes.
Once the black hole reaches this critical spin, it no longer transfers energy to the axion cloud.
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