Black holes are among the most famous objects in astronomy. They are invisible, extremely dense, and powerful enough to trap even light. Astronomers believe that black holes sit at the centers of most galaxies and power many bright X-ray sources in the sky. Over the years, strong evidence has been collected in their favor.
However, modern physics teaches us to be careful. Nature sometimes allows different objects to look almost the same from far away. In astronomy, this means that not every dark, compact object must be a true black hole. Some exotic objects predicted by theory could closely imitate black holes while being fundamentally different inside.
One of the most fascinating possibilities is a wormhole. Wormholes are theoretical tunnels in spacetime that connect two distant regions of the Universe. Surprisingly, some spinning wormholes may look almost identical to spinning black holes when observed with telescopes.
This creates an important question for science:
How can we tell whether an object is a real black hole or a wormhole pretending to be one?
A recent study by Karakonstantakis and Kluźniak addresses this question in detail. Their work shows that while wormholes can perfectly mimic black holes in many ways, the temperature of the accretion disk around them may reveal their true identity.
2. What We Know About Black Holes
Black holes are solutions of Einstein’s theory of general relativity. When a massive star collapses or when matter collects in enormous amounts at a galaxy’s center, spacetime becomes so strongly curved that an event horizon forms. Beyond this horizon, nothing can escape.
Astronomers have strong evidence for black holes:
Supermassive objects exist at the centers of galaxies, including our own Milky Way.
Stars orbiting Sagittarius A*, the object at our galaxy’s center, move as if they are influenced by a compact object with millions of solar masses.
The Event Horizon Telescope (EHT) has imaged the shadow of compact objects in M87 and the Milky Way.
Gravitational waves from merging compact objects match predictions for black hole collisions.
All of this strongly supports black holes. But there is an important limitation.
👉 We cannot directly see the event horizon using light.
This means that observations based only on electromagnetic radiation cannot prove beyond doubt that an event horizon exists.
3. Why Black Hole Mimickers Matter
If we cannot see the horizon, then other objects without horizons could, in principle, look the same. These objects are called black hole mimickers.
A black hole mimicker:
Is very compact
Has strong gravity
Looks like a black hole from far away
Lacks a true event horizon
Wormholes are among the most interesting candidates in this category.
Studying black hole mimickers is not just a theoretical exercise. It helps scientists understand:
How reliable our interpretations of observations are
Whether new physics beyond Einstein’s theory might exist
How spacetime behaves under extreme conditions
4. Wormholes: A Brief and Simple Explanation
A wormhole is a tunnel-like structure in spacetime that connects two distant regions. You can imagine it like a shortcut between two faraway places.
The idea of wormholes dates back to the early days of general relativity. At first, they were thought to be unstable or unphysical. Later, researchers showed that traversable wormholes could exist under certain conditions.
Key features of wormholes:
They have a throat, which is the narrowest part of the tunnel.
They do not necessarily have an event horizon.
In many models, spacetime continues smoothly through the throat.
Matter and light may pass through the throat.
Some wormholes require unusual forms of matter, but others appear naturally in modified theories of gravity or quantum gravity models.
5. Wormholes as Black Hole Imitators
In astrophysics, interest in wormholes has increased because some models predict wormholes that behave almost exactly like black holes.
In particular:
They can have similar mass and spin.
They can bend light in similar ways.
They may produce similar shadows.
Matter can orbit them in nearly the same way as around black holes.
If a wormhole’s throat is very close to where the black hole’s horizon would be, the differences become extremely small.
This makes wormholes very difficult to detect using ordinary observations.
6. Why Accretion Disks Are So Important
Most compact objects in astronomy are surrounded by accretion disks. These disks form when gas and dust fall toward a massive object but spiral around it instead of falling straight in.
Accretion disks are crucial because:
They convert gravitational energy into heat.
They emit radiation, often in X-rays.
They allow astronomers to study strong gravity indirectly.
In X-ray binaries, a compact object pulls matter from a companion star. In galaxies, supermassive objects accrete gas from their surroundings.
The structure and temperature of the disk depend strongly on the spacetime geometry near the compact object.
7. The Goal of the Study
Karakonstantakis and Kluźniak wanted to answer a clear question:
Can the accretion disk around a spinning wormhole be distinguished from the disk around a Kerr black hole?
To answer this, they studied:
The motion of particles around a rotating wormhole
The properties of circular orbits
The temperature profile of a thin accretion disk
They focused on a special type of Kerr-like wormhole, designed to differ from a Kerr black hole in only one part of the spacetime geometry.
8. A Kerr Black Hole and a Kerr-Like Wormhole
A Kerr black hole is a rotating black hole described by mass and spin.
The wormhole studied in this work:
Has the same mass and spin
Is stationary and symmetric
Looks like Kerr spacetime in almost every way
Differs only in the radial part of the spacetime metric
This makes it an ideal black hole mimicker.
If even this wormhole can be detected, then less perfect mimickers would be easier to identify.
9. Motion of Particles: A Perfect Disguise
The authors studied how test particles move around the wormhole. These particles represent gas in the accretion disk.
They found something remarkable:
👉 For circular orbits in the equatorial plane, the wormhole behaves exactly like a Kerr black hole.
This means:
The orbital speed is the same
The energy of the particle is the same
The angular momentum is the same
Frame-dragging (Lense–Thirring precession) is the same
Even the location of the innermost stable circular orbit (ISCO) is identical, as long as it lies outside the wormhole throat.
This result shows that:
Orbital motion alone cannot distinguish the wormhole from a black hole
Many standard methods used in X-ray astronomy would fail
In this sense, the wormhole is a perfect black hole impersonator.
10. The Key Difference: The Wormhole Throat
Despite this perfect imitation, the wormhole has one crucial feature that black holes do not have: a throat.
Important consequences of the throat:
There are no spacetime points inside the throat radius
Orbits cannot exist below this radius
Matter cannot spiral inward indefinitely
In contrast, matter around a black hole can approach the event horizon.
This difference becomes important when studying the accretion disk.
11. Accretion Disk Temperature: The Hidden Clue
The authors calculated the temperature of a standard thin accretion disk.
In such a disk:
Matter moves in nearly circular orbits
Energy is released as heat
Heat is radiated locally
The disk becomes hotter closer to the center
For a black hole, the temperature rises smoothly toward the inner edge of the disk.
For the wormhole, something different happens.
12. Temperature Suppression Near the Throat
The study found that:
👉 The disk temperature is suppressed near the wormhole throat.
Why does this happen?
The wormhole changes the radial geometry of spacetime
Disk rings near the throat have a larger physical area
The same energy is spread over a larger surface
This lowers the temperature
At the throat itself:
The disk cannot extend further
The temperature drops to zero
This creates a clear and unique signature.
13. A Natural Disk Truncation
The suppressed temperature looks like a truncated disk.
In black hole systems, truncated disks are often explained by:
A hot inner flow or corona
Changes in accretion state
In the wormhole case:
The truncation is caused by spacetime geometry
It is fixed and permanent
It does not depend on time or accretion rate
This makes it a strong and clean observational effect.
14. When Can We Detect the Difference?
The authors estimate that the temperature suppression becomes noticeable when:
The wormhole deviation parameter is not extremely small
The effect is stronger for rapidly spinning wormholes
In favorable cases:
The temperature difference can exceed 10%
This may be detectable with current or future X-ray observations
15. What Cannot Reveal the Wormhole
The study also makes clear what will not help:
Orbital frequencies
Disk luminosity at a given radius
Spectral line shifts
Frame-dragging measurements
All these are identical for circular motion around the wormhole and the black hole.
This explains why black hole candidates based on such measurements remain ambiguous.
16. Why This Work Is Important
This research is important because it shows that:
Perfect black hole mimickers may exist
Accretion disk temperatures are sensitive to spacetime geometry
Wormholes cannot hide forever if we look carefully
Theory can guide observations toward meaningful tests
It also highlights how subtle the Universe can be.
17. Limitations and Future Work
The authors note that:
Their results apply to a specific wormhole model
Other wormhole geometries may behave differently
Full predictions require detailed light-ray calculations
Even so, the temperature suppression is a local and robust effect.
18. Conclusion: Reading the True Nature of Compact Objects
Karakonstantakis and Kluźniak have shown that a spinning wormhole can imitate a Kerr black hole almost perfectly. Circular orbits, disk motion, and many observable properties are identical.
But the accretion disk remembers the truth.
The suppressed disk temperature near the inner edge is a clear physical consequence of the wormhole throat. This effect offers a realistic way to distinguish a real black hole from a wormhole pretending to be one.
As observations improve, especially in X-ray astronomy, we may finally learn whether all black holes are truly black holes—or whether some of them are gateways to something far stranger hidden within spacetime itself.
Reference: A. Karakonstantakis, W. Kluźniak, "Surface temperature of an accretion disk around a wormhole Kerr-mimicker", Arxiv, 2025. https://arxiv.org/abs/2512.07466
Technical Terms
1. Black Hole
A black hole is an object with gravity so strong that nothing—not even light—can escape once it crosses a certain boundary. This boundary is called the event horizon. Black holes form when very massive objects collapse under their own gravity.
2. Event Horizon
The event horizon is the “point of no return” around a black hole. Once something crosses this boundary, it can never come back or send information to the outside Universe. We cannot see the event horizon directly; we only see its effects on nearby matter and light.
3. Kerr Black Hole
A Kerr black hole is a black hole that is rotating. It is described by only two numbers:
Mass (how heavy it is)
Spin (how fast it rotates)
Most real black holes in space are believed to be Kerr black holes because they rotate.
4. Wormhole
A wormhole is a theoretical tunnel in spacetime that connects two different regions of the Universe. Instead of matter falling into a dead end (as in a black hole), it could pass through the tunnel and come out somewhere else.
5. Traversable Wormhole
A traversable wormhole is a wormhole that matter and light can pass through safely. Unlike black holes, it does not have an event horizon. In theory, a spaceship—or light—could travel through it.
6. Wormhole Throat
The throat is the narrowest part of a wormhole. It is like the middle of a tunnel. In the article, the throat acts as a hard inner boundary: space does not exist inside it on our side, so matter cannot orbit closer than this radius.
7. Black Hole Mimicker
A black hole mimicker is an object that looks like a black hole from the outside but is not a true black hole. It has strong gravity and compact size but does not have an event horizon. Wormholes are one example.
8. Spacetime
Spacetime combines space and time into one four-dimensional structure. Massive objects bend spacetime, and this bending is what we feel as gravity.
9. Spacetime Metric
A metric is a mathematical description of spacetime. It tells us:
How distances are measured
How time flows
How objects and light move
Different metrics describe different kinds of objects, such as black holes or wormholes.
10. Radial Metric Component (grr)
The radial metric component, written as gᵣᵣ, describes how distances change when moving directly toward or away from the center of an object.
In the article, the wormhole differs from a black hole only in this part of the metric, which affects how space is stretched near the center.
11. Test Particle
A test particle is an idealized particle with very small mass. It is used in calculations because it does not affect spacetime itself. It simply follows the rules of gravity, making it useful for studying orbits.
12. Circular Orbit
A circular orbit is a path where a particle moves around an object at a constant distance. In accretion disks, gas particles move in nearly circular orbits around compact objects.
13. Equatorial Plane
The equatorial plane is the flat plane around a rotating object where most motion occurs. Accretion disks lie in this plane because it is the most stable region for orbiting matter.
14. Innermost Stable Circular Orbit (ISCO)
The ISCO is the smallest orbit where matter can move in a stable circle. Inside this orbit, motion becomes unstable and matter quickly falls inward. The ISCO plays a key role in shaping accretion disks.
15. Accretion Disk
An accretion disk is a rotating disk of gas and dust that forms around a massive object. Friction inside the disk heats the material, causing it to emit light—often X-rays.
16. Thin Accretion Disk
A thin accretion disk is a disk that is:
Very flat (thin compared to its width)
Efficient at radiating energy
Well described by simple physical models
This type of disk is commonly used to study black holes and wormholes.
17. Disk Surface Temperature
The surface temperature of an accretion disk tells us how hot different parts of the disk are. Hotter regions emit higher-energy light (like X-rays), while cooler regions emit lower-energy light.
18. Disk Truncation
Disk truncation means that the inner part of the accretion disk is missing or cut off. In the article, the disk ends naturally at the wormhole throat, causing the temperature to drop sharply near the center.
19. Angular Momentum
Angular momentum measures how much rotational motion an object has. In orbiting matter, it determines how fast and how far from the center the object can orbit.
20. Angular Velocity
Angular velocity describes how fast something rotates or orbits. In accretion disks, it tells us how quickly matter moves around the compact object.
21. Lense–Thirring Precession
This is an effect caused by rotation of massive objects. A spinning black hole or wormhole drags spacetime around with it, causing nearby orbits to slowly twist or wobble. This effect is also called frame dragging.
22. Apsidal Precession
Apsidal precession is the slow rotation of an orbit’s shape. Instead of repeating the same ellipse, the orbit gradually shifts its orientation over time.
23. Circumferential Radius
The circumferential radius is defined by measuring the circumference of a circular orbit and dividing it by 2π. It helps compare distances in curved spacetime in a meaningful way.
24. Asymptotically Flat Spacetime
A spacetime is asymptotically flat if, far away from the object, space becomes normal and flat, like empty space far from stars and planets.
25. X-ray Binary
An X-ray binary is a system where a normal star orbits a compact object (like a black hole or wormhole). Matter from the star falls onto the compact object, forming an accretion disk that emits strong X-rays.
26. Black Hole Shadow
The shadow is a dark region surrounded by bright emission, caused by strong bending of light near a compact object. It does not directly show the event horizon but reflects how spacetime bends light.
27. Gravitational Redshift
Gravitational redshift happens when light loses energy while escaping strong gravity. The light’s wavelength becomes longer, shifting toward the red part of the spectrum.
28. Exotic Matter
Exotic matter refers to hypothetical material with unusual properties, such as negative energy. It is often required to keep wormholes open in theory.
29. Kerr-like Wormhole
A Kerr-like wormhole is a wormhole designed to closely resemble a Kerr black hole. It shares almost all properties with a rotating black hole, except for the absence of an event horizon and the presence of a throat.
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