In astronomy, a shadow is not just darkness. It is a powerful clue about how gravity works near extremely dense objects. When light passes close to a very strong gravitational field, its path bends. Some light escapes, and some gets trapped. To a distant observer, this creates a dark region called a shadow.
In recent years, images from the Event Horizon Telescope have shown the shadows of supermassive black holes. These images confirmed many predictions of Einstein’s theory of gravity. But black holes are not the only objects that could cast shadows. Other strange objects, such as traversable wormholes, may also leave visible shadow patterns.
In this article, we explain the work of Cheng, Xu, and Zhao. They studied the shadow of a rotating traversable wormhole and found new and surprising features—especially sharp points called cusps. These features could help scientists tell wormholes apart from black holes in the future.
What Is a Traversable Wormhole?
A wormhole is a theoretical tunnel in space and time. It connects two distant regions of the universe, or possibly even two different universes. A traversable wormhole is one that light—and in theory people or spacecraft—could pass through safely.
Unlike black holes, wormholes:
Do not have an event horizon
Do not trap everything forever
Can allow light to pass through the throat (the narrowest part)
Some wormholes can also rotate, just like spinning black holes. This rotation strongly affects how light moves around them.
Why Study Wormhole Shadows?
Studying wormhole shadows is important for one main reason:
👉 They may look similar to black hole shadows, but not exactly the same.
If astronomers observe a shadow that does not perfectly match black hole predictions, it could be evidence of something more exotic, like a wormhole.
Wormhole shadows help scientists:
Test Einstein’s gravity in extreme conditions
Search for alternatives to black holes
Understand how light behaves in strange spacetime geometries
How Does a Shadow Form?
The boundary of a shadow is controlled by photon orbits. These are paths where light moves in circles because gravity is extremely strong.
In a rotating traversable wormhole, Cheng, Xu, and Zhao found two main types of important photon orbits:
Unstable circular orbits outside the throat
Light circles the wormhole but can escape if disturbed
Critical orbits at the throat
Light moves right at the narrow center of the wormhole
The shadow edge is created where these two orbit families come together. This interaction is the key to understanding the strange shapes seen in wormhole shadows.
What Is a Cusp?
A cusp is a sharp, pointed feature on the edge of the shadow. It looks like a corner or spike instead of a smooth curve.
Important points about cusps:
They are real physical features, not errors
They show a sudden change in photon motion
They signal a transition in the shadow’s structure
Earlier wormhole studies often did not find cusps because they used very simple gravity models. Cheng, Xu, and Zhao showed that when gravity is described more realistically, cusps naturally appear.
The Redshift Parameter λ: A Key Factor
The researchers used a more general description of gravity near the wormhole. This description includes a parameter called λ (lambda).
In simple terms:
λ controls how quickly gravity becomes strong near the throat
Small λ → gentle gravity change
Large λ → steep gravity change
The most important discovery is a critical value of λ:
This value is special because:
It is universal
It does not depend on how fast the wormhole spins
It does not depend on the size of the throat
What happens at λc?
If λ < λc → shadow edge is smooth
If λ > λc → a sharp cusp suddenly appears
This is like a phase change, similar to water freezing into ice.
Four Types of Wormhole Shadow Shapes
By changing the spin (a) and redshift parameter (λ), the researchers mapped out all possible shadow shapes. They found four main shadow types:
1. Smooth Shadow
Occurs at small λ
Shadow boundary is round and smooth
Can look similar to a black hole shadow
2. Cuspy Shadow
Appears when λ crosses the critical value
A sharp cusp forms on the edge
Strong sign of exotic spacetime
3. Ears Touching
Happens at higher spin
Shadow develops two side “ears” that touch
Very different from black hole shadows
4. Throat Drowning
The throat-related orbits disappear from view
Shadow shape changes dramatically
Dominated by outer photon paths
These shapes form a phase diagram, similar to phases of matter.
Re-Entrant Behavior: A Strange Effect
One surprising result is re-entrant behavior. This means:
As parameters change, the throat orbit may disappear
With further changes, it can reappear again
This back-and-forth behavior shows that wormhole shadows evolve in a complex and non-simple way. Black hole shadows do not show this kind of behavior.
Connection to Deep Physics
Recent research on black hole shadows shows that cusp formation is linked to:
Topological phase transitions
Changes in mathematical properties of spacetime
Universal physical laws
The similarities between black hole cusps and wormhole cusps suggest that the same deep physics may be at work. This makes cusps especially important for understanding gravity itself.
Why This Matters for Future Observations
Astronomy is moving fast. Future telescopes will produce even sharper images of cosmic shadows.
If scientists observe:
Sharp cusps
Ear-like extensions
Unusual shadow transitions
then rotating traversable wormholes could become serious candidates for what we are seeing.
Shadow shapes may allow astronomers to:
Measure the wormhole’s spin
Estimate the redshift parameter λ
Distinguish wormholes from black holes
Conclusion: Shadows as Cosmic Clues
The study by Cheng, Xu, and Zhao shows that rotating traversable wormholes cast rich and complex shadows. Their work reveals:
A universal critical value for cusp formation
Four distinct shadow shapes
Strange re-entrant behavior
Strong differences from black hole shadows
These discoveries deepen our understanding of exotic spacetime and open a new path for testing gravity using astronomical observations.
In the future, the dark shapes we see in the sky may tell us whether we are looking at a black hole—or a doorway through spacetime itself.
Reference: Peng Cheng, Ruo-Fan Xu, Peng Zhao, "On the Cuspy Structure of Rotating Wormhole Shadows", Arxiv, 2026. https://arxiv.org/abs/2602.14182
Technical Terms
1. Shadow (of a compact object)
A shadow is a dark shape seen against bright light coming from hot gas around a very massive object.
It forms because strong gravity bends light, and some light cannot reach the observer.
The shape of the shadow tells us how space and gravity behave near that object.
2. Compact Object
A compact object is something extremely massive but very small in size.
Examples include:
Black holes
Neutron stars
Wormholes (theoretical)
Because their gravity is very strong, compact objects strongly affect light.
3. Traversable Wormhole
A wormhole is a tunnel in space and time that connects two faraway places.
A traversable wormhole means:
Light can pass through it
In theory, matter could pass safely
It has no event horizon like a black hole
It is a prediction of Einstein’s gravity, not yet observed.
4. Throat (of a Wormhole)
The throat is the narrowest part of a wormhole.
Think of it like the middle of a tunnel.
Light passing very close to the throat behaves differently than light farther away.
5. Rotating Wormhole
A rotating wormhole is a wormhole that spins around its axis.
Rotation causes:
Strong twisting of spacetime
Different bending of light on each side
More complex shadow shapes
Rotation is described using a parameter called spin.
6. Spin Parameter (a)
The spin parameter, usually written as a, tells us how fast the object is rotating.
Small a → slow rotation
Large a → fast rotation
Spin strongly affects the shape of the shadow.
7. Photon
A photon is a tiny particle of light.
In strong gravity, photons do not travel in straight lines. Instead, their paths bend or curve.
8. Photon Orbit
A photon orbit is a path where light circles around a massive object due to gravity.
These orbits are usually unstable
A tiny push can make the photon escape or fall away
Photon orbits define the edge of the shadow
9. Unstable Circular Orbit
An unstable circular orbit means:
Light can move in a circle
But the orbit is fragile
Any small disturbance breaks it
These orbits are crucial in forming shadow boundaries.
10. Critical Orbit
A critical orbit is a special photon path that separates:
Light that escapes
Light that gets trapped or diverted
At the wormhole throat, critical orbits play a major role in shaping the shadow.
11. Redshift
Redshift means light loses energy and becomes “redder” when escaping strong gravity.
In simple words:
Strong gravity slows down time
Light climbing out loses energy
Redshift tells us how intense gravity is.
12. Redshift Function
The redshift function describes how gravity strength changes with distance from the wormhole.
It controls:
How sharply gravity increases near the throat
How light paths bend
Different redshift functions lead to different shadow shapes.
13. Redshift Parameter (λ – Lambda)
The parameter λ (lambda) controls the steepness of gravity near the wormhole throat.
Small λ → gentle gravity change
Large λ → very steep gravity change
This parameter is key to whether cusps appear or not.
14. Critical Value (λc)
The critical value λc is a special number where behavior suddenly changes.
Below λc → smooth shadow
Above λc → cusp appears
This is similar to water freezing at 0°C.
15. Cusp
A cusp is a sharp, pointed feature on the edge of the shadow.
Important facts:
It is not an error
It shows a sudden change in photon behavior
It marks a phase transition in shadow shape
Cusps are strong clues of exotic gravity.
16. Smooth Shadow
A smooth shadow has a round, continuous edge with no sharp points.
Most simple black hole shadows look smooth.
17. Cuspy Shadow
A cuspy shadow has one or more sharp points on its boundary.
This shape is harder to produce and hints at non-black-hole physics, like wormholes.
18. Phase Diagram
A phase diagram is a map that shows different behaviors depending on parameters.
Here, it shows:
Shadow shapes
As spin (a) and redshift (λ) change
It is similar to diagrams showing solid, liquid, and gas phases of matter.
19. Re-Entrant Behavior
Re-entrant behavior means something:
Appears
Disappears
Then appears again as parameters change
In this case, certain photon orbits vanish and later return.
This behavior is unusual and very important.
20. Throat Drowning
Throat drowning means:
Photon orbits near the throat stop contributing to the shadow
The throat becomes “hidden” in the shadow shape
This leads to a dramatic change in appearance.
21. Topological Phase Transition
A topological phase transition is a deep mathematical change in structure, not just shape.
In shadows:
Smooth edge → cuspy edge
A fundamental change in how orbits connect
This concept comes from advanced physics but shows up clearly as cusps.
22. Observational Diagnostic
An observational diagnostic is a visible feature that helps identify what kind of object we are seeing.
Cusps, ears, and strange shadow shapes act as diagnostics to:
Tell wormholes apart from black holes
Measure their properties
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