Fermi balls are dense, dark matter objects that could help explain the formation of primordial black holes in the early universe. These objects are made up of fermions, particles that don't interact with light, making them hard to detect. Fermi balls form when dark matter particles interact through a scalar field, a force that helps bind the particles together. In a recent study, scientists explored how Fermi balls grow and eventually collapse into black holes. The researchers focused on a special part of the scalar field's equation called the λφ⁴ term, which adds a repulsive force between particles. This causes Fermi balls to reach a state called saturation faster, meaning they become dense and compact at smaller masses. Once they reach a certain size and mass, they can collapse into black holes, much like how neutron stars collapse when they get too heavy.
Dark matter is one of the biggest mysteries in science. We can't see it, but we know it's there because of its gravitational effects on galaxies and the universe as a whole. Physicists have come up with many ideas to explain dark matter, and one of the most exciting concepts involves Fermi balls. These are strange, dense objects made up of dark matter particles, and they might help us understand how primordial black holes—black holes formed in the very early universe—could have formed.
In a recent study, researchers looked into how Fermi balls form in the universe and how they might eventually collapse into black holes. They focused on a special kind of interaction in the dark sector of the universe, where heavy particles interact with a light force field. This process could explain how Fermi balls grow and eventually turn into black holes.
What Are Fermi Balls?
Fermi balls are a type of particle object made up of fermions, which are particles that follow certain rules in physics (like electrons and protons). These objects are a bit like neutron stars, which are extremely dense stars that collapse under their own gravity. But instead of being made of regular matter, Fermi balls are made of dark matter particles, which don't interact with light and are hard to detect.
Fermi balls are stable objects, meaning they can exist without falling apart. They form when particles in the dark sector of the universe interact through a special force, which is carried by a scalar field. This field helps the particles stick together and form a compact object—something like a dark matter version of a star.
How Do Fermi Balls Form?
Fermi balls could form in the early universe under the right conditions. For example:
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Dark Matter Creation: During the universe's first moments, dark matter could have been created along with regular matter. Fermi balls might form as the dark matter particles interact and group together.
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Phase Transitions in the Dark Sector: As the universe cooled down, there could have been "phase transitions" in the dark sector, where the particles went from a spread-out state to clumping together into dense objects like Fermi balls.
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Accretion of Dark Matter: Fermi balls might also form as dark matter accumulates and condenses due to gravity, just like how stars form from gas clouds.
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Long-Range Forces: In some models, long-range forces between dark matter particles could cause them to clump together and form these Fermi balls.
How Does the Scalar Field Affect Fermi Balls?
The key to understanding how Fermi balls grow and collapse lies in the interaction between dark matter particles through a scalar field. This scalar field has a special term in its equation, called the λφ⁴ term, that adds a repulsive force between the particles. This force changes how the Fermi balls behave.
When this term is included, Fermi balls reach a state called “saturation” much faster. Saturation happens when the force between the particles becomes so strong that the Fermi ball stops growing in size, even as it gains more mass. This change allows Fermi balls to become larger and puffier at smaller masses, making them easier to detect in the universe.
How Do Fermi Balls Turn Into Black Holes?
Here's the fascinating part: as Fermi balls grow, their density increases, and their size decreases. This process could eventually lead to the formation of a black hole.
When a Fermi ball’s mass becomes too large and its radius shrinks enough, it reaches a point where it falls within its own Schwarzschild radius. This is the size at which the object’s gravity becomes so strong that nothing, not even light, can escape from it. At this point, the Fermi ball collapses into a black hole.
This process is similar to how a neutron star can collapse into a black hole if it gets too massive. The presence of the λφ⁴ term allows Fermi balls to keep growing until they collapse into black holes. Without this term, the Fermi balls would stop growing before reaching this point.
Implications for Dark Matter and Black Holes
The formation of Fermi balls and their ability to collapse into black holes could help explain several important cosmic mysteries:
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Dark Matter Candidates: Fermi balls could be a form of cold dark matter (CDM). Unlike other dark matter candidates, such as weakly interacting massive particles (WIMPs), Fermi balls are large objects that could be detected through their gravitational effects, rather than by directly interacting with regular matter.
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Primordial Black Holes: The study of Fermi balls also provides a potential explanation for how primordial black holes formed in the early universe. These black holes could have been produced in the first moments of the universe, long before stars and galaxies were formed.
What’s Next?
Even though Fermi balls and their connection to primordial black holes are an exciting idea, there is still a lot we don’t know. Some areas where future research will focus include:
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Other Scalar Field Terms: Scientists are looking into other terms in the scalar field that could further change the behavior of Fermi balls, like a φ³ term that could attract the particles instead of repelling them.
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Numerical Simulations: More detailed computer simulations will help us better understand how Fermi balls interact with each other and how they evolve over time.
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Dark Matter Self-Interaction: Scientists will also study how tiny Fermi balls interact with each other. This could have important implications for understanding how dark matter behaves and interacts throughout the universe’s history.
Conclusion:
Fermi balls are an exciting possibility for explaining the formation of primordial black holes and the nature of dark matter. By studying these objects, scientists hope to unlock secrets about the early universe and how dark matter interacts with itself. If Fermi balls can grow large enough and collapse into black holes, they could provide a new way to detect and understand dark matter in the cosmos.
As researchers continue to explore these ideas, Fermi balls could play a major role in solving some of the universe’s most puzzling mysteries.
Reference: Yifan Lu, Zachary S. C. Picker, Stefano Profumo, Alexander Kusenko, "Black Holes from Fermi Ball Collapse", Arxiv, 2024. https://arxiv.org/abs/2411.17074
Technical Terms
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Fermi Balls: Fermi balls are dense objects made up of dark matter particles, which are particles that don’t interact with light and are hard to see. They are stable, meaning they don’t break apart, and are similar to neutron stars, but instead of regular matter, they’re made of dark matter. Think of them as a “dark matter star” formed from particles we can't directly detect.
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Non-Topological Solitons: A soliton is a stable, wave-like object that doesn’t lose its shape or energy over time. "Non-topological" means that the soliton doesn’t have a specific shape or structure that makes it different from regular matter. In this case, a Fermi ball is a type of soliton that doesn’t have a special shape, but is still stable and doesn’t break apart.
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Scalar Field: A scalar field is a type of force or energy field that can affect particles. It’s a concept in physics used to describe certain forces that act in a smooth, continuous way. For example, gravity is a field that affects everything with mass. In this case, the scalar field is used to explain how dark matter particles interact with each other.
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Yukawa Interaction: The Yukawa interaction is a type of force that can act between particles. In simple terms, it’s a force that gets weaker the farther apart the particles are. In this article, it’s the force that helps bind dark matter particles together inside the Fermi ball.
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Saturation: Saturation happens when the Fermi ball reaches a certain point where it can no longer grow in size, even though it might be gaining more mass. This is due to the forces between the particles reaching a balance. Once saturation happens, the Fermi ball is “full,” in a way, and its size stays constant.
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Schwarzschild Radius: The Schwarzschild radius is the size of the region around an object where gravity is so strong that nothing can escape it, not even light. Once an object’s size becomes smaller than its Schwarzschild radius, it becomes a black hole. Think of it as a point where an object’s gravity becomes so intense that it pulls everything in, even light.
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Primordial Black Holes: These are black holes that are thought to have formed in the very early universe, shortly after the Big Bang. They are different from regular black holes that form from dying stars. Primordial black holes could have formed from the collapse of extremely dense objects like Fermi balls, before stars or galaxies existed.
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Cold Dark Matter (CDM): Cold dark matter refers to a type of dark matter that moves slowly compared to the speed of light. It’s "cold" because it doesn’t have a lot of energy or movement, unlike "hot" dark matter, which would move very fast. CDM is one of the leading candidates to explain the unseen matter in the universe that affects galaxies but doesn’t emit light.
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Scalar Potential (λφ⁴ term): In physics, a potential is a way to describe the energy associated with a field (like a scalar field). The term "λφ⁴" refers to a specific mathematical term in the equation for the scalar field’s potential. This term adds a repulsive force between dark matter particles and helps explain how Fermi balls grow and collapse.
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Phase Transitions in the Dark Sector: A phase transition is a change from one state to another. For example, water changes from liquid to solid when it freezes. In the context of the dark sector, a phase transition means a change in the state of dark matter, like when dark matter particles suddenly group together to form dense objects like Fermi balls.