Original title: Tremaine-Gunn Control: Evading Bounds on Light Fermion Dark Matter
Joel Barir, Diego Blas, Anubhav Mathur, Tomer Volansky · September 2026
Scientists usually think galaxy-sized clumps of dark matter cannot be made of ultra-light particles called fermions. This rule, called the Tremaine-Gunn bound, comes from quantum physics: identical fermions refuse to squeeze into the exact same state, creating an outward push called degeneracy pressurePressure from the Pauli exclusion principle that resists squeezing particles too close together.. Because of this, very light particles would spread out too much to form small galaxies. Researchers found a way around this rule. They showed that an extra attractive force inside the dark sector, much stronger than gravity, can pull the particles together and balance that outward pressure. This allows particles as light as the eV scale to build dwarf galaxies. The force acts only on small scales and turns on late in cosmic history through a phase transitionA change in the state of matter or the universe's conditions, like the late-time shift proposed here..
Imagine packing identical chairs into a small room. There's a rule that no two chairs can occupy the same space, so you can only fit so many before they won't go in. But suppose each chair has a sticky side that lets them cling together without overlapping, allowing you to pack more than usual. In this paper, dark matter particles face a similar packing limit, but a special force acts like that sticky side, letting them group closely to form galaxies.
This changes how physicists search for dark matter, opening up a new range of extremely light particles that were thought impossible. It could help explain the nature of most of the universe's mass.
The work is a proof of concept and relies on a theoretical model with a late-time phase transition. It does not provide observational evidence or test the model against data. The mechanism is specific to fermionic dark matter and a scalar Yukawa force.
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Tremaine-Gunn Control: Evading Bounds on Light Fermion Dark Matter — Joel Barir, Diego Blas, Anubhav Mathur et al.
Abstract licence: CC0
Explanation generated by Synthesis from the abstract — it is not the original paper. This is a preprint, so it hasn't necessarily been peer reviewed.