A neutron star is the leftover core of an exploded giant, compressed to the size of a city. A teaspoon of its matter weighs a billion tons. Inside such a star, particles are packed so tightly that quantum effects start to show. One of them is the intrinsic rotation of particles, or spin. Usually, spin is averaged out and forgotten in calculations, but the authors of this study decided to see what happens if we take it seriously.

They used a theory of gravity with torsion, as if spacetime not only bends with mass but also twists around spins. In such a theory, spin stops being a passive observer and actively shapes the geometry. To describe matter, they used a spin-fluid model (Weissenhoff fluid), where each element of the medium carries its own angular momentum. The equations were reduced to Einstein's form, but with an effective addition: spin creates additional pressure and modifies the energy density.

Numerical simulations with a realistic equation of state (DD2) showed that for a positive coupling constant, spin makes the star more compact. Its radius shrinks, the maximum possible mass drops, and gravitational binding weakens. Interestingly, the difference between isotropic and anisotropic spin distributions turned out to be negligible — at least for the smooth polarization profiles considered.