A balloon in your hand springs back equally in all directions. But a bottle of soda behaves differently: along the neck it is stiff, while across it is easily squeezed. This is roughly how pressure works inside some stars: it can be unequal in different directions. Such stars are called anisotropic, and their behavior has so far been described by disparate methods.
The authors of the new work have created a unified mathematical framework that allows describing radial pulsations — the trembling of a star as it contracts and expands while maintaining a spherical shape. Within this framework, they accounted for the thermal properties of matter, the pressure difference, and even the speed at which heat spreads through the star. This is not just a beautiful theory: for the first time, three known models of heat transfer have been compared — from the simplest (like an iron heating fabric) to more complex ones where heat propagates as waves.
One of the results is a new exact model of a compact star — a solution to Einstein's equations where the radial and tangential pressures differ. Using this model, they showed that when a star becomes too dense (its compactness exceeds a certain threshold), even a slight shake causes it to contract irreversibly. This threshold — about 0.78 of the maximum possible compactness for ordinary stars — turned out to be universal across all three theories of heat transfer.
as written by the authors arXiv:2604.20960
We present a covariant and gauge-invariant formulation of the theory of radial adiabatic linear perturbations of self-gravitating, non-dissipative imperfect fluids within the theory of general relativity. By codifying the thermodynamical properties of the source into an equation of state and an ansatz on anisotropic pressure that involves both matter and kinematic variables, we obtain a set of equations that is directly applicable to a wide variety of thermodynamic theories for matter fields. As examples, we evaluate and compare the predictions of the Eckart theory, the Bemfica-Disconzi-Noronha-Kovtun theory, and the Truncated Israel-Stewart theory on the properties and evolution of radial adiabatic perturbations of stellar compact objects modeled by classical equilibrium solutions. Introducing a new solution of the Einstein field equations, and imposing causality, we propose an upper bound for the maximum compactness of dynamically stable stars with non-trivial radial and tangential pressures.
The densest matter in the universe is found inside neutron stars: a single teaspoon of such matter weighs as much as Mount Everest.
In Robert Forward's novel "Dragon's Egg," life is described on the surface of a neutron star, where enormous gravity and anisotropic pressure create exotic forms of matter.