Bulk viscosity and high-order pQCD in Quark Matter

by Saga Säppi (IEEC Barcelona)

Thursday 20 Nov 2025, 11:00 → 12:00 Europe/Amsterdam

Veltman Center (Nikhef)

Neutron stars offer a unique probe of high-density matter unlike any terrestrial experiments. Increasingly multi-faceted aspects of such matter have become relevant in recent years due to the detection of gravitational waves from collisions involving compact stars. The more dynamic events probe, for example, the transport properties of matter, leading to a need for a better theoretical description.

I will set up the seminar with a general introduction to how particle physics, and specifically quantum chromodynamics, enters in the study of distant astrophysical objects. Next, I discuss recent advances and the current state of the art of our understanding of cold (and cool) quark matter from first principles, specifically from the point of perturbative QCD, and how various effective field theories enter in the picture. In the last part of the talk, I discuss the nontrivial question of bulk viscosity in dense matter. I start with a simple model based on harmonic oscillations. I then generalise it to a more realistic case of a two-component fluid, making use of high-order, multiscale perturbative QCD as a way to obtain first-principles estimates for the bulk viscosity in quark matter. Lastly, I briefly discuss how the description readily generalises into an evolution equation of a relativistic n-component fluid.