Speaker
Description
Observations of neutron star collisions have brought about a plethora of ways to access novel properties of compact stars. Amongst them are transport phenomena: The dynamic nature of a neutron star collision probes the transport properties of extremely dense matter. Bulk viscosity is of particular importance, and is thought to leave an imprint on the gravitational wave signal emanating from the collision.
I will discuss the complex problem of understanding the physics of bulk viscosity in the quark matter phase. This requires not only a good grasp of the equation of state of the system under general conditions---in addition to density, mass and temperature also play a vital role---but also of the hydrodynamics of relativistic viscous systems.
While the simplest possible description of viscous dense matter is possible with the so-called Israel--Stewart equation of hydrodynamics, it turns out that a more accurate description requires a generalisation of the system to a two-component fluid. I will explain both the relevant equations as well as their implications in 2+1-flavour quark matter, and lastly show how the hydrodynamic framework extends elegantly to arbitrarily many components, relevant for systems with additional degrees of freedom.