Extremes 2025 abstracts

• Mark Alford (Physics, Washington University in St. Louis):  Quark matter: the high-density frontier

The densest predicted state of matter is color-superconducting quark matter, which has some affinities to electrical superconductors, but a much richer phase structure because quarks come in many varieties. This form of matter may well exist in the core of compact stars, and the search for signatures of its presence is currently proceeding. I will review the nature of color-superconducting quark matter, and discuss some ideas for finding it in nature.

• Sanjay Reddy (Physics, University of Washington): Transport Properties of Dense Matter and its Implications for Astrophysics  

I will review calculations of heat, charge, and lepton number transport in dense matter relevant to neutron star interiors. The implications of these transport processes for neutron star evolution and core-collapse supernova dynamics will be discussed. Following a survey of the current state of the field, I will highlight key open questions and outstanding challenges that remain at the forefront of research.

• Jorge Noronha (ICASU, UIUC): Emergent Viscous Hydrodynamics From a Single Quantum Particle

We present a concrete example showing how quantum decoherence in open quantum systems gives rise to hydrodynamic behavior at late times and long wavelengths. Focusing on a single non-relativistic quantum particle linearly coupled to a thermal bath of harmonic oscillators a la Caldeira and Leggett, we demonstrate that position-space decoherence renders the reduced density matrix nearly diagonal. Expanding around this diagonal limit, we derive a moment hierarchy akin to the BBGKY hierarchy which, when truncated at second order, yields transient viscous hydrodynamic equations similar to those used in heavy-ion collisions, with shear and bulk viscosities determined by decoherence and the coupling to the environment. In the asymptotic limit, these equations reduce to the Navier-Stokes equations, show here to emerge from decoherence. This suggests that even single-particle dynamics can exhibit collective behavior, offering insight into the success of hydrodynamics in small systems such as proton-proton collisions. We also comment on connections to strong-to-weak spontaneous symmetry breaking as a framework for understanding the emergence of hydrodynamics in open quantum systems.

• Dam Thanh Son (Physics, University of Chicago): The emergent "graviton" of the fractional quantum Hall effect 

In fractional quantum Hall states, electrons self-organize into a strongly interacting fluid with nontrivial emergent properties. It has recently been understood that fractional quantum Hall fluids accommodate one or several spin-2 excitations, which have been argued to be condensed-matter analogues of the graviton. In this talk we will review the origin of the idea of the graviton and the basic physics of the fractional quantum Hall effect. We then discuss a recent experiment claiming observation of "chiral graviton modes" and its broader implications.