# VandyGRAF Initiative

The Vanderbilt Initiative for Gravity, Waves, and Fluids is an interdisciplinary research venture providing mathematicians, physicists, and astrophysicists with the resources and space to connect and collaboratively work on problems of outstanding scientific merit, such as:

- General relativity: theoretical, mathematical, numerical, or experimental, including, but not restricted to, black holes, gravitational radiation, and multimessenger astrophysics.
- Fluid mechanics: theoretical, mathematical, numerical, or experimental, including, but not restricted to, relativistic fluids far from equilibrium.
- Evolution of partial differential equations related to fluids and gravity, including, but not restricted to, the geometric analysis of waves and fluids.
- The physics and mathematics of neutron star mergers and high-energy nuclear collisions.

## VandyGRAF Fall Seminar Series

**All VandyGRAF talks will take place in SC 6333, unless indicated below.**

### Andrew Chael, *Princeton University*

**December 1, 12:00 pm - 2:00 pm**

##### Polarized Images of Black Holes

*Abstract:* Extragalactic jets seen throughout the universe transport energy from small scales near a galaxy’s central supermassive black hole to large distances outside the galaxy itself. These jets may be launched via the Blandford-Znajek mechanism, where magnetic fields around extract a black hole’s spin energy. In this talk, I will discuss what polarized Event Horizon Telescope (EHT) images of synchrotron radiation from close to the event horizon of the supermassive black hole M87* tell us about black hole magnetic fields and jet launching. Polarized synchrotron radiation observed by the EHT probes the plasma properties and the structure of magnetic fields near the black hole. In a large library of simulated images from general relativistic magnetohydrodynamic (GRMHD) simulations, the only models consistent with both EHT polarized images and the observed jet power from M87* are magnetically arrested accretion disks, where near-horizon magnetic fields are coherent and dynamically important. Recent work constraining the resolved circular polarization around M87* confirms that the accretion disk is most likely magnetically arrested. The pattern of linear polarization in black hole images directly probes the direction of electromagnetic energy flux; EHT images show that electromagnetic energy flows outward on horizon scales around M87*. Future EHT observations of M87 will be sensitive enough to detect fainter emission both closer to the event horizon and farther downstream in the jet launching region. These observations will enable a definitive test of the Blandford-Znajek mechanism for powering extragalactic jets.

### Nicki Mullins, *University of Illinois Urbana-Champaign*

**December 08, 12:00 pm - 2:00 pm**

##### Stochastic fluctuations in relativistic fluids: causality, stability, and the information current

*Abstract:* The study of the quark-gluon plasma in high-energy nuclear physics has generated significant research into relativistic fluid dynamics. While hydrodynamic simulations have been used to model various phenomena in heavy-ion collisions, the commonly used formulations are incomplete, as they are inconsistent with basic thermodynamic principles. In particular, they lack the thermal fluctuations necessary to ensure the system realizes the correct equilibrium distribution. In this talk, I will discuss a new means of modeling thermal fluctuations in relativistic systems that ensure that the correlators are independent of the choice of spacetime foliation. This formulation is constructed using the recently developed information current, which is instrumental when studying causality and stability in relativistic systems. By using the information current to describe fluctuations, this formulation builds constraints associated with these effects directly into the modeling. Using this new method of describing fluctuating relativistic systems, we will construct an effective action and show that a fluctuation-dissipation relation can be realized in this action through a symmetry associated with detailed balance.