ICTS

The celebrated Arrhenius law (AL) captures the activation time of a single diffusing particle from a metastable state, yielding an exponential dependence on the activation barrier. We use Macroscopic Fluctuation Theory to extend this law to many diffusing and interacting  particles. Our analysis reveals a new universality class of activation, but also retains the Arrhenius universality depending on the nature of the inter-particle interactions. 

I will discuss two settings where the variational problem within MFT can be solved to yield explicit results on nonequilibrium fluctuations. The first concerns tracer and current statistics in a single-file gas of Brownian hard rods. Exploiting a duality with a point-particle system, we exactly compute the cumulant-generating functions and optimal fluctuation paths for two domain-wall initial ensembles. Our results reveal an intriguing connection with the height field of the dual point-particle system, linking single-file transport to interface dynamics. The second concerns steady-state density large deviations in boundary-driven diffusive systems with a bulk drive. Using an auxiliary-field transformation, we obtain perturbative results for generic systems, including in arbitrary dimensions. Our formulation reveals the dynamical origin of long-range correlations in the steady state and how the onset of non-locality depends on the bulk drive.

This pedagogical lecture will discuss a discrete formulation of macroscopic fluctuation theory (arising from the large-spin limit of interacting particle systems) and duality properties.

In the first part, I will introduce Markov duality and explain how it provides an effective tool for studying non-equilibrium stochastic systems. The harmonic process—an integrable model of heat conduction—will serve as the main example. I will review its algebraic structure and show how duality gives access to correlation functions and to an explicit characterization of the non-equilibrium stationary state. This exact microscopic information makes it possible to verify some predictions of macroscopic fluctuation theory, while suggesting the need for extensions to include singular profiles.

In the second part, I will discuss dynamical large deviations in the large-spin regime. For a large class of models, I will explain how a path-space large-deviation principle leads to a finite-dimensional Hamiltonian theory on the lattice—a discrete counterpart of the usual macroscopic fluctuation theory. The associated Hamilton equations and variational principles describe the optimal trajectories responsible for rare density and current fluctuations. I will conclude by discussing the connections among discrete and continuum MFT and fluctuation symmetries.

In active systems detailed balance is broken at the level of each individual constituent. In this talk I will show that, in several cases, the probability of nucleating the stable state from a metastable one can be computed analytically although the instanton is not the time-reversal of the relaxation dynamics. We will furthermore discuss that their stationary measure can be generically expected to be non-local whenever at least one conservation law is present, and that this property has a surprising relation with nucleation probabilities. Our results will be mostly based on the weak-noise regime of a number of field theories that were proposed to describe active systems in the past.

Macroscopic Fluctuation Theory (MFT) describes non-equilibrium large deviations through non-linear coupled PDEs called the MFT equations. Recent works, including the one by Mallick, Moriya and the lecturer[1], showed that these MFT equations for the symmetric simple exclusion process (SEP) and a few other models can be mapped to classical integrable systems and can be solved via the Inverse Scattering Methods (ISM).

In this two-hour pedagogical lecture, we provide an introduction to inverse scattering methods applied to MFT. We begin with the conventional continuous MFT for systems such as the SEP, illustrating how the underlying variational equations can be mapped to a classical integrable system called the AKNS system and can be solved through Lax pairs and spectral theory. We then extend these techniques to discrete lattice systems.

References:
[1] K. Mallick, H. Moriya, T. Sasamoto, Phys. Rev. Lett. 118, 160601 (2022).
[2] K. Mallick, H. Moriya, T. Sasamoto, JSTAT {¥bf 2024} 074001 (2024). 

We consider a system of N particles interacting via stochastic binary collisions, like the celebrated Kac's model. We discuss the large deviations asymptotic of the empirical measure and flow in a fixed time interval, reviewing some relevant results and open problems. As an application, we discuss the probability of fluctuations of the total number of collisions in the joint limit in which both the number of particles and the time interval diverge.

TBA

While Macroscopic Fluctuation Theory (MFT) has been highly successful in analyzing non-equilibrium steady states, its application to non-steady-state processes remains limited. In this talk, we will present our MFT analysis of the relaxation process of one-dimensional boundary-driven diffusive systems coupled to particle reservoirs at both ends. We derive the MFT action for the integrated current and obtain the corresponding MFT equations, together with their initial and boundary conditions. Using this formulation, we exactly derive the current variance for systems with a constant diffusion coefficient and arbitrary mobility, and the cumulant generating function for the current in Reflective Brownian Motion (RBM). Our results demonstrate that non-steady current fluctuations during the approach to a steady state can be quantitatively described within the MFT framework.
This talk is based on joint work with Tomohiro Sasamoto (arXiv:2605.27275).

I these lectures I will pedagogically cover the Ballistic Macroscopic Fluctuation Theory, a hydrodynamic fluctuation theory for systems admitting ballistic transport. As time permits, I will explain: (1) its basic physical principles, (2) its action formulation, which adapts the Macroscopic Fluctuation Theory to ballistic transport and gives equations for the large-deviation theory of currents, (3) its observable-based formulation, which connects with fluctuating hydrodynamics and gives predictions for Euler-scale correlation functions, (4) its connection with the Ballistic Fluctuation theory, which provides an alternative framework for the large-deviation theory of currents. Throughout, I will try to show the examples of many-body integrable systems concentrating on the hard rods for simplicity (as linearly degenerate systems without shocks), and TASEP (as truly non-linear systems admitting shocks). This is based on various works including with Gabriele Perfetto, Tomohiro Sasamoto and Takato Yoshimura, and with Jason Myers, M. J. Bhaseen and Rosemary J. Harris.

Genetic drift plays a crucial role in evolution by enhancing diversity within ecosystems. On the other hand, active self-propulsion can drive systems into phase separated regions promoting homogeneity within a spatial region. To analyze the intricate effects produced by such competing processes, we introduce a minimal model for a proliferating active population, where run-and-tumble migration is combined with a stochastic birth-death process. Beginning with the microscopic dynamics of the particles, we derive the corresponding hydrodynamic equations and validate them through numerical simulations. We demonstrate that the inclusion of activity leads to the emergence of novel morphological patterns, which are sensitive to the specific rules governing migration. Furthermore, we show that activity significantly alters the characteristics of invasion waves, increasing their propagation speed. However, beyond a threshold, excessive activity disrupts the wave-like nature of the invasion process. We also extend our study to two dimensions and analyze the instabilities that lead to pattern formation in such systems.

(Joint works with Gideon Amir, Ofer Busani and Ellen Saada)

Abstract: We study invariant measures and hydrodynamic limit for multilane asymmetric exclusion process.
The invariant measures problem is intermediate  between well understood 1d and far less understood multi-d.
We identify a domain of parameters where a full characterization can be obtained including 2D like blocking measures.
We then study lane by lane hydrodynamic limit under a double horizontal-vertical scalling and prove that the limit obtained coïncides with 
the limit of a hyperbolic system of conservation laws with relaxation. In the two-lane case we obtain a complete two-parameter phase diagram of
the current-density relation illustrating the competition between lanes.

Can a deterministic evolution remain random when its microscopic uncertainty vanishes? Turbulence offers a striking setting for this question: as viscosity tends to zero, infinitesimal perturbations can produce finite differences in the flow within a finite time. This phenomenon, known as spontaneous stochasticity, goes beyond the familiar sensitivity to initial conditions of deterministic chaos. I will introduce this perspective through the interplay between singular dynamics, persistent energy dissipation, and loss of predictability. Using examples ranging from an unstable shear layer to three-dimensional turbulence, I will discuss numerical evidence that uncertainty spreads from small to large scales through a statistically self-similar process, largely independent of how the perturbations are introduced. Particular attention will be paid to universality and to the anomalous scaling of error growth associated with intermittent fluctuations.

We consider systems of interacting particles described by an underdamped (second order) Langevin equation. We introduce an associated equation of fluctuating hydrodynamics, which can be interpreted as a stochastic version of a Vlasov-Fokker-Planck equation. We show that a dichotomy previously known for purely diffusive systems holds in this setting as well: solutions exist only for suitable atomic initial data, but not for smooth initial data. The class of systems covered includes several models of active matter. We will then sketch how first order fluctuating hydrodynamics can be used to learn the associated evolution operator for the hydrodynamic limit in thermodynamic form, with rigorous error estimates for the symmetric exclusion process.

We introduce a novel exclusion process with two conservation laws, mass and energy, designed to mimic continuous systems like interacting oscillators. Unlike conventional multi-species processes where only particle numbers are conserved, our model exhibits a wide variety of universality classes under nonlinear fluctuating hydrodynamics (NFH) depending on parameter choices. In this talk, we study the stationary fluctuations of these conserved quantities. We rigorously demonstrate that, under a suitable choice of parameters, the fluctuation fields converge to uncoupled stochastic Burgers equations (SBE) in the scaling limit. In addition, we present a general and rigorous proof of the diagonalizability of the macroscopic current's Jacobian matrix with distinct real eigenvalues. This mathematical justification addresses a property often assumed without proof in the physics literature, offering a firm foundation for a broad class of multi-component systems. This talk is based on a joint work with Hugo Da Cunha.

We derive the structure of macroscopic dynamical fluctuations for nonequilibrium systems satisfying local detailed balance. From that follows the structure of relaxation equations to steady nonequilibria. This approach does not start from more microscopic interacting particle systems but immediately gives the form of thermodynamically consistent relaxation, much in the spirit of Onsager's project. We give examples, including a modification of the Lorenz model to a recharge-discharge oscillator (killing its butterfly).

This is an introductory talk on key features of Generalized Hydrodynamics. In particularly, covered are generalized Gibbs ensembles including their  large deviations and the role of the time-dependent random Lax matrix.

I will discuss numerical methods for weak-noise field theories such as macroscopic fluctuation theory and the KPZ equation at short times, with a focus on computing optimal fluctuation paths and the Gaussian fluctuations around them. Instead of solving the forward-backward Hamiltonian equations directly, we treat the noise as a control. This turns the problem into a constrained optimization that only needs initial value problems, works well with automatic differentiation and standard optimizers, and can handle non-convex rate functions through an augmented Lagrangian. As an example, I will show how this approach revealed dynamical phase transitions and soliton-driven optimal paths for the average height of a KPZ interface on a ring, and how the numerics and asymptotic analysis informed each other. Finally, I will explain how to go beyond leading order and compute the prefactor of the large-deviation asymptotics, either from a Riccati equation or from a Fredholm determinant in noise space, using 1D Burgers turbulence as an example.

Irreversible aggregation is a paradigmatic nonequilibrium process, with the product-kernel serving as a canonical model for finite-time gelation. While its typical mean-field behavior is well understood, an exact theory for its rare fluctuations has remained elusive. In this talk, I will present an exact fluctuation theory for the product-kernel using a canonical path-integral formulation. By exactly solving the associated Euler-Lagrange equations, we determine the explicit instanton trajectories, revealing a dynamical phase transition where a macroscopic gel emerges as a time-dependent background potential. This framework yields the complete, non-convex LDF and the exact phase diagram. We demonstrate ensemble inequivalence between canonical and grand-canonical descriptions. Unlike equilibrium systems where inequivalence stems from non-additive Hamiltonians, here it arises purely from the global, dynamical coupling inherent in the collision rates.

We study the current fluctuations of the one-dimensional symmetric exclusion process evolving under a quenched initial condition. The macroscopic fluctuation theory is employed to probe the fluctuations and it is subsequently reformulated in terms of the potential in the inverse scattering method, leading to the boundary equation that determines the final profile of the potential. Then, we solve this nonlinear functional integral equation by applying the perturbation method up to the sufficient order to compute the fourth cumulant.