Seminars

Abstract: We study the late-time relaxation of a neutral relativistic viscous fluid in d+1 dimensions.In the long-wavelength regime, linearized hydrodynamics predicts that the sound mode at momentum nk decays as e^{-i n^2 \omega_I t}, However, nonlinear analysis gives a decay of e^{-i n \omega_I t}. We derive a closed asymptotic attractor solution in which the frequency of the n-th harmonic locks to n times the complex frequency of the fundamental mode. The amplitude envelopes for energy current J obey a simple cascading relation,J_n=\alpha_J^{n-1}J_1^n with \alpha_J is fixed by the equation of state, the longitudinal viscosity, and the fundamental wavenumber. For conformal fluids,\alpha_J=1/8\eta k, in agreement with the holographic result of arXiv:2512.07242. The existence of the attractor shows that, even near equilibrium, field powers are not equivalent to amplitude order. 

Abstract: We consider an M theory/string theory approach to support the fact that a four-dimensional effective field theory description with de Sitter isometries can exist if de Sitter space is realised as a coherent state. 

Abstract: In my talk I introduce quantum mechanics in moduli scape of string theory.  We will discuss the moduli space wave function, which describes a quantum particle moving in moduli space. We discuss the spectrum of the associated moduli space Laplacian, showing that in contains normalizable bounds states of positive energy and fixed values of the moduli fields in case the moduli space is compactifiable.  In addition we also include a potential, where quantum effects can result in moduli localised away from classical minima and in excited, positive energy states, even in cases where the classical potential exhibits a runaway behaviour.  Finally employing the well-known Wheeler-DeWitt equation of quantum gravity and combining it with  moduli space quantum mechanics,  we show how quantum effects can lead to an excited  de Sitter universe with a positive effective cosmological constant. 

Abstract: In this colloquium I will give a short introduction to Quantum Gravity and its realisation by String Theory. As known for some time, string theory predicts a huge landscape of vacua, seen as different effective theories at low energy. However, as it became evident during the recent years that the space of theories that cannot originate from string theories is much larger than the landscape, and hence usually dubbed as the so-called swampland. I will discuss some of the properties of the swampland scenario and how it has recently led to some predictions about dark energy, dark matter and primordial black holes. 

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Abstract: I will show that traversable wormholes can consistently exist in theories of quantum gravity. From the perspective of external observers, travel through the wormhole is a form of quantum teleportation. I will further discuss other surprising aspects of the nature of quantum information in dynamical spacetimes, and how emergent gravitational descriptions can give new insight into quantum phenomena. 

Abstract: There exist classical solutions with an expanding dS region connected to an asymptotically AdS region through an ER bridge, in gravitational theories admitting dS and AdS minima of the effective potential. I will explain how these are dual to pure states of the unitary CFT at the AdS boundary, that can be produced by complex path integrals, with examples in 2d and 3d gravity. These results lead to a new formalism for how to use and interpret dS/CFT. 

Abstract: I  consider the formation of a near-extremal Reissner-Nordstrom black hole by collapse, and show how to compute correlations in the outgoing Hawking radiation due to enhanced gravitational backreaction effects in the near-horizon region. This is done by reducing to the s-wave and employing the Hamiltonian formulation of Einstein-Maxwell theory coupled to a scalar field.  We show how the standard Schwarzian quantum mechanics approach arises in a suitable late time regime.

Abstract: Critical collapse provides a dynamical route from smooth initial data to a naked singularity at the threshold of black hole formation. This makes it a particularly sharp setting in which quantum effects may play a role. In this talk, I will discuss one-loop backreaction originating from the self-energy of collapsing matter in near-critical Einstein-scalar systems, using controllable continuously self-similar (CSS) backgrounds. We found that vacuum polarization produces a universal quantum growing mode whose Lyapunov exponent is fixed kinematically by self-similarity to be (D-2). This produces a growing response that competes with the classical unstable mode. Horizon tracing then shows a shift of the collapse threshold, and the emergence of trapped surfaces near the new threshold, turning the classical Type II transition into Type I-like behavior with a finite mass gap. Extending the analysis to the exterior suggests that the classically naked endpoint is instead hidden behind a quantum-generated horizon.

The exponent (D-2) grows without bound in the large-D limit, suggesting an arbitrarily steep quantum growing mode. I will also discuss how recent large-D extensions of CSS collapse support the universality of this scaling. Furthermore, the analytic large-D discretely self-similar (DSS) solutions constructed recently open a route to studying one-loop vacuum polarization directly on DSS backgrounds, paving the way toward the genuine 4D DSS Choptuik spacetime.

If time permits, I will also comment on the recently emerging picture of extremal black holes as critical endpoints of classical gravitational collapse, its connection to violations of the third law of black hole thermodynamics, and what quantum gravity might say about this picture. 

Abstract: I will begin by motivating radiative memory: a lasting change left behind by the passage of radiation. I will then describe a memory effect produced by phonon scattering in a superfluid, providing an analogue of electromagnetic and gravitational memory. A localized scattering event generates a far-field pressure pulse whose time integral vanishes but whose first temporal moment is nonzero. I will explain how this effect is controlled by the soft factor for phonon emission, including corrections from nonlinear dispersion.

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