# Upcoming seminars

**Cosmo:**Wednesday, 17/07/2024, 11:00

**Robert Brandenberger**(McGill University, Montreal)

*Searching for Cosmic Strings in New Observational Windows*

Cosmic strings are predicted in many particle physics models beyond the Standard Model. If such a model describes our universe, a network of strings inevitably forms in the early universe and persists to the present time. Cosmic strings lead to nonlinear density fluctuations at early times and will dominate the nonlinear structure at high redshifts. I will discuss how cosmic string loops dominate the halo mass function and can lead to early galaxy formation and to the formation of supermassive black holes at high redshifts. Long string segments lead to wakes which leave behind characteristic imprints in 21-cm redshift maps.

**Strings:**Monday, 05/08/2024, 14:00, Room 226

**Sebastian Murk**(OIST, Okinawa)

*Light rings and causality for nonsingular ultracompact objects sourced by nonlinear electrodynamics*

In this seminar, we will explore observational signatures of nonsingular ultracompact objects regularized by nonlinear electrodynamics (NED). Due to the phenomenon of birefringence, photons of different polarizations propagate with respect to two distinct metrics, which manifests itself in the appearance of additional light rings surrounding the ultracompact object. I will outline the physical consequences of this result and illustrate them based on three regular black hole models commonly considered in the literature. We will discover that nonsingular horizonless ultracompact objects sourced by NED possess an odd number of light rings and discuss the viability of NED as an effective description of their properties. Time permitting, we will compare the phase velocities of polarized light rays propagating in nonsingular NED geometries to that of the Schwarzschild spacetime and demonstrate that regularizing the singularity by means of a theory that does not adhere to the Maxwell weak-field limit may lead to the emergence of acausal regions. [Based on arXiv:2406.07957]

**Strings:**Monday, 26/08/2024, 14:00, Room 226

**Matthew Blacker**(DAMTP, Cambridge)

*Cosmological Wheeler DeWitt States and their Holographic Interpretation*

In the canonical quantization of gravity, the Hamiltonian constraint of general relativity is promoted to the Wheeler DeWitt (WDW) equation – the quantum cosmological analogue of the Schrödinger equation. Solving the WDW equation is challenging, but by assuming homogeneity and isotropy we can construct WDW states via the Hamilton-Jacobi formalism of classical mechanics. Such an approach lends itself towards a holographic interpretation, as the Hamilton-Jacobi equation also describes the holographic renormalization group flow. The first technical point of this talk is to construct a semiclassical wavefunction (in 3+1 dimensional pure gravity with a positive cosmological constant) which strongly localises on the de-Sitter Schwarzschild spacetime. The second aim is to conjecture a holographic duality between these WDW states and a family of quantum theories in the static patch, and extract information about those dual theories from the WDW state. These first two results are as presented in [2304.06865]. We will then, time permitting, establish a link between this approach and that of the TT-bar deformation, as presented in [2406.02508].

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