Jean Alexandre (KCL)
"Three-dimensional exact Wilsonian approach on a cosmological background."
We derive a three-dimensional exact Wilsonian approach for a scalar field (the inflaton), on a classical cosmological background. The resulting Wilsonian flow for the inflaton potential allows us to go beyond the usual adiabatic approximation for quantum corrections. The effective dynamics of the inflaton is then coupled to the metric via (modified) Friedman equation, in order to study the evolution of the Early Universe. [Based on arXiv:2511.05296, J. A, L. Heurtier and S. Pla]
Fay Dowker (Imperial College London)
"To get to semiclassical gravity, we need to solve the cosmological constant problem"
I will argue that from a path integral perspective, the cosmological constant problem presents itself in a particular form. The gravitational path integral includes spacetimes that are nothing like the large smooth spacetimes of semiclassical gravity. There is evidence that these "unruly" spacetimes threaten to dominate the gravitational path integral due to the fact that there are so many of them compared to the large smooth spacetimes.
This is a struggle between Action and Entropy, if you will, and
I will explain why this is the path integral form of the cosmological constant problem. I will use causal set quantum gravity as an example of how (one aspect of) the cosmological constant problem might be solved in quantum gravity.
Hector Antonio Fernandez Melendez (University of Southampton)
"Testing modified gravity with Bose-Einstein condensates"
Ivette Fuentes (University of Southampton)
[TBA]
Stefano Galanda (University of York)
"The Semiclassical Einstein-Klein-Gordon System: Asymptotic Analysis of Minkowski Spacetime"
We establish in this talk the linear instability of the semiclassical Einstein–Klein–Gordon system linearised around the Minkowski vacuum spacetime. The proof relies on formulating a forcing problem for both metric and state perturbations within the space of past-compact sections. The metric perturbations are shown to be governed by a higher-order, nonlocal hyperbolic partial differential equation. By relegating the nonlocal contributions to subleading order, we establish the well-posedness of this forcing problem. Furthermore, we provide a rigorous asymptotic analysis for physically admissible choices of the renormalisation constants. We prove that the system exhibits a late-time linear instability: the metric perturbations grow exponentially, bounded strictly by a universal scale, thereby indicating a quantum backreaction-driven transition toward a de Sitter cosmological spacetime. Provided the parameters governing the system are restricted to a physically relevant regime, this universal scale is compatible with the measured expansion of our universe.
Steffen Gielen (University of Sheffield)
"Unitarity in unimodular time and singularity resolution"
In quantum gravity, the concept of time evolution is ambiguous due to general covariance. While there is no simple general resolution of this issue, in symmetric models with a preferred foliation one can work in unimodular gravity, where the preferred "time" is proportional to the spacetime four-volume, and is conjugate to the cosmological constant.
One can require that this form of time evolution be unitary.
I will show consequences of imposing unitarity in the setting of spherically symmetric black holes: the singularity is resolved by a transition to an asymptotic white hole. In a semiclassical approximation, one can obtain an analytical "quantum metric"
Atsushi Higuchi (University of York)
"Curious symmetries for free bosonic nonzero-spin fields in de Sitter spacetime"
Some symmetries for free bosonic nonzero-spin fields in four-dimensional de Sitter spacetime generated by conformal Killing vectors are presented. For the massless fields these and the Killing symmetries form an so(4,2) algebra. For the massive fields these symmetries do not seem to form a finite Lie algebra and appear to lead to infinitely many symmetries.
Daan W. Janssen (University of York)
"Making measurements in spacetime"
In semi-classical gravity, physically meaningful observables are diffeomorphism invariant, and field theories coupling to gravity are generally covariant (in particular yielding a covariantly preserved stress-energy tensor). Local measurement theories in quantum field theory often rely on breaking covariance in some way, by specifying a coupling between measurement probe and the system that is to be measured in some fixed spacetime region, as well as involving observables that fail to be diffeomorphism invariant. I will present ongoing work towards a covariant formulation of a measurement theory that is compatible with semi-classical gravity.
Karapet Mkrtchyan (Imperial College London)
"Manifest duality symmetry and Lorentz covariance in linearized gravity"
We present the first formulation of linearized gravity in four dimensions which is manifestly Lorentz
covariant and democratic, i.e. treats the two frames related by electric-magnetic duality on equal
footing. Four-dimensional linearized gravity belongs to a class of short (singleton) representations
of the four-dimensional conformal algebra so(2, 4). Our key insight is to use this algebra as the AdS5
isometry and realize the massless spin-2 field as edge modes of a five-dimensional topological field
taking values in a specific finite-dimensional representation of so(2, 4). The desired four-dimensional
action is then found by a covariant boundary reduction procedure.
Diego Pardo Santos (KCL)
"Entropy of asymptotically flat eternal quantum black holes in 2D"
Semi-classical dilaton gravity in (1+1)-dimensions remains one of the only arenas where quantum black holes can be exactly constructed, fully accounting for backreaction due to quantum matter. In this work, we present a comprehensive analysis of the entropy of static asymptotically flat quantum black holes both analytically and numerically. We first analyse eternal quantum black hole solutions in a one-parameter family of analytically solvable models interpolating between Russo-Susskind-Thorlacius and Bose-Parker-Peleg gravity. In the Hartle-Hawking state, we show that the Wald entropy of the semi-classical static black holes equals the entropy of classical black holes plus the entropy of the thermal bath generated by the quantum state. We then numerically construct eternal black hole solutions to semi-classical Callan-Giddings-Harvey-Strominger gravity and find that their thermal behaviour is qualitatively different from their analytic counterparts.
Joshua Procter (KCL)
"A positive mass theorem with averaged energy conditions"
The positive mass theorem shows that the ADM mass of an asymptotically flat spacetime is non-negative provided the dominant energy condition holds. However, pointwise energy conditions are known to be violated in a range of physically important settings, including non-minimally coupled classical fields and quantum field theory. In this talk, I will review Witten’s spinorial proof of the positive mass theorem before presenting a generalisation in which the pointwise dominant energy condition is replaced by an averaged energy inequality on the initial data. The proof follows the Witten spinor approach while incorporating the weaker energy assumption through suitable coercive estimates. I will then discuss examples showing how the required averaged inequality arises for the classical non-minimally coupled scalar field and from quantum energy inequalities, illustrating how positivity of the ADM mass can be established even in situations where the classical dominant energy condition is violated.
Patricia Ribes Metidieri (University of York)
"Partner Systems as a Probe of Entanglement Structure in Quantum Fields"
In this talk, I will present recent results on the definition and properties of partner systems in free quantum field theories in Gaussian states, both pure and mixed. I will then show how this framework can be used to characterize the distribution of quantum correlations in Minkowski spacetime and in the cosmological patch of de Sitter spacetime. These results provide new insights into how entanglement is distributed across spacetime and into the extent to which such correlations may be operationally accessible.
Kostas Tzanavaris (Max Planck Institute AEI)
"The Free Boundary Problem in General Relativity"
Even if a complete theory of fundamental physics were discovered tomorrow, one would still have to confront the question of initial conditions. Are they independent inputs, to be specified by hand, or can they be derived from a deeper principle?
In this talk, I will describe such a principle in the setting of general relativity. Rather than prescribing initial data at a spacelike singularity, we treat the singularity as a free boundary of spacetime and ask that the gravitational action be stationary under unconstrained variations there. This yields boundary conditions that restrict the class of admissible cosmological singularities. In particular, we show that conformally regular singularities are selected, BKL and Kasner-like singularities are ruled out, and, for linear perturbations around FLRW, the boundary conditions select the reflection-invariant modes which ensure conformal regularity.
Adam Wilkinson (University of Nottingham)
"Field back-action and energy conservation in the Unruh effect"
Unruh-DeWitt detectors have long served as a powerful tool for investigations into quantum fields in curved spacetimes, and the question of how these detectors perturb their environment has a long history of debate, beginning with early investigations from Unruh and Wald. I will be presenting a recent approach to field back-action completed in collaboration with Leo J. A. Parry, Jorma Louko, and William G. Unruh, which calculates perturbations in the two-point correlator of a field interacting with a particle detector, yielding a causal and covariant description of field back-action.
Using point-splitting renormalisation, we apply this framework to the computation of the stress-energy tensor of a field in flat spacetime, initialized in the Minkowski vacuum, interacting with a uniformly accelerated detector, a scenario in which the detector is known to thermalise to the Unruh temperature. We find an energy balance between the two systems: the energy flux in a small sphere around the detector exactly matches the rate of energy change within the detector. For a ground state detector, this corresponds to an inward flux of energy that matches the energy gained during the Unruh effect. Further, this inward flux is accompanied by regions of negative energy density in the field in both Rindler and Minkowski coordinates.