We will explain the construction of the euclidean vacuum state for linearized gravity on de Sitter spacetime by a rigorous version of Wick rotation. We will discuss issues related to gauge invariance, positivity and invariance under de Sitter isometries.

The analysis of quantum states in non-smooth spacetimes has two main motivations. First, there are several models of physical phenomena that require spacetime metrics with finite regularity. These include models of gravitational collapse, astrophysical objects and general relativistic fluids. Second, the well-posedness of Einstein’s equations, viewed as a system of hyperbolic PDE requires...

We consider how to formulate semiclassical problems in the context of the AdS/CFT correspondence, based on the proposal of Compere and Marolf [arXiv:0805.1902]. Our prescription involves the effective action with self-action term for boundary dynamical fields, which can be viewed as imposing mixed boundary conditions for the gravity dual. We derive the semiclassical Einstein equations sourced...

I review the status of the asymptotic safety program which posits a nonperturbative ultraviolet completion of gravitational quantum field theory. In particular, I put emphasis (i) on systematic extensions of approximation schemes, (ii) on the role of field redefinitions, and (iii) on observable consequences.

The latter naturally connects the asymptotic safety program to an effective field...

Motivated by a long-standing aim to understand the emergence of spacetime and its relation to entanglement in the context of gauge/gravity duality, we study the relations between subsystem entanglement entropies. These quantities are delimited by the so-called holographic entropy cone, characterized conveniently by holographic entropy inequalities or alternately by certain extreme states. ...

It is well known that the expectation value of the stress-energy tensor for a quantum field must be renormalized. While there exists a well-understood formal resolution to the renormalization problem, the practical implementation is technically difficult in black hole spacetimes. The first successful computation of the renormalized stress-energy tensor in a black hole spacetime dates back to...

"Facets of Unitarity Violation in Dynamical Spacetimes" by Ka Hei Choi (Ludwig Maximilian University of Munich);

"Gravity and the Superposition Principle" by Hristu Culetu (Ovidius University);

"The master Dyson-Schwinger equation and the Wilsonian renormalization group flows in a generally covariant setting" by Andras Laszlo (Wigner Research Centre for Physics, Budapest);

"Uniformly...

The event horizon of a dynamical black hole is generically a non-smooth hypersurface. I shall describe the types of non-smooth structure that can arise on a horizon that is smooth at late time. This includes creases, corners and caustic points.

I shall discuss ``perestroikas'' of these structures, in which they undergo a qualitative change at an instant of time. A crease perestroika gives an...

During this talk we shall discuss some properties of the semiclassical Einstein equation.

This equation is used to model the backreaction of quantum matter on classical backgrounds.

In this talk we shall in particular analyze the case of backgrounds which describe cosmological spacetimes.

The quantum field we shall consider is massive and satisfies a linear equation with a generic coupling...

In order to study physical effects of quantum fields on curved spacetimes, one needs appropriate

Hadamard states to describe the fields. In this talk, we present a rigorous construction, including

the proof of the Hadamard property, of the Unruh state for the free scalar field on slowly rotating

Kerr-de Sitter spacetimes. We sketch how this state can be used to compute the stress-energy...

Black hole spacetimes harbor intricate internal structures, featuring geometry that extends through an inner horizon to another external universe. This regularity of the inner horizon, which plays the causal role of a Cauchy horizon, challenges predictability within black holes. The strong cosmic censorship conjecture offers a solution, asserting that the Cauchy horizon becomes sufficiently...

A characteristic feature of semiclassical theories of gravity involving the backreaction of a quantum matter field is the presence of higher order derivatives in the dynamical equations. Hence, the appearance of pathological "runaway solutions" is often argued, i.e., solutions of the linearized equations around a background spacetime that grow exponentially in time. In this talk, this issue is...

I will show that the repulsive effects associated to the zero-point energies of quantum

fields are capable of supporting ultracompact stars that overcome the compactness

limits present in general relativity for any sphere in hydrostatic equilibrium. These

objects are self-consistent solutions in semiclassical gravity that incorporate

the backreaction of the renormalized stress-energy...

Many results in general relativity rely crucially on classical energy conditions inflicted on the stress-energy tensor. Quantum matter, however, violates these conditions since the energy density can fluctuate and in particular become arbitrarily negative at a point. Nonetheless quantum matter should have some reminiscent notion of stability, which can be captured by so-called quantum (weak)...

Some of the quantum properties of rotating black holes in (3+1)-dimensional spacetimes remain unresolved. However, studying higher-dimensional cases may provide insight into their behaviour. In this talk, we investigate the behaviour of a massive scalar field in a Kerr-AdS (4+1)-dimensional spacetime. Specifically, we focus on the existence of a Hartle-Hawking state, which is a vacuum state...

We review the procedure to construct ground and KMS states, for real scalar fields whose dynamics is dictated by the Klein-Gordon equation, on standard static Lorentzian manifolds with a time-like boundary. We observe that this construction, depending on which boundary condition we fix on the boundary, does not always lead to a bi-distribution $w_2\in \mathcal{D}'(M\times M)$ for the two-point...

It is well-known that the (1+1) dimensional Schwarzschild and spatially flat FLRW spacetimes are conformally flat. This work examines entanglement harvesting from the conformal field vacuums in these spacetimes between two Unruh-DeWitt detectors, moving along outgoing null trajectories. In (1+1) dimensional Schwarzschild spacetime, we considered the Boulware and Unruh vacua for our...

A fresh attempt towards a quantum field theory of gravitons

interacting with matter is based on the Fock space quantization of

Wigner's helicity 2 representation, rather than canonical quantization

of a massless rank-2 tensor field. In this way, all issues with

indefinite state spaces, overshooting degrees of freedom, gauge fixing,

etc. are avoided. It turns out that quantum consistency...

The construction of a quantum gravity theory remains a challenge. One of the difficulties stems from the lack of sufficient experimental evidence. As a first step toward the quantum gravity experiment, Bose et al. proposed a low-energy experiment to test if Newtonian gravity can generate quantum entanglement or not. However, they assumed that only gravity is mediating in the system, and their...

The role of entropy, in particular entanglement entropy, in quantum field theory has become increasingly prominent, and entropy has appeared in relation with several primary research topics in QFT: area theorems, c theorems, the AdS/CFT correspondence, quantum null energy inequalities etc. While von Neumann entropy, the basic concept in information theory, is divergent in QFT and can thus only...

Recently, various experiments have been proposed to verify quantum entanglement induced by Newtonian gravitational interactions. However, no feasible setup has yet been found that is certainly achievable with existing techniques. To search for an optimal setup, we compute the logarithmic negativity of two oscillators with arbitrary quadratic potential and coupled by gravity. We find that...

Since the discovery of the Bekenstein-Hawking formula, there had been many attempts to derive the entropy of black holes from the entanglement between the degrees of freedom inside and outside the event horizon. This entanglement entropy reproduces the area-law, but it suffers from divergences in the continuum limit. In this talk, I show how to derive the Bekenstein-Hawking entropy from the...

We study the renormalised stress-energy tensor (RSET) for a massless, conformally

coupled scalar field on four dimensional anti-de Sitter space-time (adS4). As ads4 is

not a globally hyperbolic space-time, we impose boundary conditions on the space-time

boundary to have a well posed quantum field theory. We use Dirichlet, Neumann and

Robin (mixed) boundary conditions applied to the scalar...

We show that if a massive body is put in a quantum superposition of spatially separated states, the mere presence of a black hole in the vicinity of the body will eventually destroy the coherence of the superposition. This occurs because, in effect, the gravitational field of the body radiates soft gravitons into the black hole, allowing the black hole to harvest "which path'' information...

A long-standing problem in QFT and quantum gravity is the construction of an “IR-finite" S-matrix. In the gravitational case, the existence of these “infrared divergences” is intimately tied to the “memory effect” (i.e. the permanent displacement of test masses due to the passage of a gravitational wave). In this talk, I shall explain the origin of these connections and illustrate that the...

We consider the global Hadamard condition and the notion of Hadamard parametrix whose use is pervasive in algebraic QFT in curved spacetime (see references in the main text). We point out the existence of a technical problem in the literature concerning well-definedness of the global Hadamard parametrix in normal neighbourhoods of Cauchy surfaces. We discuss in particular the definition of the...

In this talk I will discuss how the algebraic approach to (perturbative) quantum field theory can benefit the search for the theory quantum gravity through several different approaches. I will discuss the problem of constructing gauge invariant observables, the asymptotic safety program and consequences of potential discreteness of spacetime at small scales.

We present a regularization prescription for Hadamard two-point functions defined on the boundary of a lightcone, which can be used to analyze renormalized quantities for linear scalar fields in a curved background. This can be applied to formulate the semi-classical Einstein equations as a characteristic initial value problem. Furthermore, we shall discuss how these tools can be used to...

We will discuss some newly found solutions to the full massless semiclassical Einstein equation (SCE) in a cosmological setting (with Λ=0).

After a short introduction to the relevant notions we present the SCE in a particular shape which allows for the construction of certain vacuum states. These states may be viewed as as the least possible generalization of the Minkowski vacuum to general...

What are the essential aspects of quantum theory needed in order to understand compact relativistic objects? Relying solely on universal properties of QFTs at high energies, we show that as a star contracts towards its Buchdahl radius the effects of the trace anomaly become macroscopic at densities much below the Planck scale. As a consequence the unstable modes of scalar fields disappear,...

We confirm that Hawking radiation from an analogue black hole in a Bose-Einstein condensate is spontaneous, thermal, and stationary. Furthermore, we follow the time evolution of the Hawking radiation, and compare and contrast it with the predictions for real black holes. We observe the ramp up of the Hawking radiation, similar to a real black hole. The end of the spontaneous Hawking radiation...

We investigate the roots of unitarity violations of quantum field theory in dynamical spacetimes

and its connection to back-reactions and measurement processes. Within the framework of effective

field theories, local observables require compact configuration spaces given by the domain

of validity of the effective description and in accordance with detector specifications. Using the...

We use C.Kiefer’s idea that QG means any theory in which the superposition principle (SP) is applied to the gravitational field. For masses above the Planck mass, we conjecture that the

Schrodinger equation should contain the universal constant G and c, instead of the Planck constant h. The wave packet expansion depends on G, c and the acceleration $Gm/σ^2_0$, where $σ_0$ is the initial width...