Renormalization of Perturbative Quantum Gravity

Abstract: General Relativity and Quantum Theory are the two main achievements of physics in the 20th century. Even though they have greatly enlarged the physical understanding of our universe, there are still situations which are completely inaccessible to us, most notably the Big Bang and the inside of black holes: These circumstances require a theory of Quantum Gravity — the unification of General Relativity with Quantum Theory. The most natural approach for that would be the application of the astonishingly successful methods of perturbative Quantum Field Theory to the graviton field, defined as the deviation of the metric with respect to a fixed background metric. Unfortunately, this approach seemed impossible due to the non-renormalizable nature of General Relativity. In this talk, I aim to give a pedagogical introduction to this topic, in particular to the Lagrange density, the Feynman graph expansion and the renormalization problem of their associated Feynman integrals. Finally, I will explain how this renormalization problem could be overcome by an infinite tower of gravitational Ward identities, as was established in my dissertation and the articles it is based upon, cf. arXiv:2210.17510 [hep-th].

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Gravitational redshift induces quantum interference

Abstract: General relativity and quantum mechanics are the two frameworks through which we understand Nature. To date, they have remained valid to great extent in their respective domains. Regardless of the myriad of attempts to find a unified theory that can describe all of observable phenomena, the quest for unification continues. One avenue for investigating the overlap of general relativity and quantum mechanics that is less ambitious but can still provide potentially observable and measurable predictions is that of quantum field theory in curved spacetime viewed through the lens of quantum information. In recent years, a great deal of attention has been given to this approach, which has provided novel and intriguing insights into phenomena that can be tested in the laboratory. We present an investigation in the quantum nature of the gravitational redshift, seeking to understand which are the expected quantum dynamics that lead to the effective classical observable effect. We discuss the classical regime and show that more intriguing aspects are expected. We conclude discussing potential for detection in space-based experiments.

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N-body simulation of the large-scale structure formation: manifestation of the cosmic screening effect

Abstract: In my talk, I consider the large-scale structure formation within the cosmic screening approach. The main feature of this approach is that a careful analysis of the perturbed Einstein equations leads to the conclusion that there is an exponential cutoff of the gravitational interaction on large (of the order of 2–3 Gpc) cosmological scales. This is a purely relativistic effect associated with the non-linearity of Einstein's equations. To confirm this effect numerically, we perform the N-body simulation employing the relativistic code “gevolution” modified to our approach. First, we obtain power spectra for scalar and vector metric perturbations to show that both “gevolution” and “screening” approaches are in very good agreement between each other. However, the “screening” code consumes less computational time, saving almost 40% of CPU (central processing unit) hours. Then, we perform N-body simulations of the power spectra of the mass density contrast in a box with a comoving size of 5.632 Gps. We find that these spectra cease to depend on time for scales beyond the cosmic screening length. This is a clear manifestation of the cosmic screening effect.

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Could Buchdahl star be a Virial star?

Abstract: Buchdahl star is the most compact object without horizon and it is defined by the potential Φ(R) = 4/9 and black hole by Φ(R) = 1/2, where Φ(R) = (MR−Q^2/2)/(R^2+a^2) for Kerr-Newman charged and rotating object. It turns out that in terms of gravitational and non-gravitational energy, the former is defined when gravitational energy is half of non-gravitational energy while the latter when the two are equal. It would then be argued that the equilibrium of Buchdahl star may be governed by the Virial theorem, average kinetic energy being half of potential energy.

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General Relativistic Magnetohydrodynamic Simulations of Relativistic Jets Interacting with Stellar Winds

Abstract: The celebrated Unruh effect states that observers in uniform linear acceleration experience Minkowski vacuum in relativistic quantum field theory as a thermal state, in a temperature proportional to the proper acceleration. Observers in uniform circular acceleration experience a similar effect, with a more subtle notion of an effective temperature, but, crucially, with better prospects of experimental verification, especially so in condensed matter analogue spacetime systems that are realisable in the laboratory. This talk reviews the theoretical distinctions between linear and circular acceleration and the recent experimental proposals to observe the circular motion Unruh effect in the laboratory. The talk is based on 2007.09523, 2007.07160 and 2302.12023.

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General Relativistic Magnetohydrodynamic Simulations of Relativistic Jets Interacting with Stellar Winds

Abstract: Recent observations made by the Event Horizon Telescope have allowed scientists to present the first interpretations of the galactic center black holes, Sgr A* and M87*. The fundamental properties associated with their horizon-scale images are being tested by a library of general relativistic magnetohydrodynamic simulations with plasma sources. These has open new questions regarding their physical behavior. Firstly, observations on M87* show knots over densities along the jet arms of the galaxy. Secondly, on Sgr A* a lack of jet formation and remnant flare activity on the surrounding atmosphere was observed. In this work, we investigate the interactions between relativistic jets and stellar winds through simulations using three different magnetically arrested disk (MAD) models. The simulations were conducted for a Kerr black hole with dimensionless spin a=15/16, surrounded by a Fishbone-Monecrief tori with constant angular momentum and a weak magnetic field loop. The simulations included randomized configurations of 4, 8, and 12 stellar winds surrounding the tori, and were simulated for times up to t=10,000M. Saturation periods were recorded for each model, and the magnetization and rest-mass density distributions were analyzed. Also, the opening angles of the jets were measured using the Engauge Digitizer program. The simulations demonstrated accretion processes of clumps near the event horizon, as well as advection of plasma and magnetic fields around the accretion disk. These regions resulted in reduced relativistic jet velocities (v~0.8c) and narrowed final jet apertures. Furthermore, we explore the possibility of plasmoid formation that could emit extra radiation around the models. A classification of three classes of plasmoids was made for the following states: disk-jet boundary, far jet sheath region, and accretion process. Long-lasting plasmoids formed from magnetic reconnection within a range of 5rg-200rg with a maximal lifetime of 700M. Nevertheless, we show that most plasmoids converted into high-temperature stellar wind densities that cooled down over time. Their high magnetization and low density allowed them to become gravitationally unbound, providing optimal conditions for flaring activity.

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The Wheeler-DeWitt interpretation of the TTbar deformation

Abstract: In this work we explore the TTbar deformation and its holographic interpretation in terms of the Wheeler-DeWitt equation. We begin with a bibliographical review of relevant concepts of the holographic principle and the AdS/CFT correspondence. This summary serves as theoretical context for the following sections, where we continue by introducing important aspects of the deformation of a two dimensional CFT generated by coupling a TTbar operator. The most significant part is the holographic interpretation in the bulk side of the theory. Afterwards, this is contextualized in terms of holographic renormalization and quantum gravity, where the correspondence between the flow generated by the TTbar coupling and the bulk is given by a radial Wheeler-DeWitt equation. The final part of this work consists of applying these calculations and techniques to a quantized BTZ black hole in the minisuperspace approximation. The calculated Wheeler-DeWitt equation is then solved semiclassically. We provide a holographic interpretation of the solution in terms of the TTbar deformation through holographic renormalization. This interpretation is later supported by the energy spectrum and the asymptotic limit of the gravitational theory.

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