Bulletin of the American Physical Society
APS April Meeting 2021
Volume 66, Number 5
Saturday–Tuesday, April 17–20, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session E17: Quantum Gravity IILive
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Sponsoring Units: DGRAV Chair: Jorge Pullin, Louisiana State University |
Saturday, April 17, 2021 3:45PM - 3:57PM Live |
E17.00001: The S-matrix and Infrared Divergences in Quantum Gravity Gautam Satishchandran, Robert Wald We study the construction of an ``infrared finite'' S-matrix in Quantum Gravity. Infrared divergences in Quantum Gravity (and QFT) are a direct consequence of a classical observable known as the Memory Effect. The memory effect implies that ``out'' scattering states live in an uncountably infinite set of unitarily inequivalent Hilbert spaces (one for each memory effect). In order to construct an ``IR finite'' S-matrix we seek a ``in'' and ``out'' Hilbert space of scattering states which is (1) separable, (2) invariant under the asymptotic symmetry group and (3) preserved under scattering. The analogous problem in QED is solved by building a Hilbert space of ``dressed states''. We clarify that this procedure fails in Quantum Gravity and we argue that, in contrast to QED, there is no natural Hilbert space of ``in'' and ``out'' scattering states in any IR finite description of Quantum Gravity. Nevertheless, the ``in'' and ``out'' Algebra of Observables are well defined and, in the absence of a preferred Hilbert space, an IR finite description of Q.G. requires the formulation of an ``S matrix'' as a map on algebraic states. We present some progress towards this construction. [Preview Abstract] |
Saturday, April 17, 2021 3:57PM - 4:09PM Live |
E17.00002: Asymptotic quantum fields at spatial infinity Kartik Prabhu, Gautam Satishchandran I will consider the asymptotic behaviour of massless quantum fields at spatial infinity in Minkowski spacetime. The bulk fields can be written in terms of massive de Sitter fields on the hyperboloid of directions at spatial infinity. This gives a formulation of the QFT purely in terms of fields defined at spatial infinity which can be generalised to any asymptotically-flat spacetime. Further, the gravitational field near spatial infinity can also be formulated in this manner giving an asymptotic formulation of quantum gravity. I will also comment on the asymptotic charges in the quantum theory which label the Hilbert spaces corresponding to different superselection sectors and the possible relation to asymptotic quantization on null infinity. [Preview Abstract] |
Saturday, April 17, 2021 4:09PM - 4:21PM Live |
E17.00003: Anomalies in the CMB from a cosmic bounce Dimitrios Kranas, Ivan Agullo, Vijayakumar Sreenath We propose a cosmological model that can provide a common origin to several of the anomalous features observed at large angular scales in the cosmic microwave background (CMB). More concretely, we show that a power suppression, a dipolar asymmetry, and a preference for odd-parity correlations, with amplitude and scale dependence in consonance with observations, are expected from this scenario. The model also alleviates the tension in the lensing amplitude. These features originate from the indirect effect that non-Gaussian correlations between CMB modes and super-horizon wavelengths induce in the power spectrum. In the model we propose, a cosmic bounce precedes the inflationary era. Adopting a phenomenological approach for the profile of the bounce, we keep our analysis as general as possible but we complement it by mentioning well-established theories where our model can be materialized. [Preview Abstract] |
Saturday, April 17, 2021 4:21PM - 4:33PM Live |
E17.00004: Holography Abhors Visible Trapped Surfaces Aasmund Folkestad, Netta Engelhardt In this talk, I will explain how the consistency of the holographic dictionary implies a hallmark prediction of the weak cosmic censorship conjecture: that in classical gravity, trapped surfaces lie behind event horizons. We will see that causal wedge inclusion requires the formation of event horizons outside of strong gravity regions. Few assumptions are made beyond the absence of evaporating singularities in strictly classical gravity. Finally, I comment on the implication that spacetimes with naked trapped surfaces do not admit a holographic dual and speculate on the dual CFT interpretation of a trapped surface. [Preview Abstract] |
Saturday, April 17, 2021 4:33PM - 4:45PM Live |
E17.00005: Effective Spin Foam Models for Four-Dimensional Quantum Gravity and Their Dynamics Hal Haggard, Seth Asante, Bianca Dittrich A number of approaches to four-dimensional quantum gravity, such as loop quantum gravity and holography, situate areas as their fundamental variables. However, this choice of kinematics can easily lead to gravitational dynamics peaked on flat spacetimes because of how regions are glued in the gravitational path integral. I will introduce a family of `effective' spin foam models that incorporate a quantum area spectrum, impose gluing constraints as strongly as possible, and leverage the discrete general relativity action to specify amplitudes. These effective spin foam models avoid flatness and exhibit good semiclassical behavior in a restricted regime of the parameter space. These models are the numerically fastest models to-date and I will report results on a triangulation including an inner edge. [Preview Abstract] |
Saturday, April 17, 2021 4:45PM - 4:57PM Live |
E17.00006: Reconciling a quantum gravity minimal length with lack of photon dispersion Douglas Singleton, Michael Bishop, Joey Contreras, Jaeyeong Lee Generic arguments lead to the idea that quantum gravity has a minimal length scale. A possible observational signal of such a minimal length scale is that photons should exhibit dispersion. In 2009, the observation of a short gamma ray burst seemed to push the minimal length scale to distances smaller than the Planck length. This poses a challenge for such minimal distance models. Here we propose a modification of the position and momentum operators, ${\hat x}$ and ${\hat p}$, which lead to a minimal length scale, but preserve the photon energy-momentum relationship $E = p c$. In this way there is no dispersion of photons with different energies. Additionally, this can be accomplished without modifying the commutation relationship $[{\hat x}, {\hat p}] = i \hbar$. [Preview Abstract] |
Saturday, April 17, 2021 4:57PM - 5:09PM Live |
E17.00007: Bohmian Quantum Gravity Thomas Andersen The recent experimental proposals by Bose et al. and Marletto et al. (BMV) outline a way to test for the quantum nature of gravity by measuring gravitationally induced differential phase accumulation over the superposed paths of two 10-14 kg masses. BMV outline the expected outcome of these experiments for semi-classical, quantum gravity and collapse models. BMV finds that both semi-classical and collapse models predict a lack of entanglement in the experimental results. This work predicts the outcome of the BMV experiment in Bohmian trajectory gravity - where classical gravity is assumed to couple to the particle configuration in each Bohmian path, as opposed to semi-classical gravity where gravity couples to the expectation value of the wave function, or of quantized gravity, where the gravitational field is itself in a quantum superposition. In the case of the BMV experiment, Bohmian trajectory gravity predicts that there will quantum entanglement. This is surprising as the gravitational field is treated classically. A discussion of how Bohmian trajectory gravity can induce quantum entanglement for a non-superposed gravitational field is put forward. [Preview Abstract] |
Saturday, April 17, 2021 5:09PM - 5:21PM Live |
E17.00008: Discrete gravity from effective spin foam models Seth Asante, Bianca Dittrich, Hal Haggard Recently, a new and `effective’ family of spin foam models describing quantum gravity dynamics with discrete area spectrum have been introduced. These models are built directly from the geometrical variables of spacetime and have been shown to be amenable to numerical computations. The simplicity of these models allows to clarify some issues appearing in semi classical analysis of generic spin foam models. I will describe first steps toward testing quantum gravity equations from these models on non-trivial spacetime boundaries. The numerical evaluations reveal a very rich structure of amplitudes and also the expectation values of certain observables results from an interplay between various parameters of the model. [Preview Abstract] |
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