Group Seminar by Susanne Schander

Speaker: Susanne SchanderPlace: 241 Nicholson

Date: Wednesday Nov 19th

Time: 13:30

Title: Backreactions in Quantum Cosmological Perturbation Theory

Abstract:

Cosmological perturbation theory has a long tradition for describing the early phases of the Universe. As the observations of the CMB radiation suggest, it is reasonable, at least as a first approximation, to implement cosmological inhomogeneities as small perturbations around homogeneous and isotropic FRW solutions. In these approaches, backreactions between the inhomogeneities and the background are usually neglected. There is an ongoing debate about how and to which extend these backreactions affect the large scale structure of the Universe. Even at a purely classical level, there is no conclusive answer to this question yet.

In my talk, I am going to present a new systematic formalism for implementing backreactions in cosmological perturbation theory, in which both, the perturbations and the homogeneous degrees of freedom are considered as quantum degrees of freedom. As a more realistic theory of quantum fields on quantum cosmological space times, it can help to close the gap between a full theory of quantum gravity and symmetry-reduced models of quantum cosmology, and to confront these theories with observations. Our results show that quantum backreactions imply non-trivial corrections that are potentially phenomenologically significant.

Group Seminar by Adrià Delhom I Latorre

 Speaker: Adrià Delhom I Latorre
 Place: 241 Nicholson

 Date: Wednesday Nov 20th

 Time: 13:30

Title: Curvature-based Metric-affine gravity

Abstract:  I will give an overview of metric-affine gravity theories, focusing in those with actions built with the Ricci tensor, trying to emphasize the phenomenological aspects of these theories that are similar to those encountered in quantum gravity theories. First, I will review the different frameworks that fall within metric-affine gravity (teleparallel, gauge gravity, etc.). Focusing on actions built upon curvature tensors (curvature-based), I will then explain why low-curvature modifications to GR, that could explain the accelerated expansion, are ruled out by experiments. Then I will show why metric-affine higher order curvature theories can be free from ghosts and the role played by projective symmetry on this. After that, I will give some results involving black hole like objects that arise naturally in several metric-affine models are described by non-singular spacetimes (in the sense of geodeticall completeness). I will also present some bouncing cosmological models within these theories and how the LQC dynamics can be mimicked by an f(R) theory. Finally, I will show how in some curvature-based metric-affine gravity the matter sector develops non-linear interactions that can be used to constrain these theories.

Online seminar by Marcin Kisielowski

Speaker: Marcin Kisielowski

Place: 241 Nicholson

Date: Wednesday Nov 13th

Time: 11 AM

Title: Spin-foam model for gravity coupled to massless scalar field

Abstract: In my talk I will present a spin-foam model for gravity coupled to massless scalar field which we derived from the Canonical Loop Quantum Gravity. We generalized to the full theory the scheme first proposed in the context of Loop Quantum Cosmology by Ashtekar, Campiglia and Henderson, later developed by Henderson, Rovelli, Vidotto and Wilson-Ewing.

Group Seminar by Muxin Han

Speaker: Muxin Han
Place: 241 Nicholson
Date: Thursday, October 31
Time: 1:30 PM

Title: Effective dynamics from full loop quantum gravity

Group Seminar by Andrea Dapor

Speaker: Andrea Dapor
Place: 241 Nicholson
Date: Thursday, October 24
Time: 1:30 PM

Title: Recent developments in LQG effective dynamics

Online Seminar by Saeed Rastgoo

Speaker: Saeed Rastgoo

Place: 241 Nicholson

Date: Thursday May 9th

Time: 11 AM

Title: Effective polymer dynamics of Schwarzschild interior via path integral: challenges and new results

Abstract: I will present a systematic method of deriving the effective polymer Hamiltonian of Schwarzschild black hole interior, using path integral, which leads to the appearance of new triad corrections in the effective Hamiltonian. These new terms present some issues such as dependence of physical quantities on fiducial variables. We propose two prescription to solve these issues. Upon using these prescription, not only the issues are resolved, but also we get new results including two types of modified (larger) minimum radius at bounce.

Online Seminar by Anzhong Wang

Speaker: Anzhong Wang

Place: 241 Nicholson

Date: Thursday May 2nd

Time: 11 AM

Title: Understanding Quantum Cosmology from loop quantum gravity

Group Seminar by Laura Herold

Speaker: Laura Herold
Place: 241 Nicholson
Date: Thursday, April 11
Time: 1:30 PM

Title: Gaussian dust as a reference field and corrections to the Mukhanov-Saskai equation

Online Seminar by Muxin Han

Speaker: Muxin Han
Place: 241 Nicholson
Date: Thursday, March 28
Time: 1:30 PM
 

Title: Emergent Gravity from Spinfoam Models

Abstract: Recently, many new insights of quantum gravity are brought by the program of emergent gravity from quantum information. An idea in this program is that gravity, which is geometrical and smooth, might emerge as low energy excitations from fundamentally entangled qubits (or generally qudits), which are algebraic and discrete. In this talk, I demonstrate that spinfoam models, on one hand, are made fundamentally by entangled qudits, while on the other hand, emerge smooth gravitational fields in a low energy limit. The result suggests the spinfoam model to be a working example of the above idea in the emergent gravity program.

Online Seminar by Suzanne Lanery

Speaker: Suzanne Lanery

Place: 241 Nicholson

Date: Thursday March 21th

Time: 13:30

Title: Projective State Spaces for Loop Quantum Cosmology

Online Seminar by Philip Hoehn

Speaker: Philip Hoehn

Place: 241 Nicholson

Date: 14.03.198

Time: 10:30 AM

Title:  From quantum reference systems to quantum general covariance

Abstract: Reference frames (or, more generally, systems) provide the vantage points from which to describe the remaining physics. Treating them fundamentally as quantum systems is inevitable in quantum gravity, where coordinates are a priori unavailable, but also in quantum foundations once accepting that all frames are physical systems. Both fields thus face the question of how to describe physics from the perspective of quantum reference systems and how the descriptions relative to different such choices are related. I will summarize a recent systematic method for such switches, which works in analogy to coordinate changes on a manifold, except that these `quantum coordinate changes' proceed between different Hilbert spaces. This method relies on a new quantum reduction procedure, relating Dirac and reduced quantization, sets the stage for a quantum version of general covariance and applies to both temporal and spatial reference systems. In particular, I will illustrate how one can use this method to switch between different relational clocks in the quantum theory and thereby address the so-called "multiple choice problem of time". (Based on arXiv:1809.00556, 1809.05093, 1810.04153, 1811.00611.)

Online Seminar by Eugenio Bianchi

 

Speaker: Eugenio Bianchi

Place: 241 Nicholson

Date: Thursday Feburary 7th

 Time: 1:30 PM

Title: Entanglement entropy of Bell-network states in LQG