Quantum Gravity

503 talks

S005

Collection TypeSeries

Source RepositoryPIRSA

Displaying 349 - 360 of 503

From spin foams to anyons and back again - Joint Condensed Matter/Quantum Gravity Seminar

Bianca Dittrich Perimeter Institute for Theoretical Physics
December 05, 2013

PIRSA:13120048
General Relativity & Quantum Cosmology
Divergences in Spinfoam Quantum Gravity

Aldo Riello Perimeter Institute for Theoretical Physics
November 28, 2013

PIRSA:13110063
General Relativity & Quantum Cosmology
Dimensional reduction in the sky
November 21, 2013

PIRSA:13110059
General Relativity & Quantum Cosmology
Finite regions, spherical entanglement, and quantum gravity

Hal Haggard Bard College
November 14, 2013

PIRSA:13110049
General Relativity & Quantum Cosmology
Matter matters in asymptotically safe quantum gravity
November 07, 2013

PIRSA:13110048
General Relativity & Quantum Cosmology
A causal set action

Lisa Glaser University of Vienna
October 31, 2013

PIRSA:13100063
General Relativity & Quantum Cosmology
Asymptotically safe matrix models

Tim Koslowski Technical University of Applied Sciences Würzburg-Schweinfurt
October 24, 2013

PIRSA:13100067
General Relativity & Quantum Cosmology
Constructing theories from duality: my response to John Wheeler
October 17, 2013

PIRSA:13100072
General Relativity & Quantum Cosmology
Superselection Sectors in AdS/CFT - Joint Quantum Gravity and String Theory Seminar

Aron Wall University of Cambridge
October 10, 2013

PIRSA:13100066
General Relativity & Quantum Cosmology
The World is Discrete

Olaf Dreyer Freelance Consultant
September 19, 2013

PIRSA:13090073
General Relativity & Quantum Cosmology
Holography without Strings - Joint Quantum Gravity and String Theory Seminar

Donald Marolf University of California, Santa Barbara
September 12, 2013

PIRSA:13090060
General Relativity & Quantum Cosmology
Strong Binary Pulsar Constraints on Lorentz Violation in Gravity

Enrico Barausse SISSA International School for Advanced Studies
August 22, 2013

PIRSA:13080040
General Relativity & Quantum Cosmology

From spin foams to anyons and back again - Joint Condensed Matter/Quantum Gravity Seminar

Bianca Dittrich Perimeter Institute for Theoretical Physics
December 05, 2013

PIRSA:13120048
General Relativity & Quantum Cosmology
Spin foams provide models for quantum gravity and hence quantum space time. One of the key outstanding questions is to show that they reproduce smooth space time manifolds in a continuum limit.I will start with a very short introduction to spin foams and the structure of quantum space time they encode. I will explain how the investigation of the continuum limit via coarse graining and renormalization techniques led as to consider anyonic spin chains and a classification of ground states in systems with quantum group symmetries.I will then present new results on the continuum limit of spin net models, that allow us to draw first conclusions about the large scale dynamics of spin foams.
Based on: B.D., W. Kaminski, Topological lattice field theories from intertwiner dynamics, arXiv:1311.1798, B.D., S. Steinhaus, Time evolution as refining, coarse graining and entangling, to appear, B.D. M. Martin-Benito, S. Steinhaus, The refinement limit of quantum group spin net models, to appear
Divergences in Spinfoam Quantum Gravity

Aldo Riello Perimeter Institute for Theoretical Physics
November 28, 2013

PIRSA:13110063
General Relativity & Quantum Cosmology
The most relevant evidences in favour of the Lorentzian EPRL-FK spinfoam model come from its capibility of reproducing the expected semiclassical limit in the large spin regime. The main examples of this are the large spin limit of the vertex amplitude, later extended to arbitrary triangulations, and that of the spinfoam graviton propagator, which was calculated on the simplest possible two complex. These results are very promising. Nonetheless, their relevance may be endangered by the effects associated to radiative corrections. In this seminar, I will focus on the role played by the simplest diverging graph, the so called 'melon graph', which is known to play a fundamental role in tensorial group field theories. In particular, I will discuss its most divergent part and its geometrical interpretation. I will finally comment on the result, with particular attention to its physical consequences, especially in relation with the semiclassical limit of the spinfoam graviton propagator.
Dimensional reduction in the sky
November 21, 2013

PIRSA:13110059
General Relativity & Quantum Cosmology
In several approaches to quantum-gravity, the spectral dimension of spacetime runs from the standard value of 4 in the infrared (IR) to a smaller value in the ultraviolet (UV). Describing this running in terms of deformed dispersion relations, I show that a striking cosmological implication is that that UV behavior leading to 2 spectral dimensions results in an exactly scale-invariant spectrum of vacuum scalar and tensor fluctuations. I discuss scenarios that break exact scale-invariance and show that the tensor to scalar ratio is fixed by the UV ratio between the speed of gravity and the speed of light. Cosmological perturbations in this framework display a wavelength-dependent speed of light, but by transforming to a suitable "rainbow frame" this feature can be removed, at the expense of modifying gravity. In particular it turns out that the following concepts are closely connected: scale-invariance of vacuum fluctuations, conformal invariance of the gravitational coupling, UV reduction to spectral dimension 2 in position space and UV reduction to Hausdorff dimension 2 in energy-momentum space.
Finite regions, spherical entanglement, and quantum gravity

Hal Haggard Bard College
November 14, 2013

PIRSA:13110049
General Relativity & Quantum Cosmology
An exciting frontier in physics is to understand the quantum nature of gravitation in finite regions of spacetime. Study of these regions from ``below'', that is, by studying the quantum geometry of finite regions emerging from loop gravity and spin networks has recently resulted in a new road to the quantization of volume and to evidence that there is a robust gap in the volume spectrum. In this talk I will complement these results with recent work on conformal field theories in a particular finite region, a spherical ball of space. This new view afforded from ``above" gives insights into entanglement and the Reeh-Schlieder theorem, allows calculation of the entanglement spectrum, and suggests a new route to constructing the Minkowski vacuum out of independent finite regions in quantum gravity.
Matter matters in asymptotically safe quantum gravity
November 07, 2013

PIRSA:13110048
General Relativity & Quantum Cosmology
The Functional Renormalisation Group technique has received great attention in recent times proving itself as a powerful tool to describe the high energy behaviour of gravitational interactions.
Its key ingredient is a nontrivial fixed point of the theory renormalization group flow which controls the behavior of the coupling constants in the ultraviolet regime and ensures that physical quantities are safe from divergences. I will briefly review the main ingredients of the gravitational asymptotic safety framework before focusing on the effect of massless minimally coupled scalars, fermions and vector fields on the gravitational fixed point. I will then set bounds on the type and number of such fields requiring the existence of a UV attractive fixed point. I will also discuss the dynamically generated quantum-gravity scale in asymptotic safety, which is the transition scale to the fixed-point regime, and its variation as a function of matter degrees of freedom. To conclude I will also consider the case of higher dimensional models.
A causal set action

Lisa Glaser University of Vienna
October 31, 2013

PIRSA:13100063
General Relativity & Quantum Cosmology
Causal set theory is discrete, fully covariant theory of quantum gravity. The discrete framework makes it necessary to reformulate continuum concepts. One of these concepts is that of a derivative operator. It is possible to define a derivative operator in causal sets that in the continuum limit agrees with the d'Alembertian for a scalar field. This operator can be used to define a causal set action, which enables Monte-Carlo simulations. In this seminar I will present this operator and action and then show some results of Monte-Carlo simulations in 2 dimensions.
Asymptotically safe matrix models

Tim Koslowski Technical University of Applied Sciences Würzburg-Schweinfurt
October 24, 2013

PIRSA:13100067
General Relativity & Quantum Cosmology
The functional renormalization group is a tool in the systematic search for Euclidean QFTs that works with very little input: All one needs to specify is a field content, symmetries and a notion of locality. The functional renormalization group then allows one to scan this theory space for bare actions for which the path integral can be performed nonperturbatively. These actions appear as fixed points (and relevant deformations) of the renormalization group flow (so-called asymptotic safety). Such a systematic search has so far not been performed for the tensor model approach to quantum gravity. We investigate matrix models for 2-dimensional gravity and the Grosse-Wulkenhaar model, which is a simple tensor model for a 4D noncommutative scalar field theory, as a first step. I will report on work, done in collaboration with Astrid Eichhorn and Alessandro Sfondrini, where we confirmed asymptotic safety purely through functional RG methods. Based on the lessons learned from these models I will summarize how the usual continuum RG approach has to be generalized for the investigation of tensor models and conclude with a recipe for the investigation of general tensor models.
Constructing theories from duality: my response to John Wheeler
October 17, 2013

PIRSA:13100072
General Relativity & Quantum Cosmology
The late physicist John Wheeler, was renowned for his Socratic method of conducting physics discussions. "Why is general relativity the way it is? What makes it special?" were reportedly questions one should expect in his presence. There are different answers to these questions, each requiring a set of assumptions - which Wheeler would likely question again - and each bringing with it new insights into physics as a whole. This talk will put forward new principles for deriving general relativity. Perhaps more than is the case with other construction principles, the principles introduced are not limited to the derivation of general relativity, requiring only an specification of the theory's phase space in order to be applicable. To be less enigmatic, one defines observable equivalence between physical theories in the Dirac constraint setting, and then the principle merely searches theory space for two equivalent (or dual) fully constrained theories. We find it quite remarkable that such a complex theory as general relativity emerges, when no initial presupposition even on the existence of space-time is made. If there is time (and I will argue there is!) I will discuss another issue that also distinguishes the present set of assumptions: the possibility that they are in a concrete sense ``self-selected", which I like to think would be more satisfactory to Wheeler's line of questioning (or at least give him more to chew on).
Superselection Sectors in AdS/CFT - Joint Quantum Gravity and String Theory Seminar

Aron Wall University of Cambridge
October 10, 2013

PIRSA:13100066
General Relativity & Quantum Cosmology
AdS/CFT is a duality relating the degrees of freedom in a D dimensional bulk gravity theory to a (D-1) dimensional theory living on the boundary. I will argue that in fact the boundary theory contains only a subset of the bulk observables. For each state of the boundary theory, there are multiple bulk states dual to it, which can be operationally distinguished by observers who fall across event horizons. Based on arXiv:1210.3590.
The World is Discrete

Olaf Dreyer Freelance Consultant
September 19, 2013

PIRSA:13090073
General Relativity & Quantum Cosmology
We argue that the scale-free spectrum that is observed in the cosmic microwave background is the result of a phase transition in the early universe. The observed tilt of the spectrum, which has been measured to be 0.04, is shown to be equal to the anomalous scaling dimension of the correlation function. The phase transition replaces inflation as the mechanism that produces this spectrum. The tilt further indicates that there is a fundamental small length scale in nature that we have not yet observed in any other way.
Holography without Strings - Joint Quantum Gravity and String Theory Seminar

Donald Marolf University of California, Santa Barbara
September 12, 2013

PIRSA:13090060
General Relativity & Quantum Cosmology
A defining feature of holographic dualities is that, along with the bulk equations of motion, boundary correlators at any given time t determine those of observables deep in the bulk. We argue that this property emerges from the bulk gravitational Gauss law together with bulk quantum entanglement as embodied in the Reeh-Schlieder theorem. Stringy bulk degrees of freedom are not required and play little role even when they exist. As an example we study a toy model whose matter sector is a free scalar field. The energy density (\rho) sources what we call a pseudo-Newtonian potential (\Phi) through Poisson's equation on each constant time surface, but there is no back-reaction on the matter. We show the Hamiltonian to be essentially self-adjoint on the domain generated from the vacuum by acting with boundary observables localized in an arbitrarily small neighborhood of the chosen time t. Since the Gauss law represents the Hamiltonian as a boundary term, the model is holographic in the sense stated above.
Strong Binary Pulsar Constraints on Lorentz Violation in Gravity

Enrico Barausse SISSA International School for Advanced Studies
August 22, 2013

PIRSA:13080040
General Relativity & Quantum Cosmology
Binary pulsars are excellent laboratories to test the building blocks of Einstein's theory of General
Relativity. One of these is Lorentz symmetry which states that physical phenomena appear the same for all inertially moving observers. We study the e ect of violations of Lorentz symmetry in the orbital evolution of binary pulsars and that it induces a much more rapid decay of the binary's orbital period due to the emission of dipolar radiation. The absence of such behavior in recent observations allows us to place the most stringent constraints on Lorentz violation in gravity, thus verifying one of the cornerstones of Einstein's theory much more accurately than any previous gravitational observation.

Title	Speaker(s)	Date	Talk Type	Info
From spin foams to anyons and back again - Joint Condensed Matter/Quantum Gravity Seminar	Bianca Dittrich	2013-12-05	Scientific Series	View details
Divergences in Spinfoam Quantum Gravity	Aldo Riello	2013-11-28	Scientific Series	View details
Dimensional reduction in the sky		2013-11-21	Scientific Series	View details
Finite regions, spherical entanglement, and quantum gravity	Hal Haggard	2013-11-14	Scientific Series	View details
Matter matters in asymptotically safe quantum gravity		2013-11-07	Scientific Series	View details
A causal set action	Lisa Glaser	2013-10-31	Scientific Series	View details
Asymptotically safe matrix models	Tim Koslowski	2013-10-24	Scientific Series	View details
Constructing theories from duality: my response to John Wheeler		2013-10-17	Scientific Series	View details
Superselection Sectors in AdS/CFT - Joint Quantum Gravity and String Theory Seminar	Aron Wall	2013-10-10	Scientific Series	View details
The World is Discrete	Olaf Dreyer	2013-09-19	Scientific Series	View details
Holography without Strings - Joint Quantum Gravity and String Theory Seminar	Donald Marolf	2013-09-12	Scientific Series	View details
Strong Binary Pulsar Constraints on Lorentz Violation in Gravity	Enrico Barausse	2013-08-22	Scientific Series	View details

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From spin foams to anyons and back again - Joint Condensed Matter/Quantum Gravity Seminar

Divergences in Spinfoam Quantum Gravity

Dimensional reduction in the sky

Finite regions, spherical entanglement, and quantum gravity

Matter matters in asymptotically safe quantum gravity

A causal set action

Asymptotically safe matrix models

Constructing theories from duality: my response to John Wheeler

Superselection Sectors in AdS/CFT - Joint Quantum Gravity and String Theory Seminar

The World is Discrete

Holography without Strings - Joint Quantum Gravity and String Theory Seminar

Strong Binary Pulsar Constraints on Lorentz Violation in Gravity

From spin foams to anyons and back again - Joint Condensed Matter/Quantum Gravity Seminar

Divergences in Spinfoam Quantum Gravity

Dimensional reduction in the sky

Finite regions, spherical entanglement, and quantum gravity

Matter matters in asymptotically safe quantum gravity

A causal set action

Asymptotically safe matrix models

Constructing theories from duality: my response to John Wheeler

Superselection Sectors in AdS/CFT - Joint Quantum Gravity and String Theory Seminar

The World is Discrete

Holography without Strings - Joint Quantum Gravity and String Theory Seminar

Strong Binary Pulsar Constraints on Lorentz Violation in Gravity