Quantum Gravity

The possibility of observing quantum gravitational phenomena, viewed as remote until not long ago, is increasingly considered to be plausible.  A potentially observable phenomenon is the decay of black holes via a quantum gravitational tunneling akin to standard nuclear decay.  Loop quantum gravity can be used to compute the corresponding lifetime.  This could be much shorter than the Hawking radiation time, rendering the effect astrophysically relevant. Preliminary estimates indicate that centimeter-size primordial black holes should be exploding today and predict impulsive high energy as well as microwave signals tantalizing similar to the Fast Radio Bursts recently observed by the Areceibo and Parkes radio telescopes.  The predicted signal has a characteristic distance/frequency relation which will allow to test the Planck star hypotheses in the near future.

We introduce the construction of a new framework for probing discrete emergent geometry and boundary-boundary observables based on a fundamentally a-dimensional underlying network structure. Using a gravitationally motivated action with Forman weighted combinatorial curvatures and simplicial volumes relying on a decomposition of an abstract simplicial complex into realized embeddings of proper skeletons, we demonstrate model phenomenology such as a minimal volume-scale cutoff, a positive-definite cosmological constant as a regulator for non-degenerate geometries, and naturally emergent simplicial structures from Metropolis network evolution simulations with no restrictions on attachment rules or regular building blocks. Properties which echo results from both the spinfoam formalism and causal dynamical triangulations in quantum gravity will also be illustrated. We conclude with prospects for future investigations into the model, discussing ongoing simulations, modifications to the structure of the theory, and broader applications of the framework.

General relativity is invariant under diffeomorphisms, and

excitations of the metric corresponding to diffeomorphisms

are nonphysical.  In the presence of a boundary, though --

including a boundary at infinity -- the Einstein-Hilbert

action with suitable boundary terms is no longer fully

invariant, and certain diffeomorphisms are promoted to

physical degrees of freedom.  After briefly describing how

this happens in (2+1)-dimensional AdS gravity, I will

report on work in progress on the asymptotically flat case,

for which the newly dynamical diffeomorphisms are the

superrotations, and the boundary action is related to

coadjoint orbits of the Virasoro group and Hill's equation.

We present results from a study of Euclidean dynamical triangulations in an attempt to make contact with Weinberg's asymptotic safety scenario. We find that a fine-tuning is necessary in order to recover semiclassical behavior, and that once this tuning is performed, our simulations provide evidence in support of the asymptotic safety scenario for gravity.  We discuss our motivation for the tuning and present our numerical results. Finally, we discuss what our simulations imply for the dimension of the ultraviolet critical surface, which sets the number of free parameters in the theory.

In the geometric models of matter, proposed in a joint paper with Michael Atiyah and Nick Manton, static particles like the electron or proton are modelled by Riemannian 4-manifolds. In this talk I will explain how the spin degrees of freedom appear in the geometric framework. I will also discuss a proposal for time evolution in one particular model, namely the Taub-NUT model of the electron.

In the geometric models of matter, proposed in a joint paper with Michael Atiyah and Nick Manton, static particles like the electron or proton are modelled by Riemannian 4-manifolds. In this talk I will explain how the spin degrees of freedom appear in the geometric framework. I will also discuss a proposal for time evolution in one particular model, namely the Taub-NUT model of the electron.

 

The fact that the Einstein-Hilbert action, by itself, does not lead to a well-posed variational principle has become textbook knowledge. It can be made well-posed by the addition of suitable boundary terms. There are many boundary terms available in the literature, of which the most famous and most widely used is the Gibbons-Hawking-York (GHY) boundary term. The GHY term is ostensibly defined only for a non-null boundary. There have been very few efforts in the literature to extend its definition to null boundaries. The speaker will present his group's work towards finding a boundary term to render the Einstein-Hilbert action well-posed in the presence of null boundaries. He shall discuss a proposed boundary term, associated boundary conditions, outstanding issues and possible applications.

I will present recent result on constructing effective quantum gravity theory as a locally covariant QFT. The approach I advocate uses the BV formalism for dealing with the gauge freedom and Epstein-Glaser renormalization to control the UV divergences. I will show how gauge invariant observables that satisfy a weak notion of locality can be constructed in this framework and I will sketch the argument for perturbative background independence. Recently these ideas were applied to models relevant in cosmology.

In this talk I discuss the effects of nonlinear backreaction of small scale density inhomogeneities in general relativistic cosmology. It has been proposed that in an inhomogeneous universe, nonlinear terms in the Einstein equation could, if properly averaged and taken into account, affect the large scale Friedmannian evolution of the universe. In particular, it was hoped that these terms might mimic a cosmological constant and eliminate the need for dark energy. After reviewing some of these approaches, and some of their flaws, I will describe a perturbative framework (developed with R. Wald) designed to properly take into account these effects. In our framework, we assume that the spacetime metric is "close"---within 1 part in 10^4, except near strong field objects---to a background metric of FLRW symmetry, but we do not assume that the background metric satisfies the Friedmann equation. We also do not require that spacetime derivatives of the metric be close to derivatives of the background metric. This allows for significant deviations in geodesics, and very large curvature inhomogeneities. A priori, this framework also allows for significant backreaction, which would take the form of new effective matter sources in the Friedmann equation. Nevertheless, we prove that if the matter stress-energy tensor satisfies the weak energy condition, then large matter inhomogeneities on small scales cannot produce significant backreaction effects on large scales, and in particular cannot account for dark energy. As I will also review here, with a suitable ‘dictionary,’ Newtonian cosmologies provide excellent approximations to cosmological solutions to Einsteinʼs equation (with dust and a cosmological constant) on all scales. Our results thereby provide strong justification for the mathematical consistency and validity of the LCDM model within the context of general relativistic cosmology. While our rigorous framework makes use of 1-parameter families and weak limits, in this talk I will provide a simple heuristic discussion that places emphasis on the manner in which "averaging" is done, and the fact that one is solving the Einstein equation.