Talks

I will present three results: 1. Violation of positivity of energy in the extension of classical General Relativity provided by Ashtekar's polynomial formulation by virtue of its admittance of initial data with degenerate (densitized) triads. 2. Consistency of Propagation in the sense articulated by Lee with the apparent ``ultralocal'' action of the Hamiltonian constraint in LQG. 3. An explicit demonstration of the consistency of area discreteness with Lorentz Invariance for an LQG type quantization of free scalar field theory on 1+1 flat spacetime formulated in a diffeomorphism invariant disguise. The first two results were obtained as answers to direct questions from Lee, the third is a new result and all three are motivated by issues connected with Dynamics, and hence, Time in LQG.

Title

Battle of the Big Bang: The New Tales of Our Cosmic Origins

Abstract

The story of the universe’s origins is one of the greatest mysteries in science. From the explosive birth of the cosmos to the enigmatic nature of time and space, we are continually challenged by paradoxes that defy current understanding. In this public talk, we shall explore the triumphs and debates that shape modern cosmology, and the novel quests to uncover our cosmic origins. I will also highlight the pioneering contributions of my friend and colleague, Lee Smolin. His groundbreaking ideas on quantum gravity, the nature of time, and cosmological evolution have sparked new directions in the search for a deeper theory of the universe. Together, we will journey through the frontiers of physics, exploring how new theories and astronomical observations might offer clues to the next great paradigm shift. Whether you’re a science enthusiast or just curious about the cosmos, join us for a conversation on the past, present, and future of our understanding of the universe.

About the Speaker  

Niayesh Afshordi is a theoretical astrophysicist and professor at the University of Waterloo and Perimeter Institute for Theoretical Physics. His research explores the frontiers of cosmology, gravity, and quantum physics, with a focus on black holes and the origins of the universe. He is the co-author of the forthcoming popular science book Battle of the Big Bang: The New Tales of Our Cosmic Origins (University of Chicago Press, 2025), which reexamines the history and future of the cosmos through the lens of cutting-edge theory and observation.

In this talk, I will investigate the possibility of time being a fundamental physical entity in quantum gravity. I will briefly review the different notions of time in classical and quantum mechanics and then focus on explaining the fundamental clash that exists between the notions of time in Quantum Field theory and general relativity. This clash, related to the so-called problem of time, is at the core of the fundamental difficulty of quantizing gravity. I will review how recent results in Quantum gravity allow a surprising resolution of this fundamental puzzle through the presence of a quantum anomaly, which promotes time to a fundamental quantum observable in quantum gravity.

Spinfoams define a path integral for quantum gravity based on topological quantum field theory (TQFT). Following Lee’s early intuitions about the implementation of the holographic principle in the formalism, I will explain how the bulk dynamics arises as a network of boundary theories and draw lessons about the bulk-boundary relations. Diving into the example of 3d quantum gravity, this is illustrated by a beautiful holographic-like duality with the 2d inhomogeneous Ising model, with an underlying emergent supersymmetry, opening an intriguing interplay with the world of statistical physics with an elegant geometric formula for the zeroes of the Ising partition function.

I will briefly review an idea spurring from Lee, Stephon and Joao for an action with a varying cosmological constant and at most only three new free parameters, based on a `quasi-topological principle', and show how one can determine its number of degrees of freedom at the non-perturbative level, and some of the interesting features that arise in the analysis.

The original spinfoam model, the Ponzano-Regge spinfoam model of 3D Euclidean gravity with zero cosmological constant, has two properties in seeming contradiction: (1.) Its connection formulation consists in the imposition of exact flatness, and (2.) Its sum-over-spins formulation has, as its leading order large spin asymptotics, Regge calculus, modified to include an additional local discrete orientation variable for each tetrahedron, which, when fixed inhomogeneously, leads to critical point equations for the edge lengths which do not imply flatness, but rather allow spikes. We explore possible resolutions to this paradox which may also have relevance for the semiclassical regime of both Euclidean and Lorentzian 4D spinfoams, with both zero and non-zero cosmological constant, in which a similar sum over local orientations appears.