Talks

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.

What if the cosmic clock actually ticks? In this talk, I will explore growing evidence—from the structure of the cosmos to the frontiers of quantum gravity—that time may not be continuous, but fundamentally discrete. By rethinking the flow of time as a sequence of quantized events, we uncover surprising connections that challenge our classical intuitions and open new paths in fundamental physics.

If the Hamiltonian constraint were ever violated (in the early Universe, at high energies, etc), its subsequent restoration could never erase a memory effect of the original violation. Depending on the technicalities of restoration, the memory effect may be as boring as something which mimics standard dark matter, or an anisotropic extension of dark matter... Or, as crazy as something that acts like a dust fluid capable of attracting other matter but being attracted to nothing. Attracting without being attracted: we discuss how it might be possible in a relativistic theory of gravity, its implications to the foundations of physics (specifically Dirac's algebra of constraints underpinning relativistic physics), and what its observational hallmarks might be.

I report on recent progress of investigations of in-vacuo dispersion for GRB neutrinos, also highlighting the connection with a previous study done in collaboration with Lee Smolin investigating in-vacuo dispersion for GRB photons. The present status of IceCube neutrino observations provides preliminary encouragement for a scenario based on in-vacuo dispersion and the KM3-230213A neutrino recently annouced by KM3NeT fits rather naturally within the in-vacuo-dispersion scenario motivated by IceCube data.

The covariant dynamics of Loop Quantum Gravity answers the question of what we can in principle predict and observe in quantum gravity. It features a characteristic discrete quantum evolution, as proposed in a seminal paper by Smolin and Markoupoulou in 1997. Spinfoam transition amplitudes are local exactly thanks to this form of discreteness. Another property of the amplitudes is to be exactly finite when their formulation includes a cosmological constant: this adds further to the fundamental discreteness of spacetime quanta, and again it is a feature rooted in Lee’s Smolin seminal works.

I will examine a foundational assumption in quantum probability assignments, that experimental data arise from an identically and independently distributed (i.i.d.) ensemble. However, this assumption becomes problematic in the regime of quantum gravity. I will outline a proposal to resolve this issue by leveraging the tool of quantum reference frames in the measurement context.

In a series of papers [Phys. Rev. D 102, 124027 (2020)], [Phys. Rev. D 102, 124028 (2020)] I had the pleasure to explore with Cortês, Elitzur and Smolin realist formulations of quantum theory which obey either retrocausality or disordered causality. After briefly presenting these works, I will attempt to extend them by addressing time as a quantum observable using the Page-Wootters formalism. Within this framework, the clock is treated as a quantum subsystem, endowing the remainder of the system with a notion of time through entanglement. I will show the fruitfulness of this approach by deriving new time-energy uncertainty relations [Quantum 6, 683 (2022)] and by elucidating aspects of dynamical nonlocality [Phys. Rev. A 105, 042207 (2022)]. Furthermore, I will demonstrate that in the post-Newtonian regime, perspective-dependent non-unitarity and a distinct form of time–asymmetry naturally emerge when the clocks experience acceleration or gravitational effects [Commun. Phys. 5, 298 (2022)]. Finally, I will discuss the more recent works viewing all the above as spatiotemporal quantum reference frames [Phys. Rev. A 109, 032205 (2024)], [arXiv:2503.20090].