Scientific Series

Models for gravitationally induced decoherence can be formulated within the framework of open quantum systems, in which gravity is chosen as the environment and quantised together with the system under consideration. The effective dynamics of the system under the influence of the environment are encoded in the so-called master equation. Existing phenomenological models often take the Lindblad equation as the starting point for the master equation, while from the perspective of quantum gravity approaches, an underlying microscopic model from which a field-theoretical master equation can be derived seems to be the more accessible approach, which also makes it possible to establish a connection between microscopic and phenomenological models.

Perturbative calculations with light and polynomially self-interacting scalar fields in de Sitter spacetime are commonly plagued by infrared divergences as the scalar fluctuations become strongly coupled on large scales. Using techniques familiar from stochastic inflation, we treat super-Hubble fluctuations non-perturbatively through a probability distribution featuring composite operators of the light scalar. We find that these composite operators behave like late-time conformal primaries and admit a power-law four-point function in position space that is strikingly simple. We attribute physical meaning to these composite operators and conjecture that light interacting scalars in de Sitter in the stochastic picture hadronize into a tower of composite scalars on superhorizon scales. We further investigate whether these composite scalars can be described by a weakly-coupled effective field theory.

Two puzzling periods of accelerated expansion are thought to bookend the timeline of our Universe: cosmic inflation in the beginning, and dark energy domination in recent times. New observations from experiments such as DESI and the Simons Observatory are enabling unprcedented insights into both of these epochs. In this talk, I will highlight the critical and cross-cutting role of gravitational lensing in realizing this scientific promise, focusing on recent joint analyses of CMB lensing and galaxy clustering in redshift space, as well as delensing of CMB B-mode polarization. Along the way, I will discuss several enabling methodological developments, including a new understanding of extragalactic CMB foregrounds, fingers-of-god, pixel-free estimators, and more.

The original spinfoam amplitude, Ponzano-Regge, has two properties in seeming contradiction: (1.) It can be written as an integral of a product of Dirac delta functions imposing that holonomies be exactly flat, and (2.) In its original sum-over-spins form, its leading order large spin asymptotics consist in 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 necessarily imply flatness, but allow spikes. Of course, this apparent contradiction between flatness and spikes appears only for triangulations with bubbles, for which both of these formulations of the model are divergent and ill-defined anyway, and this may be the resolution of the paradox. However, we explore the possibility of another resolution of this paradox which may also have relevance for the semiclassical regime of 4D spinfoams, in which a similar sum over local orientations appears.

As part of a visit to Perimeter of a delegation from the ELI Beamlines laser facility in the Czech Republic, Dr. Bulanov will speak about potential topics for collaboration between Perimeter Institute and 
ELI theorists on topics related to high energy laser physics. To highlight the interplay between theory and experiment, Dr. Bulanov will briefly mention two experiments where this was realized and is currently planned at the ELI Beamlines facility.

This lecture discusses the stratification of regions in the space of real symmetric matrices. The points of these regions are Mandelstam matrices for momentum vectors in particle physics. The kinematic strata are indexed by signs and rank two matroids. Matroid strata of Lorentzian polynomials arise when all signs are nonnegative. We describe the posets of strata, for massless and massive particles, with and without momentum conservation. This is joint work with Veronica Calvo and Hadleigh Frost.

High school students learn how to express the solution of a quadratic equation in one unknown in terms of its three coefficients.  Why does this formula matter? We offer an answer in terms of discriminants and data. This lecture invites the audience to a journey towards non-linear algebra.