Origins of the Universe (2022)

 
Swampland Group
Lead Organizer: Cumrun Vafa, Harvard University

Three talks were given on the topic of swampland and its relation to cosmology. These talks were given by Miguel Montero, Georges Obied and Irene Valenzuela.

The Swampland Program aims to uncover the universal constraints that quantum gravity produces at low energies. These are not often apparent from a low-energy effective field theory (EFT) point of view, and one often argues for these from arguments involving black hole evaporation. The resulting constraints can be checked against the plethora of existing string theory constructions. As an illustration of the power of the approach, Montero’s talk focused on two of these constraints. The first was the “no global symmetries” conjecture and its recent application in higher dimensional supersymmetric quantum gravities to fully establish a match between the existing string theory constructions and what can be predicted from swampland principles. The second constraint, the “festina lente” (FL) bound, is an extrapolation of the weak gravity conjecture (WGC) to de Sitter space, resulting in constraints on the particle spectrum there. The standard model complies with the simplest application of this bound, but there are many more phenomenological implications to be explored.

Obied’s talk explored the interplay between swampland conjectures and models of inflation and light dark matter. It included a review of the WGC and the related FL bound. These have implications for light darkly and milli-charged particles and can rule out a large portion of parameter space. The FL bound also implies strong restrictions on the field content of our universe during inflation and presents an opportunity for inflationary model building. At the same time, it rules out some popular models like chromo-natural inflation and gauge-flation. Finally, another swampland conjecture related to Stückelberg photon masses is introduced and applied to models of massive dark photons in astroparticle physics where a large portion of parameter space is again ruled out.

Valenzuela’s talk provided a possible explanation of some of the main swampland conjectures based on finiteness of black hole entropy. Small charged black holes can lead to a violation of Bekenstein bound unless the EFT cut-off decreases as dictated by the swampland conjectures. It is also explained how several swampland criteria may be more fundamentally a consequence of the finiteness of quantum gravity amplitudes. Finally, she showed how a generalization of these ideas can lead to an upper bound on neutrino masses in terms of the cosmological constant, providing new insights into the naturalness issues observed in our universe.
 

Field-theory Aspects of Gravity and Cosmology Group
Lead Organizer: Alberto Nicolis, Columbia University

Three talks were given on the topic of Field-theory Aspects of Gravity and Cosmology presented by Paolo Creminelli, Alberto Nicolis and Federico Piazza.

The coefficients of the operators of an EFT are constrained to satisfy certain inequalities, under the (mild) assumption that the ultraviolet (UV) completion satisfies general requirements of causality and unitarity. These “positivity” constraints have been the subject of intense investigation for the last 15 years, since they prove rigorously that some low energy theories cannot be UV completed. We begin the extension of these ideas to theories where the Lorentz symmetry is spontaneously broken, as it happens in cosmology and condensed matter physics. As a starting point, we focus on a conformal field theory at finite charge. The low energy excitations of this system are described by an EFT for the Goldstone of the spontaneously broken charge: a conformal superfluid. We show that the coefficients of this EFT are constrained since, similarly to what happens in the Lorentz invariant case, one can run a dispersive argument for the 2-point function of the conserved current and of the stress-energy tensor.

The so-called trans-Planckian problem is the statement that in present-day cosmological observables there could be signatures of physics beyond the Planck scale, because of the exponential “stretching” of modes that took place during inflation. A popular counterargument is that, as long the Hubble rate during inflation is sub-Planckian, the expansion of the universe is adiabatic as far as trans-Planckian modes are concerned, and thus can only have exponentially small effects on them. We give quantitative support to this view by performing explicit computations in a new set of coordinates, which highlights that, for local physics, the expansion of the universe can be dealt with in perturbation theory.

The gravitational field and a certain number of other “reference” fields representing observers are put in a highly quantum state. It is possible to introduce an “equal time” distance operator among any pair of observers. The expectation value of such an operator has interesting properties. At small separation, it behaves as a Riemannian distance itself. At larger separations, however, it can show departures from Riemannian distances in that it can fail to be additive along a given geodesic. Possible Lorentzian extensions of this have been discussed. It has been argued that, in the Lorentzian setup, non-additivity of distances will imply violations of causality of some sort, building up at large separations.
 

Modern Inflationary Cosmology Group
Lead Organizer: Eva Silverstein, Stanford University

Modern Inflationary Cosmology Group participants reported on recent results, which included constraints on cosmological parameters including equilateral non-Gaussianity from large-scale structure (independent of CMB), calculations of non-perturbative non-Gaussianity and aspects of the wave functional of fields in cosmology, particle production and controlled realizations of warm inflation, examples of broad classes of string-theoretic de Sitter and anti-de Sitter solutions including concrete realizations of control parameters, calculations of the de Sitter horizon microstates in terms of black hole ones using a generalized solvable T-Tbar deformation (in place of BPS methods), and applications of early universe inflationary models to formulate a broad new class of optimization algorithms.
 

New Theoretical Avenues for the Early Universe Group
Lead Organizer: Mark Trodden, University of Pennsylvania

Five talks were given on topics relevant to this group. Four of the talks were by group members (Mark Trodden, Justin Khoury, Kurt Hinterbichler and Sam Wong), and the fifth was given by symposium participant Ruth Gregory with strong expertise in this area. Trodden and Khoury opted to deliver half-hour talks to ensure full slots for more junior presenters.

Trodden’s talk focused on how fundamental physics considerations can help address recent cosmological tensions — particularly the Hubble tension, in which measurements made using data from different cosmic epochs yield different results. Trodden described how some of the fine-tuning problems of the early dark energy solution to the Hubble tension can be addressed using couplings to other fields already present in cosmology (specifically neutrinos as well as dark matter), discussing the formulation, the cosmology and the constraints on such models, arising from both observational and theoretical considerations.

Probabilities in eternal inflation are usually defined in terms of frequencies, but this approach has failed to yield a unique answer. In his talk, Khoury presented a different approach, based on Bayesian reasoning. By adopting the least informative priors, he was led to well-defined, time-reparametrization invariant probabilities for occupying different vacua. Remarkably, these probabilities favor vacua whose surrounding landscape topography is that of a deep funnel, akin to folding funnels of naturally occurring proteins. Furthermore, by modeling the landscape as a random network, Khoury argued that the probabilities favor regions that are nearly tuned at the directed percolation phase transition.

Hinterbichler presented recent work in collaboration with Simons postdoc Kara Farnsworth and Ondřej Hulik, and forthcoming work in collaboration with Diego Hofman, Austin Joyce and Greg Mathys. New results included findings on conformal vs. scale invariance in various dimensions at the infrared fixed point of the renormalization group (RG) flow of Einstein gravity, including the result that conformal symmetry arises only in four dimensions. Other results included 1-form generalized global symmetries at the infrared (IR) fixed point of gravity that arise from gauge invariant conserved currents, as well as dualities and new anomalies that arise among them.

In the first half of Wong’s talk, he presented constraints on correlation functions from non-linearly realized symmetries, demonstrating how these symmetries lead to soft theorems about correlators. He also discussed their applications to fluctuations in primordial cosmology and superfluid theories. In the second half, Wong presented applications of black hole perturbation theory to probing hairy black holes and theories of gravity beyond GR, moving on to discuss how to resolve the black hole quasinormal mode instability.

Gregory also discussed a range of issues concerning the physics of black holes, particularly in de Sitter space, focused on their thermodynamic properties and on their behavior in the presence of scalar field.
 

Fiend and String Theory Investigations of Gravity and Cosmology
Speakers:
Austin Joyce, University of Chicago
Tony Padilla, University of Nottingham
Andrew Tolley, Imperial College

The members of the group covered various aspects of cosmology and gravitation descending from the top-down string-theory constructions (Padilla) and emerging in the bottom-up approach based on unitarity and analyticity of the low energy field theory S-matrix amplitudes (Joyce, Tolley).

The presentations made connections with the topics covered in other groups and stimulated further discussions. While Leah Jenks did not give a presentation, she defended her Ph.D. thesis during the symposium week and engaged in discussions of the presented topics with other participants.

J. Richard Bond
University of Toronto

Extending stochastic inflation to include coherent pulses to explain instability-driven multi-field simulation ensembles of concentrated primordial non-Gaussianity

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Paolo Creminelli
ICTP Trieste

Positivity constraints in Lorentz-breaking EFTs

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G. Bruno De Luca
Stanford University

Hyperbolic compactification of M-theory and dS quantum gravity

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Ruth Gregory
King’s College London

Slow roll with a black hole

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Justin Khoury
University of Pennsylvania

Criticality in Eternal Inflation

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Mehrdad Mirbabayi
ICTP

Shapes of non-Gaussinaity in warm inflation

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Miguel Montero
Harvard University

A Tour of the Swampland

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Jakob Moritz
Cornell University

Small Cosmological Constants in String Theory

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Alberto Nicolis
Columbia University

Cosmology as a small perturbation and the trans-Planckian problem

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Georges Obied
Harvard University

Inflation and light Dark Matter constraints from the Swampland

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Tony Padilla
University of Nottingham

Dark Energy and Quantum Gravity

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Federico Piazza
Centre de Physique Théorique

Understanding deviations from classicality in gravity

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Eva Silverstein
Stanford University

MIC Overview

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Mark Trodden
University of Pennsylvania

Couple Early Dark Energy

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Irene Valenzuela
CERN and IFT UAM-CSIC

Swampland, Black Holes and Cosmological Bounds

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Sam Wong
University of Pennsylvania

Ripples of the universe: primordial fluctuations and blackhole ringdown

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