Upcoming Journal Club Meetings
Title: Completeness Relation and Casimir Operator for the Virasoro Algebra
Speaker: Jean-François Fortin (Universite Laval)
Date: 2025-12-10, 14:00:00
Abstract
We discuss two basic objects of 2d local conformal field theories: the completeness relation and the Casimir operator. First, we propose an explicit expression for the inverse Shapovalov form in terms of the singular vectors and their conformal dimensions, that directly implies a novel expression for the completeness relation. We show that this result can be leveraged in the Virasoro OPE and the computation of Virasoro conformal blocks. Second, we determine generic Casimir operators for arbitrary algebras by deriving a recurrence relation in terms of their inverse Shapovalov form and solving it through repeated substitution. For the Virasoro algebra, our answer leads to a partial inversion of the Virasoro/Heisenberg dictionary and might be of interest for integrable models.
Title: Cosmic Parity Violation in the Large-scale Structure of the Universe
Speaker: Sha Azyzy (Max Planck Institute for Astrophysics)
Date: 2025-12-03, 13:30:00
Abstract
Parity-odd four-point correlation functions, or trispectra, of cosmic matter density fields provide a unique probe of fundamental symmetries in cosmology. Trispectra of primordial matter density fluctuations produced in the early universe are modified by the subsequent nonlinear structure formation. In this work, we compute the nonlinear evolution of the parity-odd matter trispectrum to one-loop order, i.e., to third order in density fluctuations, within the framework of effective field theory of the large-scale structure of the universe. By analyzing the different terms in the perturbation series, we demonstrate the structure of infrared divergence cancellations, as required by the equivalence principle. We also derive the forms of the counterterms required to renormalize the ultraviolet divergences. Adopting a specific model for a primordial parity-odd trispectrum, we numerically compute the leading-order effects of nonlinear gravitational evolution and study its impact on baryonic acoustic oscillations within the signal. These calculations are essential for comparing the observed trispectra of nonlinear cosmic density fields with theoretical expectations. (based on arXiv:2510.06164)
Title: Gravitational Waves from Graviton Production in the Early Universe
Speaker: Yong Xu (McGill University)
Date: 2025-11-26, 13:30:00
Abstract
The early universe naturally produces gravitons through their unavoidable couplings to the evolving cosmological background. Once generated, these gravitons propagate freely, forming a stochastic gravitational-wave (GW) background. Within an inflationary framework, gravitons can be sourced after inflation through multiple channels—including non-thermal, semi-thermal, and thermal processes. Each channel leaves distinct imprints on the GW spectrum, providing a direct probe of the universe’s state before Big Bang Nucleosynthesis (BBN). In this talk, I will present recent advances in understanding graviton production across these channels. I will also demonstrate how the resulting GW backgrounds act as powerful cosmic messengers that probe the unknown physics before BBN.
Title: TBA
Title: Parity and Baryogenesis in Theories Beyond Standard Cosmology
Speaker: Tatsuya Daniel (McGill University)
Date: 2025-11-12, 13:30:00
Abstract
Motivated by the open questions of quantum gravity, the matter-antimatter asymmetry, and the dark sector, I will talk about theories beyond the standard cosmological and gravitational paradigms, in particular the theoretical and observational prospects of such theories. First, I will explain a parameterization I developed to use gravitational waves for constraining a wide class of beyond-GR theories in explicitly parity-violating and parity-invariant contributions. Such theories can contain candidates for the dark sector, including axions, Kalb-Ramond dark matter, U(1) vector fields, and the dilaton. Then, I will discuss how a general class of spontaneous baryogenesis models, which involve a scalar field coupled to a baryon current, may satisfy cosmological isocurvature constraints under certain conditions.
Title: Aspects of Cosmic Strings and Their Gravitational Waves
Speaker: Tobias Schröder (McGill University)
Date: 2025-11-05, 13:30:00
Abstract
After a brief introduction to cosmic strings, I discuss the gravitational wave background (GWB) generated by a network of these objects. I demonstrate that strings with very low tension can produce GWB spectra that differ qualitatively from those of higher-tension strings. Specifically, low-scale string spectra exhibit a distinct oscillatory pattern with amplitude dips at integer multiples of the frequency of the first dip. Despite the low tensions, such a GWB is potentially detectable in future experiments. The analysis up to this point assumes completely stable strings. Turning away from the GWB, I consider metastable strings that are stabilized within parts of a symmetry-breaking chain but not in the overall symmetry breaking. I construct field configurations describing the unwinding of such metastable strings, which terminate on monopoles carrying partially unconfined flux. This can provide a foundation for future studies of their decay rates.
Title: The Quantum Hall Effect for Gravity and the Normalizability of Graviton States
Speaker: Heliudson Bernardo (University of Lethbridge)
Date: 2025-10-29, 13:30:00
Abstract
I will talk about expected properties of topological theta vacua and how they can be implemented in the Wheeler-DeWitt approach to quantum gravity in the presence of a cosmological constant. I will also explain how the physics of the Hamiltonian constraint in the self-dual formulation is analogous to the quantum Hall effect, with the cosmological constant playing the role of a quantum gravitational Hall resistivity. Then I will discuss graviton states and revisit the normalizability of the wavefunction of the universe in the self-dual formulation.
Title: TBA
Title: Axion U(1)-warm inflation
Speaker: Vahid Kamali (McGill University)
Date: 2025-10-08, 13:30:00
Abstract
In this talk, I will present our new proposal for warm inflation driven by axion-like particles interacting with gauge fields. I will discuss the dissipation mechanism that arises from thermal fluctuations of the electromagnetic fields. Our results show consistency with CMB observations even for a natural sub-Planckian axion decay constant, f less than Mp . I will also argue that this framework provides a useful setup for studying thermal axion dark matter.
Title: Ultraheavy dark matter: How to build it and how to find it
Speaker: Melissa Diamond (McGill University)
Date: 2025-10-01, 13:30:00
Abstract
This talk will cover recent and ongoing work around heavy and composite dark matter candidates. This will cover a few formation mechanisms, as well as some possible detection techniques. We will discuss how self-interacting dark sectors can lead to the formation of dark compact structures. The typical size and mass of the structures formed reflect the microphysics of the dark matter and its interactions. If the dark matter composing these structures interacts with photons, they will cause stars to appear to dim when passing across the line of sight to the star. We can search for this effect using existing data from microlensing searches. I will also touch on ongoing work around how primordial black holes can upscatter gravitational waves causing distortions to the stochastic gravitational wave background, an effect simillar to the SZ effect in the CMB.
Title: Maximizing the Interaction Strength
Speaker: Miguel Correia (McGill University)
Date: 2025-09-24, 13:30:00
Abstract
Is QCD the most strongly interacting theory consistent with the basic principles of causality, unitarity, and crossing symmetry? This foundational question motivated the original bootstrap program of the 1960s, which ultimately failed due to the need of uncontrolled approximations. In this talk, I will revisit the original question with the use of the modern non-perturbative S-matrix Bootstrap. I will derive a universal bound on the scattering cross-section at finite energies and present the conjectured "Froissart Amplitude," which asymptotically saturates this bound. I will contrast these results with experimental data from proton–proton scattering.
Title: QCD Equation of State, Sign Problem, and the Lattice
Speaker: Guy Moore (Darmstadt, Tech. U.)
Date: 2025-09-10, 13:30:00
Abstract
The equation of state of high-density QCD is important astrophysically.
Unfortunately, our best nonperturbative tool, lattice QCD, fails because of a sign problem which I will explain.
Then I will propose a partial solution -- we can use the lattice to place a rigorous upper bound on the pressure
as a function of chemical potential, which can be used to constrain the equation of state.
I show that, at large chemical potentials where perturbation theory should work, the bound becomes sharp,
with the first corrections arising at $\alpha_s^3$, with an already-calculated coefficient.
Title: Synergies between CMB spectral distortions and 21cm cosmology: A dark photon case study.
Speaker: Bryce Cyr (Massachusetts Institute of Technology)
Date: 2025-09-03, 13:30:00
Abstract
The frequency spectrum of CMB photons contains key information which can allow us to see beyond the veil of the last scattering surface, providing us with a way to precisely reconstruct our thermal history to much higher redshifts. In this talk, I will provide a gentle and intuitive introduction to CMB spectral distortion theory, before presenting new constraints on dark photons through the resonant conversion process. Following this, I will loosely motivate a connection between dark photons and late time universe anomalies, such as the ARCADE-2 radio background excess and the EDGES global 21cm signal. This will allow me to discuss an effect known as soft photon heating, which is a crucial ingredient in understanding the interplay between radio backgrounds and 21cm signals.
Title: Looking at bulk points in general geometries
Speaker: Joydeep Chakravarty (McGill University)
Date: 2025-04-23, 11:00:00
Abstract
The holographic correspondence predicts that certain strongly coupled
quantum systems describe an emergent, higher-dimensional bulk spacetime
in which excitations enjoy local dynamics. We consider a general
holographic state dual to an asymptotically AdS bulk spacetime, and
study boundary correlation functions of local fields integrated against
wavepackets. We derive a factorization formula showing that when the
wavepackets suitably meet at a common bulk point, the boundary
correlators develop sharp features controlled by flat-space-like bulk
scattering processes. These features extend along boundary hyperboloids
whose shape naturally reveals the bulk geometry. We discuss different
choices of operator ordering, which lead to inclusive and
out-of-time-ordered amplitudes, as well as fields of various spins and
masses.
Title: DM @ Finite Temperature
Speaker: Saniya Heeba (McGill University)
Date: 2025-04-16, 11:00:00
Abstract
The microphysical properties of Dark Matter (DM), such as its mass and coupling strength, are typically assumed to retain their vacuum values when considering DM behaviour at a range of scales. However, DM interactions in different astrophysical and cosmological environments may be impacted by the properties of the background which in turn can substantially affect both DM production and the detection prospects for any given model. In the recent years, this has generated a lot of interest in calculating DM observables at finite temperature and density.
In this talk, I will provide an overview of what these effects are and how to calculate these observables at finite temperature.
Title: What is the graviton pole made of?
Speaker: Kelian Häring ( University of Amsterdam)
Date: 2025-04-09, 11:00:00
Abstract
In this talk, I will start by reviewing dispersion relations in the presence of gravity. This leads to a dispersion relation for the graviton pole. Then, I will explore the physical mechanisms responsible for generating this pole in the dispersion relations. These mechanisms are characterized by an energy scale at which the graviton pole is generated at fixed impact parameters. At large impact parameters, the eikonal model of high-energy gravitational scattering is identified as a universal mechanism that generates the graviton pole in dispersion relations. At smaller impact parameters, the graviton pole can arise from stringy higher-spin resonances. If time permits, I will discuss a Tauberian theorem for the graviton pole, which must be satisfied by any UV completion of gravity that admits twice-subtracted dispersion relations. This talk is based on https://arxiv.org/abs/2410.21499.
Title: Bubble wall velocity and gravitational waves from first-order QCD phase transition
Speaker: Benoit Laurent (McGill University)
Date: 2025-04-02, 11:00:00
Abstract
Although the QCD phase transition is a crossover in the standard model, nonstandard effects such as a large lepton asymmetry are known to make it first order, with possible applications to gravitational wave production. This process is sensitive to the speed of the bubble walls during the phase transition, which is difficult to compute from first principles.
In this talk, I will take advantage of recent progress on wall speed determinations to provide a simple estimate which constrains the wall speed to be significantly lower than what has been used in previous literature. This, in turn, strongly suppresses the production of gravitational waves, to a level that is just out of reach of the most sensitive projected experiment for this signal.
Title: The boundary imprint of local bulk dynamics in general geometries
Speaker: Keivan Namjou (McGill University)
Date: 2025-03-26, 11:00:00
Abstract
I will explore the boundary imprint of bulk lightcones in spacetimes with boundaries, motivated by the holographic correspondence. These lightcones can be observed whenever a localized event takes place in the bulk. The associated boundary surfaces (hyperboloids) reveal the bulk conformal metric. Moreover, studying the amplitude of the process allows us to fix the conformal factor. I will explain how to study these processes and their corresponding boundary surfaces using a Hamilton-Jacobi description, and investigate the implications regarding causality.
Title: Supersizing hydrodynamical simulations of reionization using EFT
Speaker: Katelin Schutz (McGill University)
Date: 2025-03-19, 11:00:00
Abstract
I will show that perturbative techniques inspired by effective field theory (EFT) can be used to “paint on” the 21 cm field during reionization using only the underlying linear density field. This procedure is accurate to within O(10%) on large scales and thus can be used to enlarge or “supersize” hydrodynamical simulations. In particular, the EFT provides a mapping between the linear density field and the nonlinear 21 cm field, which I will review in this talk. I will show how this mapping can be reliably extracted from relatively small simulation volumes using the THESAN suite of simulations as a test case. Specifically, if we fit the EFT coefficients in a small ∼ 5% sub-volume of the simulation, we can accurately predict the 21 cm field in the rest of the simulation given only the linear density field. I will show that our technique is robust to different models of dark matter and differences in the sub-grid implementation of reionization in the simulation.
Title: False and real decoherence of primordial perturbations
Speaker: Junsei Tokuda (McGill University)
Date: 2025-03-12, 11:00:00
Abstract
Quantum fluctuations of the metric are amplified during inflation, producing the macroscopic perturbations observed in the late universe. To clarify whether these fluctuations retain their quantum coherence, we study the decoherence of super-Hubble modes induced by gravitational nonlinearities during inflation. We show that cubic gravitational couplings, constrained by the soft theorem, lead to a UV-divergent decoherence rate at one loop, arising from violent zero-point fluctuations in the deep UV. We argue that these divergences are unobservable, as they vanish when considering time-smeared observables that better reflect realistic measurements. We then compute the finite decoherence rate induced by an excited environment, which remains robust against such smearing. This talk is based on work in preparation.
Title: What if string theory has a de Sitter excited state?
Speaker: Keshav Dasgupta (McGill University)
Date: 2025-02-19, 11:00:00
Abstract
So far, despite many heroic attempts, no accelerating solutions have been demonstrated in string theory.
In this talk I’ll discuss why there may not exist such solutions at the vacuum level in string theory, and
point out that only way they would exist in string theory is if we view them as excited states.
Reading suggestion: Section 8 of https://arxiv.org/pdf/2404.11680 summarizes our understanding in a
non-technical language.
Title: Investigating cosmic strings using large-volume hydrodynamical simulations in the context of JWST’s massive UV-bright galaxies
Speaker: Hao Jiao (McGill University)
Date: 2025-02-12, 11:00:00
Abstract
Recent observations from the James Webb Space Telescope (JWST) have uncovered an unexpectedly large abundance of massive,
UV-bright galaxies at high redshifts z ≳ 10, presenting a significant challenge to established galaxy formation models within the
standard ΛCDM cosmological framework. Cosmic strings, predicted by a wide range of particle physics theories beyond the Standard
Model, provide a promising potential explanation for these observations. They may act as additional gravitational seeds in the early
universe, enhancing the process of high-redshift structure formation, potentially resulting in a more substantial population of massive,
efficiently star-forming galaxies. We numerically investigate this prediction in large-volume hydrodynamical simulations using the
moving-mesh code arepo and the well-tested IllustrisTNG galaxy formation model. We evaluate the simulation results in the context
of recent JWST data and find that sufficiently energetic cosmic strings produce UV luminosity and stellar mass functions that are in
slightly to substantially better agreement with observations at high redshifts. Moreover, we observe that the halos seeded by cosmic
strings exhibit a greater efficiency of star formation and enhanced central concentrations. Interestingly, our findings indicate that the
simulations incorporating cosmic strings converge with those from a baseline ΛCDM model by redshift z ∼ 6. This convergence
suggests that the modified cosmological framework effectively replicates the successful predictions of the standard ΛCDM model at
lower redshifts, where observational constraints are significantly stronger. Our results provide compelling evidence that cosmic strings
may play a crucial role in explaining the galaxy properties observed by JWST at high redshifts while maintaining consistency with
well-established models at later epochs.
Title: Cosmic axion strings from string theoretic axions
Speaker: Jim Cline (McGill University)
Date: 2025-02-05, 11:00:00
Abstract
Axion cosmic strings can produce the dominant component of axion dark
matter if the Peccei-Quinn phase transition occurs after inflation. This
allows one to link the axion mass to its relic density and thereby predict
the axion mass. It has been argued that QCD axions derived from string
theory (which can have a natural protection mechanism against PQ-symmetry
breaking effects from quantum gravity) generically cannot form axion
strings, and thus would not have this advantage. Here I demonstrate a
simple counterexample where the axion arises from a U(1) gauge symmetry in
a warped compactification from 5D. The cosmic strings have parametrically
lower tension than their conventional field theory counterparts.
Title: Recent progress in Landau Analysis: Schwinger, Baikov, and implementations
Speaker: Mathieu Giroux (McGill University)
Date: 2025-01-29, 11:00:00
Abstract
The purpose of this talk is to review recent progress in Landau analysis, which aims to predict the singularity structure of Feynman integrals before explicitly evaluating them. In the first part, I will discuss the advantages and disadvantages of formulating this problem in both Schwinger parameter space and momentum (Baikov) space. In the second part, I will explain how the latter approach extends the powerful unitary-based method of arXiv:2406.05241 beyond two-particle cut-reducible graphs. To demonstrate its efficiency, I will present new results for multi-loop and multi-scale Feynman integrals, derived using an automatized Mathematica implementation in preparation.
Title: Do Observations Prefer Thawing Quintessence?
Speaker: Guillaume Payeur (McGill University)
Date: 2025-01-22, 11:00:00
Abstract
In light of recent observations by the Dark Energy Spectroscopic Instrument (DESI), we study evidence for thawing quintessence over a cosmological constant as dark energy, with emphasis on the effect of the choice of priors. Working with a parametrization for the equation of state parameter motivated by the theory, we analyse the DESI BAO data jointly with Planck 2018 and Pantheon+ supernovae data, and find a preference for thawing quintessence compared to a bare cosmological constant only if we use priors which are heavily informed by the data itself. If we extend the priors to physically better motivated ranges, the evidence for thawing quintessence disappears.
Title: Learning the Morphology and Small-Scale Structure of the Galactic Center Gamma-ray Excess
Speaker: Yitian Sun (McGill University)
Date: 2025-01-15, 11:00:00
Abstract
Since its discovery, debates have been ongoing about the Galactic Center gamma-ray Excess (GCE) and its nature -- whether it arises from millisecond pulsars or annihilating dark matter. To distinguish these hypotheses, studies focused on examining the GCE’s morphology and small-scale photon clustering. However, most studies have relied on fits using inflexible templates for the signal and the background, which have been shown to be unreliable due to mis-modeling.
In this talk, I will present a first attempt at modeling the GCE with a Gaussian process -- a flexible, non-parametric morphological model. I will then present an analysis of the small-scale structure featuring a unprecedented level of template flexibility. I will introduce our pipelines enabled by powerful yet fully interpretable machine learning techniques, discuss the results and their implications for the current status of our understanding of the GCE, and outline the future directions of our work.
Title: Gravity and a universal cutoff for field theory
Speaker: Simon Caron-Huot (McGill University)
Date: 2024-12-11, 11:00:00
Abstract
Many arguments indicate that in the presence of a large number of fields, local field theory must "break down" at a length scale that is parametrically larger than the Planck length, the so-called species scale. Aiming to clarify what happens then, we analyze the 2->2 scattering amplitudes of gravitons and contrast calculable field-theoretic contributions with possible short-distance completions, such as string theory or Planck-scale strongly-coupled physics. The latter reveal a phenomenon of "high-spin onset" which we argue provides a simple diagnosis for the (in)applicability of local field theory. Based on arXiv:2408.06440.
Title: Wave Interference in Self-Interacting Fuzzy Dark Matter
Speaker: Christian Capanelli (McGill University)
Date: 2024-12-04, 11:00:00
Abstract
In the Fuzzy Dark Matter (FDM) scenario, the dark matter is composed of an ultra-light scalar field with a coherence length on the order of a kiloparsec. Naively, the early-time field configuration will exhibit order unity density fluctuations that are characteristic of wave-interference between free-streaming field modes. However, scalar fields generically have quartic self-interactions that can modify the field's dispersion. In this talk, I will describe the role that even feeble self-interactions have on the FDM free streaming and resulting density fluctuations. Namely, for a "warm" initial condition (motivated by post-inflationary ALP misalignment), I will characterize the effects of both attractive and repulsive self-interactions on the density power spectrum, studying the evolution over three relevant dynamical time scales. Finally, I will speculate on the application of our work to more realistic astrophysical environments.
Title: A Superfluid Dark Matter Cosmic String Wake
Speaker: Aline Favero (McGill University)
Date: 2024-11-27, 11:00:00
Abstract
We study the effects of superfluid dark matter on the structure of a cosmic string wake, considering both the effects of regular and quantum pressure terms. We consider the total fluid to consist of a combination of baryons and dark matter. Hence, we are also able to study the effects of superfluid dark matter on the distribution of baryons inside the wake. We focus on parameter values for the superfluid dark matter which allow a MONDian explanation of galaxy rotation curves.
Title: Plasmon production of axions and dark photons in anisotropic plasmas: the effect of magnetic fields
Speaker: Nirmalya Brahma (McGill University)
Date: 2024-11-20, 11:00:00
Abstract
Some of the most stringent constraints on physics beyond the Standard Model(BSM) arise from considerations of particle emission from astrophysical plasmas. However, many studies assume that particle production occurs in an isotropic plasma environment. This condition is rarely (if ever) met in astrophysical settings, for instance due to the ubiquitous presence of magnetic fields. In anisotropic plasmas, the equations of motion are not diagonal in the usual polarization basis of transverse and longitudinal modes, causing a mixing of these modes and breaking the degeneracy in the dispersion relation of the two
transverse modes. This behavior is captured by a 3 \times 3 mixing matrix \pi^{IJ} , determined by projecting the response tensor of the plasma \Pi^{\mu\nu} into mode space, whose eigenvectors and eigenvalues are related to the normal modes and their dispersion relations. In this work, we provide a general formalism for determining the normal modes of propagation that are coupled to axions and dark photons in an anisotropic plasma. As a key example, we work with magnetized plasmas and provide analytic approximations for the normal modes and their dispersion relations assuming various plasma conditions that are relevant to astrophysical environments. These approximations will allow for a systematic exploration of the effects of plasma anisotropy on BSM particle production.
Title: The Regge Limit of the AdS Virasoro-Shapiro Amplitude
Speaker: Clement Virally (University of Oxford)
Date: 2024-11-13, 11:00:00
Abstract
In this talk, we consider the Regge limit of the Virasoro-Shapiro amplitude. This amplitude is understood through the AdS/CFT correspondence as a 4-point correlator in N=4 SYM. At strong 't Hooft coupling, which corresponds to a small curvature limit in the bulk, we can expand it perturbatively. We show that, following a few integral transforms, the correlator in the Regge limit can be computed from the action of appropriate differential operators on the flat space result. We finally discuss the relation of this result to the worldsheet representation of the AdS Virasoro-Shapiro amplitude.
Title: Formation of defects associated with both spontaneous and explicit symmetry breaking
Speaker: Michiru Uwabo (Ochanomizu University/IBS)
Date: 2024-11-06, 11:00:00
Abstract
We discuss formation of cosmic strings associated with a spontaneously broken approximate U(1) symmetry by performing classical field-theoretical
simulations. An original U(1) symmetry is explicitly broken down to its
subgroup Z_N even before spontaneous breaking takes place. We estimate the
ratio of explicit breaking to that of spontaneous breaking for which
topological defects for N=1 and N=2 are formed. For N=1, a cosmic string
attached to a single domain wall can be formed when the amount of the explicit breaking is three orders of magnitude smaller than that of the spontaneous breaking. For N=2, no matter how large the explicit breaking is, domain walls are inevitably formed as long as the temperature of the Universe is high enough to restore Z_2 symmetry. In that case, cosmic strings are also inevitably formed as long as the amount of the explicit breaking is smaller than that of the spontaneous breaking.
Title: Dynamical Chern-Simons gravity: Anomalies, gravitational axions, and neutrinos
Speaker: Heliudson Bernardo (Brown University)
Date: 2024-10-30, 11:00:00
Abstract
Motivated by the interplay between the scalar-gravitational Pontryagin term in dynamical Chern-Simons (dCS) gravity and the baryon-lepton (B-L) number symmetry in the Standard Model (SM), I will revisit some aspects of the dCS classical and quantum symmetries. It has been argued that dCS gravity can be reduced to Einstein gravity after "rotating away" the gravitational-Pontryagin coupling into the phase of the Weinberg operator -- analogous to the rotation of the axion zero-mode into the quark mass matrix. I will show that this is not the case, precisely because of the B-L chiral gravitational anomaly. Then I will introduce a minimal extension of the SM where the coupling of the dCS scalar with right-handed neutrinos induces both the scalar-Pontryagin coupling and an axion-like phase in the dimension-five Weinberg operator at low energies.
Title: Photon self-energy at all temperatures and densities in all of phase space
Speaker: Hugo Schérer (McGill University)
Date: 2024-10-02, 11:00:00
Abstract
Dark matter could reside in a dark sector that contains a dark photon, a spin-1 gauge boson that would couple weakly to the Standard Model photon. This coupling results in interesting phenomenology in a variety of physical contexts. In many environments, such as the early universe or stars, finite temperature effects modify the properties of particles, including their dispersion relations. In particular, the Standard Model photon acquires an effective mass, or self-energy. This impacts its coupling to the dark photon in qualitatively new ways.
In an isotropic background comprised of free charges, the transverse and longitudinal modes of the photon acquire large corrections to their dispersion relations, described by the in-medium photon self-energy. Previous work has developed simple approximations that describe the propagation of on-shell photons in plasmas of varying temperatures and densities. However, off-shell excitations can also receive large medium-induced corrections, and the on-shell approximations have often been used in an effort to capture these effects. In this work we show that the off-shell self-energy can be qualitatively very different than the on-shell case. We develop analytic approximations that are accurate everywhere in phase space, especially in classical and degenerate plasmas.
Title: Spherical symmetry in effective loop quantum gravity
Speaker: Edward Wilson-Ewing (University of New Brunswick)
Date: 2024-09-25, 11:00:00
Abstract
I will give a brief overview of the quantization of the spherically symmetric Lemaître-Tolman-Bondi spacetime in loop quantum gravity, and apply the resulting effective dynamics to black hole collapse models and to cosmology.
Title: Glauber-Sudarshan States and Wheeler-De Witt equation
Speaker: Bohdan Kulinich (McGill University)
Date: 2024-09-18, 11:00:00
Abstract
Arguably, one of the possible resolutions to the question of the four-dimensional de Sitter space within M-theory would be to view a transient four-dimensional de Sitter phase in string theory as an excited state over a supersymmetric warped Minkowski background and not as a vacuum state. It seems that using the resurgent structure of observables computed from the path integral over a specific class of excited states called the Glauber-Sudarshan states, four-dimensional de Sitter in the flat slicing can be constructed.
I will discuss what happens to the Glauber-Sudarshan states that are off-shell, i.e. states that do not satisfy the Schwinger-Dyson equations. We argue that these Glauber-Sudarshan states, including the on-shell ones, are controlled by a larger envelope wave functional, namely a wave functional of the universe, which surprisingly satisfies a Wheeler-De Witt equation.
Title: Statistics of three-dimensional black holes from Liouville line defects
Speaker: Viraj Meruliya (McGill University)
Date: 2024-09-11, 11:00:00
Abstract
Low energy effective theories of gravity capture statistics of observables in the UV complete theory. In this talk, I will explore an explicit example in higher dimensions involving thin shells of matter that can collapse to form black holes. Specifically, I will describe these solutions in 3D gravity and demonstrate how the behavior of black holes and wormholes can be reproduced from the dual 2D conformal field theory (CFT) using the large-c conformal bootstrap. In the CFT framework, thin shells correspond to line defects. The crossing equations yield a universal formula for the average high-energy matrix elements of a line defect in any compact, unitary 2D CFT with central charge c>1. I will discuss the connections between three distinct quantities: the statistics of line defects in holographic CFTs, individual matrix elements of a line defect in Liouville CFT, and the on-shell action of black holes and wormholes in 3D gravity. Assuming Gaussian statistics for the line defects, I will also show how a specific class of wormholes can be reproduced by the dual CFT.
Title: TBA
Title: A new observable for holographic cosmology
Speaker: Keivan Namjou (McGill University)
Date: 2024-09-04, 11:00:00
Abstract
The double-cone geometry is a saddle of the gravitational path integral, which explains the chaotic statistics of the spectrum of black hole microstates. This geometry is the usual AdS-Schwarzschild black hole, but with a periodic identification of the time coordinate; the resulting singularity at the black hole horizon is regulated by making the geometry slightly complex. In this talk, I will consider generalizations of the double-cone geometry, in three dimensions, which include the Lorentzian cosmology that sits between the event horizon and the black hole singularity, and discuss what we can learn about the observables in the dual conformal field theory from these geometries.
Title: Gravity of String Theory: Formalism and Test
Title: 2D Conformal Field Theories and Resurgence
Speaker: Viraj Meruliya (McGill)
Date: 2024-04-03, 11:00:00
Abstract
We consider two-dimensional Conformal Field Theories (CFTs) in the semi-classical limit of large central charge-c. Observables within this regime are amenable to analysis via a 1/c expansion, revealing an asymptotic series that demands the inclusion of non-perturbative effects. Techniques like Borel resummation can be used to systematically study these effects. These ideas will be concretely applied to study 1) Sphere 4-Point Conformal Blocks in Generic CFTs, where we explore how non-perturbative effects manifest as contributions from non-vacuum operators and 2) Structure constants in Liouville theory in which case these effects are understood as complex solutions to Liouville equation.
Title: Accretion onto Oscillating Cosmic String Loops
Speaker: Jiao Hao (McGill )
Date: 2024-03-20, 11:00:00
Abstract
Cosmic string loops could act as non-linear seeds in the early universe and play an important role in explaining many phenomena which are in tension with the standard ΛCDM model. Hence, the accretion process onto cosmic string loops should be studied in detail. Most previous works view loops as point masses and ignore the impact of the finite loop size. We utilize the Zel’dovich approximation to calculate the non-linear mass sourced by a static extended loop with a time-averaged density profile derived from the trajectory of the loop oscillation, and compare the result with what is obtained with a point-mass source. We find that the finite size of a loop mainly affects the evolution of turnaround shells during the early stages of accretion, converging to the point mass result after a critical redshift. The total accreted mass surrounding a loop is suppressed relative to the point mass case. As an immediate extension, we also qualitatively analyze the accretion onto moving point masses and onto moving extended loops. In addition to the reduction in the nonlinear mass, the loop finite size also changes the shape of the turnaround surface at the early stages of accretion.
Title: Brane motion in a compact space: adiabatic perturbations of brane-bulk coupled fluids
Speaker: Fangyi Guo (McGill)
Date: 2024-03-06, 11:00:00
Abstract
When a brane is moving in a compact space, bulk-probing signals originating at the brane can arrive back at the brane outside the light cone of the emitting event. In this work, we study how adiabatic perturbations in the brane fluid, coupled to a bulk fluid, propagate in the moving brane. In the non-dissipative regime, we find an effective sound speed for such perturbations, depending on the brane and bulk fluid energy densities, equations of state, and brane speed. In the tight-coupling approximation, the effective sound speed might be superluminal for brane and bulk fluids that satisfy the strong energy condition. This has immediate consequences for brane-world cosmology models.
Title: Cosmological Production of Kalb-Ramond-like Particles
Speaker: Christian Capanelli (McGill)
Date: 2024-02-14, 11:00:00
Abstract
The Kalb-Ramond field most notably appears in the context of string theory, but has largely been unexplored in the context of cosmology. In this work, motivated by the Kalb-Ramond field in string theory, and antisymmetric tensor fields that emerge in effective field theories, we study the primordial production of interacting massive Kalb-Ramond-like-particles (KRLPs). KRLPs contain features of both dark photon and axion models, which can be appreciated via their duality properties. We study early-universe production of KRLPs via the freeze-in mechanism, considering a `dark photon-like' interaction, an `axion-like' interaction, and a `Higgs portal' interaction, as well as production via cosmological gravitational particle production. We find that as a dark matter candidate, KRLPs can be produced by all of the above mechanisms and account for the relic density of dark matter today for a wide range of masses.
Title: Static sphere observers and geodesics in Schwarzschild-de Sitter spacetime
Speaker: Mir Mehedi Faruk (McGill)
Date: 2024-02-07, 11:00:00
Abstract
We will study null radial geodesics in Schwarzschild-de Sitter spacetime connecting two conjugate static sphere observers, i.e. free-falling observers at a fixed radius in between the two horizons (based on arXiv:2312.06878). We explicitly determine the changes in the causal structure with respect to these natural observers as a result of the inward bending of the black hole singularity, as well as the outward bending of asymptotic infinity. Notably, the inward and outward bending changes as a function of the black hole mass, first increasing towards a maximum and then decreasing to vanish in the extreme Nariai limit.
Title: Effective mass and symmetry breaking in the IKKT matrix model from compactification
Speaker: Samuel Laliberté (McGill)
Date: 2024-01-31, 11:00:00
Abstract
The IKKT model is a promising candidate for a non-perturbative description of Type IIB superstring theory. A modified version of this model, with a Lorentz invariant mass term, is known to have interesting early universe solutions. This mass term, however, is often introduced by hand or serves as a regulator in the theory. In this talk, I will show that a Lorentz invariant mass term can arise naturally in the IKKT model under certain choices of compactification. When six dimensions are compactified, the S(1,9) symmetry of the Lorentzian IKKT model is naturally broken to SO(1,3) x SO(6). This opens the way for solutions of the IKKT model where three spacial dimensions become large, and six others stay small.
Title: Distance conjecture, species scale, and the pattern
Speaker: Brian McPeak (McGill )
Date: 2024-01-24, 11:00:00
Abstract
It has been observed in examples in string theory that moving massless moduli infinite distances in field space has the effect of causing a tower of states to become light, and causing the species scale to become light, exponentially fast in the distance. In this talk, I will review these two observations – the so-called Distance Conjecture and Species Scale Distance Conjecture – and I will discuss recent work uncovering a universal "Patten" relating the two.
Title: fslauh
Title: q-CFTs from Quantum Groups
Speaker: Shounak De (Brown University)
Date: 2023-11-29, 11:00:00
Abstract
The uniqueness of the Veneziano amplitude has attracted much theoretical attention since its inception in the 1960s. While there have been proposals for various deformations to the Veneziano amplitude that satisfy the bootstrap constraints, a worldsheet realisation of such models has remained elusive. In this talk, we shall outline a systematic construction of q-deformed CFTs whose underlying conformal group is the quantum group SU(1,1)_q. After reviewing existing literature on the set of observables emanating from such q-CFTs, we will discuss the construction of q-deformed amplitudes derived using invertible coproduct maps called Drinfel’d twists. Upon a suitable analytic continuation, we show that these twisted amplitudes are manifestly dual resonant. We will conclude by discussing a potential worldsheet realisation (or the lack thereof) of a particular q-deformation of the Veneziano amplitude, the Coon amplitude, in the space of such worldsheet q-CFTs. The talk will be based on 2309.07214.
Title: Crossing beyond scattering amplitudes
Speaker: Mathieu Giroux (McGill)
Date: 2023-11-22, 11:00:00
Abstract
Introduced in the mid-50s, crossing symmetry in interacting quantum field theory suggests that particles and anti-particles traveling back in time are indistinguishable. This perspective has significant practical implications for perturbative computations and for the S-matrix bootstrap. To prove this property rigorously, one needs to show that on-shell observables across different channels are boundary values of the same analytic function. For the simplest cases of 2-to-2 and 2-to-3 scattering, the known non-perturbative proofs rely heavily on physical principles like (micro)causality, locality, and unitarity, but also on a significant amount of several variables complex analysis, which makes their extension to arbitrary multiplicity quite challenging. In this talk, we review recent progress regarding the implications of crossing symmetry in quantum field theory, assuming analyticity. The story begins by asking what can be measured asymptotically in quantum field theory? Among the answers to this question are conventional (time-ordered) scattering amplitudes, but also a whole compendium of inclusive measurements, such as expectation values of gravitational radiation and out-of-time-ordered correlators. We show that these asymptotic observables can be related to one another through new versions of crossing symmetry, and propose generalized crossing relations together with the corresponding paths of analytic continuation. Throughout the talk, we show how to apply crossing between different observables in practice, both at tree- and loop-level. (Based on: 2308.02125 and 2310.12199)
Title: Nonperturbative Anomalous Thresholds
Speaker: Miguel Correia (McGill)
Date: 2023-11-15, 11:00:00
Abstract
Feynman diagrams (notably the triangle diagram) involving heavy enough particles contain branch cuts on the physical sheet - anomalous thresholds - which, unlike normal thresholds and bound-state poles, do not correspond to any asymptotic n-particle state. "Who ordered that?" In this talk I will show that anomalous thresholds arise as a consequence of established S-matrix principles and an extra assumption: analyticity in the mass. I will review how the anomalous threshold appears in the triangle diagram and then derive a nonperturbative version that resums the relevant class of diagrams and how it can be tested.
Title: Fuzzy sphere regularization of 3D CFTs
Speaker: Yin-Chen He (Perimeter Institute)
Date: 2023-11-01, 11:00:00
Abstract
Conformal Field Theory (CFT) represents a class of quantum field theories that has profound applications across various physics domains, from critical phenomena in statistical mechanics to quantum matter, quantum gravity, and string theory. In this talk, I will introduce our recently proposed 'fuzzy (non-commutative) sphere regularization' scheme, a method that addresses and offers a solution to the longstanding need for a non-perturbative approach to 3D CFTs. I will first elucidate its fundamental concepts and then diving into illustrative examples, particularly, the 3D Ising transition. Specifically, we showcase that this scheme is not only potent—revealing a wealth of universal data on 3D CFTs otherwise inaccessible through existing methods—but also efficient, as the necessary computations can be performed on a laptop within an hour. Our innovative scheme not only heralds a new era for the study of CFTs but also hints at a profound interplay between non-commutative geometry and both CFTs and QFTs at large.
Title: Higher-point lightcone bootstrap in the comb channel: double-twist data and triple-twist anomalous dimensions
Speaker: Lorenzo Quintavalle (Laval University)
Date: 2023-10-18, 11:00:00
Abstract
It is a well-established fact that any conformal field theory with a gap in the twist spectrum must contain families of multi-twist operators, whose spectrum at large spin approaches that of generalized free theory. In this talk, we aim to discuss how the lightcone bootstrap can be applied to five- and six-point correlation functions in the comb channel to constrain the behavior of double- and triple-twist operators. Our analysis yields new expressions for large-spin OPE coefficients involving two double-twist operators, as well as the leading-order anomalous dimension matrix for triple-twist operators. The latter offers valuable insight into how the degeneracy of triple-twist primaries in the free-theory limit gets lifted by the inclusion of interactions.
Title: Dark Matter at Finite Temperature
Title: Liouville theory and the Weil–Petersson geometry of moduli space
Speaker: Keivan Namjou (McGill)
Date: 2023-10-04, 11:00:00
Abstract
Conformal field theory is a powerful tool in the study of geometry, with applications ranging from mirror symmetry to spectral theory and quantum chaos. Liouville theory is a specific conformal field theory that provides a natural means to study the Weil–Petersson geometry of the moduli space of Riemann surfaces. In the semiclassical limit, the Liouville path integral computes the Kähler potential, giving access to the metric and associated geometric quantities. In this talk, I will review Liouville theory, the geometry of hyperbolic surfaces and their moduli space, and explain how the study of Liouville theory in the semiclassical limit allows us to describe these geometries.
Title: Phantom fluid cosmology: constraints and direct detection
Speaker: Jim Cline (McGill)
Date: 2023-09-27, 11:00:00
Abstract
Phantom fields have been widely invoked as a source of dark energy
in cosmology, but rarely taken seriously as quantum theories. The
vacuum is automatically unstable to production of negative-energy
ghost particles plus normal particles, requiring such theories to be
effective only, below some UV cutoff. I will present recent cosmological
constraints arising from the vacuum instability, both at the level of
the homogeneous background, and the density perturbations. We find
that the fluid of particles produced from vacuum decay can ameliorate but
not solve the notorious Hubble tension problem, and cannot fully replace
the cosmological constant as a source of dark energy. The vacuum decay
can be a source of boosted dark radiation, which might be detectable
if it interacts with ordinary matter. I will show that this could fit
recent excess events reported by the DAMIC experiment at SNOLAB.
Title: Superfluid dark matter flow around cosmic strings
Speaker: Aline Faverot (McGill)
Date: 2023-09-13, 11:00:00
Abstract
We consider a cosmic string moving through a gas of superfluid dark matter (SFDM) particles and analyze how it affects the dark matter distribution. We look at two different cases: first, a cosmic string passing through an already condensed region, and second, through a region that is not yet condensed. In the former, the string induces a weak shock in the superfluid, and the Bose-Einstein condensate (BEC) survives. In the latter, a wake of larger density is formed behind the string, and we study under which conditions a BEC can be formed in the virialized region of the wake. By requiring the thermalization of the DM particles and the overlap of their de Broglie wavelengths inside the wake, we obtain an upper bound on the mass of the dark matter particles on the order of 10 eV, which is compatible with typical SFDM models.