Perimeter Institute Recorded Seminar Archive

Cosmological surveys can reveal new particles through signals hidden among astrophysical foregrounds. This talk examines axion conversion of microwave photons in magnetized galactic halos, using Planck, unWISE and ACT observations with improved component separation. It then turns to inflationary particle signatures in galaxy clustering, including a constraint on the primordial four-point amplitude from DESI galaxies and quasars. The discussion highlights both the reach of these measurements and the systematic challenges of analysing extremely large scales.

Starting with a cyclic A-infinity category, this talk constructs enumerative invariants with both open and closed sectors. The approach extends Costello's closed-sector construction through appropriate differential graded Lie algebras and trivializations of circle actions. Applications include a unital, cyclic version of the Bogomolov–Tian–Todorov theorem and a construction of quantum A-infinity algebras. The presentation describes ongoing joint work with Lino Amorim.

Gravitational-wave detections show an excess of binary black-hole mergers around 35 solar masses. This talk compares their mass ratios, spins and redshifts with the predictions of competing formation mechanisms. The central question is whether one astrophysical pathway can account for the observed features or whether several populations contribute to the excess.

Consistency requirements on scattering amplitudes can constrain quantum gravity without a perturbative expansion. This talk applies analyticity, crossing and unitarity to maximally supersymmetric gravitational theories in ten and eleven dimensions. It extends bounds from the leading correction to higher-order effective couplings, comparing the allowed regions with string-theory and M-theory predictions. A nearby kink in eleven dimensions suggests a possible route toward characterizing the M-theory scattering matrix through consistency alone.

A weak attractive interaction confined to dark matter can outweigh gravity over subgalactic distances. This talk studies its effects on collapse and stable bound structures when the force is screened beyond roughly kiloparsec scales. Delaying its onset until low cosmic density avoids early-Universe and galaxy-structure constraints. For fermionic matter, the additional attraction can support much lighter particles against degeneracy pressure; a density-triggered phase transition provides a concrete delayed-activation mechanism.

Can quantum uncertainty resolve cosmological singularities within the Wheeler–DeWitt description itself? Introducing an observer's proper time turns the problem into evolution resembling scattering in a central potential. The talk describes a smooth quantum bounce for a radiation-filled universe, with large scale-factor uncertainty, and explains why anisotropy introduces a critical attractive potential. That harder regime motivates extending the classical BKL account of singularities into quantum theory.

Representing quantum states through measurement probabilities can preserve subsystem structure or faithfully track nearby states, but not necessarily both. This creates robustness problems for axiomatic reconstructions based on generalized probabilistic theories. The talk introduces an operational alternative that treats a state as an observer's knowledge, aiming to compare possible physical theories while avoiding instability under small changes of the theory.

Operational definitions identify subsystems through the transformations an agent can perform. This talk classifies complementary, bicommutant-closed subsystems using unitary channels, whose disregard for global phase permits possibilities beyond conventional operator subalgebras. The classification involves graded subalgebras. Some resulting subsystems are nontrivial yet support no measurements, raising a foundational choice between excluding them and accepting agents that act without being able to observe.

This talk reexamines black-hole information through the completeness of observables at infinity in gravitational theories. That completeness challenges radial factorization of the bulk state space and assumptions underlying familiar information-loss arguments. Within this account, information is already represented asymptotically. Restricting an observer's available measurements can nevertheless produce Page curves, interpreted as redistribution among accessible observables rather than information newly escaping the hole.

Accurate waveforms for a small black hole entering a much larger one remain difficult near merger. This talk presents a staged treatment of the first-order merger and ringdown that can isolate nonlinear contributions for later corrections. It also analyses the transition to plunge in rotating backgrounds, where catastrophe theory explains why equatorial and inclined trajectories share the same Painlevé equation. Together these approaches expose structure useful for extending self-force calculations.

Some protected operators exist only at particular gauge-group ranks, a phenomenon connected conjecturally to black-hole microstates. This talk studies such fortuitous operators in ABJM theory at large Chern–Simons level. Harmonic representatives make the operators explicit, while analytic continuation of the two-loop Hamiltonian tracks them from finite rank toward the large-rank limit. Selected operator-product coefficients complement the trajectory analysis.

Lyman-alpha galaxies offer a way to map structure beyond redshift two and connect galaxy surveys with microwave-background lensing. This talk demonstrates selection from IBIS medium-band images and spectroscopic follow-up with DESI. Joint clustering measurements of these galaxies and quasars constrain bias, small-scale velocity effects and radiative-transfer contributions, testing the modelling needed for future high-redshift surveys.

Early active black holes seen by JWST, pulsar-timing evidence for a gravitational-wave background and stellar tidal disruptions probe different parts of black-hole growth. This talk presents L-GalaxiesBH, which follows black holes and their host galaxies from early formation to the present. Combining these observations constrains growth models, including the abundance of rapidly growing early objects, massive binaries nearby and elusive intermediate-mass systems.

Resolving singular Spin(7) spaces links exceptional geometry with gauge-theory vacuum structure. This talk builds torsion-free structures on compact orbifold resolutions from local ALE data encoded by harmonic self-dual forms. It explains the relation to supersymmetric gauge equations and asks when local solutions extend globally. Gluing methods and Dirac operators identify analytic obstructions, with explicit examples attaining full Spin(7) holonomy.

Euclidean wormholes can make a theory appear nonfactorizing when it has effectively averaged over hidden alternatives. This talk develops a categorical analogue for spatial wormholes. In that formulation, apparent failure of state-space factorization can instead arise from restricting an underlying holographic field theory to neutral states, offering a different interpretation of Einstein–Rosen bridges.

Future extreme-mass-ratio inspiral detections could identify how compact objects reach massive black holes. This talk compares expected eccentricities, inclinations and component masses for gas-free and gas-assisted formation. It examines how population measurements might separate channels and reveal properties of active-galaxy disks, including lifetimes and turbulence, as well as accretion behaviour and stellar black-hole populations in nuclear clusters.

Deep networks display recurring phenomena such as grokking, scaling behaviour, generalization despite overparameterization and failures outside training distributions. This talk surveys controlled settings where those behaviours can be explained, suggesting that language models may combine many tractable, largely interpolative mechanisms. It then distinguishes that scientific account from a more speculative argument about the role of industrial computation and human labour, and what these imply for claims of broadly extrapolative intelligence.

Primordial fluctuations need not lose their quantum character as quickly as commonly assumed. Using inflationary effective theory beyond slow roll, this talk constructs Wigner functions whose negative interference fringes diagnose nonclassical behaviour. Those features can strengthen outside the horizon, and in some motivated regimes their creation outpaces environmental decoherence. The resulting phase-space structure suggests cosmological observables that might retain evidence of the fluctuations' quantum origins.

Poisson sigma models connect geometric deformation with quantum field theory. This talk extends them into higher-dimensional theories with holomorphic and topological directions and relates the construction to quantization of factorization algebras. It develops interfaces, boundaries and defects, then uses Koszul duality to connect the models with quantum-group structures, including examples involving Lie bialgebras, W-algebras and Yangians.

This talk tests asymptotic safety using perturbative methods that retain gauge invariance and allow Lorentzian calculations. Unlike mass-independent prescriptions, the proposed schemes preserve power-law sensitivity through modified dimensional regularization or gradient-flow techniques. Heavy-quark decoupling in QCD supplies a demonstration. Applying the approach to gravity yields a one-loop ultraviolet fixed point with indications of perturbative accessibility, which is compared with functional-renormalization-group evidence.