Astrophysics seminars

The behaviour of massive scalar waves near black holes links spacetime geometry, stability and long-time evolution. After reviewing Klein-Gordon instabilities on rotating Kerr backgrounds, the seminar presents polynomial decay results for localized initial data on Schwarzschild spacetime despite stable trapping. The proof brings in analytic-number-theory bounds on exponential sums.

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.

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.

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.

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.