Astrophysics seminars

Uddipan Banik derives a dynamical account of dark-matter halo density profiles using the Boltzmann–Poisson equations for an expanding Universe populated by subhalos of many masses. A self-similar solution links density slope to halo mass growth; including gravitational scattering and subhalo capture gives gamma = 6/(s + 2). Initial collapse with s = 2 produces a prompt cusp of slope 3/2. Hierarchical growth connects the slope to the primordial power spectrum and recovers the Syer–White relation during matter domination. Stable clustering selects an inner Navarro–Frenk–White cusp of slope one, while stalled accretion approaches the outer slope of three. The framework connects these profiles to successive stages of halo assembly.

Paco Villaescusa examines how simulations and artificial intelligence change the extraction of physical knowledge from precision cosmological surveys. The talk asks how much information lies in large-scale structure beyond power spectra, including nonlinear scales and higher-order statistics, and introduces the Quijote and Backlight simulation programmes. It then addresses the uncertain astrophysical processes that complicate small-scale inference through CAMELS, a framework combining cosmology, galaxy formation and machine learning. DREAMS is presented as a route to predicting dark-matter observables while accounting for baryonic uncertainties. The final part considers coupling simulations with increasingly capable AI: Denario is introduced as an agentic research system that generates hypotheses, designs analyses, writes and executes code, interprets results and refines research directions. The discussion considers what coordinated populations of agents and simulations spanning physical scales could make possible.

Taillte May investigates how unstable black-hole quasinormal-mode spectra affect observed gravitational-wave signals. Small changes in a gravitational potential can substantially shift the frequencies normally used to test astrophysical black holes. A simple time-domain model reveals a delay before those shifts appear, tied to the echo travel time between the perturbation and the main potential. In the perturbative regime, the frequency change after one echo agrees with the conventional frequency-domain prediction. Beyond that regime, the intermediate waveform cannot be represented by a single constant frequency shift.

Arefe Abghari tests whether the reported excess in the quasar number-density dipole challenges the Cosmological Principle. If large-scale homogeneity and isotropy hold, the motion responsible for the cosmic microwave background dipole should also predict the dipole in distant-source counts. The analysis examines quasar selection and dipole-estimation systematics. Higher multipoles in the quasar density map, plausibly caused by survey and selection effects, can explain the apparent discrepancy. The talk argues that these effects need careful control before interpreting the measurements as evidence against the standard cosmological model.

Claudio Manzari connects astrophysical observations and precision flavor measurements to two unresolved aspects of CP symmetry. For the strong CP problem, the talk considers QCD axions produced in a supernova core and converted to gamma rays by surrounding magnetic fields. A Galactic supernova could expose this distinctive transient, motivating the GALAXY network of telescopes monitoring the full sky. The second part examines the origin of flavor-sector CP violation through precise measurements of the CKM unitarity triangle. Possible hints of spontaneous CP violation are discussed alongside their implications for quark flavor and the fundamental symmetries of matter.

Ania Liu examines gravitational-wave lensing as a probe of compact objects, dark matter and cosmic large-scale structure. Propagation over cosmological distances can distort signals, but identifying lensing in observations requires disentangling those distortions from uncertain waveform models and other astrophysical effects. The talk describes this phenomenology and the ambiguities it creates for interpretation, then presents recent methods and results from lensing searches using LIGO–Virgo–KAGRA data.

Fabian Walter presents the Deep Synoptic Array, a Nevada radio survey telescope within the Eric and Wendy Schmidt Observatory System, with major surveys planned from 2029. Operating across 0.7–2 GHz, its design combines inexpensive antennas and ambient-temperature receivers with a digital radio camera that produces images in real time. The programme targets approximately 31,000 square degrees at 500 nJy, aiming to detect a billion star-forming galaxies and active supermassive black holes while measuring neutral-hydrogen properties in millions of galaxies. Time-domain goals include roughly 100,000 fast radio bursts, more than 20,000 new pulsars, about a million image-plane transients and multi-year timing of 200 pulsars for nanohertz gravitational-wave studies. The talk connects these capabilities to multimessenger astronomy, cosmic history, time-domain astrophysics and the dark sector, including synergies with other all-sky surveys.

Tien-Tien Yu examines observational signatures of ultralight scalar dark matter with quadratic couplings to Standard Model fields. The couplings change fundamental constants over time, affecting primordial light-element production, the microwave-background power spectrum and tests of the equivalence principle. The talk follows these effects from the early Universe to present experiments and explains how current cosmological measurements constrain this class of scalar-dark-matter models.

Subha Majumdar presents a six-dimensional Milky Way phase-space catalogue designed to map stellar motions and dark matter beyond the reach of Gaia parallaxes alone. Gaia astrometry is combined with spectrophotometric distances and radial velocities from fourteen surveys, including DESI, SDSS-BOSS, APOGEE and LAMOST. The resulting catalogue contains about 33 million tracers and detailed distance–velocity measurements for roughly half a million halo stars. It characterizes distant clusters, dwarf galaxies and stellar streams, enabling mass modeling, kinematic studies, abundance mapping, Galactic archaeology and dark-matter searches. An application constructs a continuous Galactic rotation curve extending to 250 kiloparsecs.

Giada Santoro examines strong-field gravity and compact-object structure using GW250114, a gravitational-wave event with signal-to-noise ratio around eighty. The unusually strong signal supports precise tests of departures from general relativity and detailed measurements of black-hole ringdown. No tidal deformability is detected: the analysis places a ninety-percent upper bound of 34.8 on the effective tidal-deformability parameter. This is consistent with the vanishing tidal response predicted for Kerr black holes.

Anna Tsai studies how precise pulsar distances can improve localization of individual gravitational-wave sources with pulsar timing arrays. A coherent map-making method uses distance information to approach the diffraction limit, potentially reaching angular precision of about two arcminutes and enabling electromagnetic counterpart searches. At a signal-to-noise ratio of ten, approximately nine pulsars can reach this limit. The resolution improves sharply as more well-timed pulsars have accurately known distances. Since the distance of PSR J0437−4715 is already measured to subparsec precision, the talk motivates coherent analyses that fully incorporate pulsar-distance information.

Sharan Banagiri uses gravitational-wave observations to investigate how stellar-mass binary black holes form. The first part of the fourth LIGO–Virgo–KAGRA observing run more than doubled the number of detections, revealing new population features and clarifying earlier ones. The talk surveys notable and puzzling subpopulations, relates them to formation channels, and connects the evidence with transient phenomena and compact-object astrophysics. As the sample grows, these identifiable subpopulations can support a broader physical account of binary black-hole formation.

Lisa Dang explores how observations reveal the three-dimensional atmospheres and climates of close-in exoplanets. Kepler and TESS have established a diverse population that tests theories of planetary formation and evolution. Tidally locked short-period planets offer strong atmospheric signals but their large day–night contrasts make one-dimensional interpretations inadequate. Measurements from JWST and precise ground-based observatories can expose these spatial differences and determine whether an atmosphere is present. The talk reviews discoveries about intensely irradiated planets and explains how the same observational techniques now investigate temperate rocky worlds and their potential habitability.

Pratika Dayal examines galaxy and black-hole formation during the first billion years, when the first galaxies ended the cosmic dark ages and began reionizing intergalactic hydrogen. JWST has revealed unexpectedly abundant, massive black holes, reaching about one hundred million solar masses within the first six hundred million years, challenging formation models. These observations can constrain the progress and spatial structure of reionization in preparation for 21-centimeter measurements. Early galaxies also test alternatives to cold dark matter. The talk considers the gravitational-wave event rates that early black holes could produce for the future LISA mission.

Recent high-profile scientific achievements by machine learning (ML) and especially deep learning (DL) systems have reinvigorated interest in ML for automated scientific discovery (eg, Wang et al. 2023). Much of this work is motivated by the thought that DL methods might facilitate the efficient discovery of phenomena, hypotheses, or even models or theories more efficiently than traditional, theory-driven approaches to discovery. This talk considers some of the more specific obstacles to automated, DL-driven discovery in frontier science, focusing on gravitational-wave astrophysics (GWA) as a representative case study. In the first part of the talk, we argue that despite these efforts, prospects for DL-driven discovery in GWA remain uncertain. In the second part, we advocate a shift in focus towards the ways DL can be used to augment or enhance existing discovery methods, and the epistemic virtues and vices associated with these uses. We argue that the primary epistemic virtue of many such uses is to decrease opportunity costs associated with investigating puzzling or anomalous signals, and that the right framework for evaluating these uses comes from philosophical work on pursuitworthiness.

Francis Halzen explains how the IceCube telescope uses detectors buried deep in Antarctic ice to observe high-energy neutrinos from the cosmos. The lecture describes construction of the observatory, the measurement of particle energies and directions, and the distinction between atmospheric backgrounds and neutrinos arriving from distant astrophysical sources. It connects the discovery of cosmic neutrinos with the search for the powerful accelerators that produce cosmic rays. Examples include the association with a flaring blazar and evidence identifying the active galaxy NGC 1068 as a neutrino source. Halzen discusses the role of rotating supermassive black holes, coordination with other observatories, and prospects for extending the detector.

How can black holes be established both as a consequence of gravitational theory and as objects in the sky? Roger Penrose examines the mathematical development of gravitational collapse and space-time singularities, explaining why black-hole formation does not depend on perfectly symmetric idealisations. Reinhard Genzel and Andrea Ghez trace the observational effort to establish a massive compact object at the centre of the Milky Way. Their accounts connect infrared observations, improvements in imaging through atmospheric distortion, and long-term measurements of stellar motions around Sagittarius A*. Together, the lectures present the theoretical and observational foundations of the 2020 physics prize: the robustness of black-hole formation in general relativity and the evidence for the Galactic Centre’s supermassive black hole.