GW day at UNIGE

Friday 23 May 2025, 11:30 → 17:30 Europe/Zurich

Ecole de Physique - Université de Genève

11:50 → 12:50 Phase transitions and gravitational waves with supercooling (cosmology seminar)

https://cosmology.unige.ch/content/tba-33

Models with classical scale invariance provide interesting candidates for GW signals from phase transitions, that could be observable with LISA. They typically feature a significant amount of supercooling, and correspondingly a very large phase transition strength. Computations of phase transition parameters often feature slow perturbative convergence, resulting in large errors. A remedy for this issue is to construct an appropriate effective theory for bubble nucleation, this is typically done in the framework of high-temperature dimensional reduction. In this talk, I discuss the first application of this procedure to a model with a large amount of supercooling, and compare the results to the frequently applied daisy-resummation. We will see that the gravitational wave predictions get affected significantly, confirming the importance of an appropriate use of effective field theories, including functional determinants for the gauge modes.

Speaker: Jorinde van de Vis (CERN)

12:50 → 14:00 Lunch

14:00 → 14:30 Exploring nanoHz gravitational waves with pulsar timing arrays

Two years ago, pulsar timing arrays reported evidence of a gravitational-wave background consistent with emission from supermassive black hole binaries at the centers of galaxies—although more intriguing sources have been proposed by many. I celebrate the surprising opportunity of nanoHz gravitational-wave detection via pulsar timing, discuss our quest for statistical evidence and the fight against systematics, present new AI-inspired tools and methods for pulsar-timing-array data analysis, and speculate about future milestones.

Speaker: Michele Vallisneri (ETH Zurich)

14:30 → 14:45 Discussion

14:45 → 15:15 Emergence of Milky Way structure in the first year of LISA data

Tens of thousands of gravitational-wave (GW) sources are expected to be detected by the Laser Interferometer Space Antenna (LISA) during its mission. The largest majority of such sources will be persistent signals from the Milky Way (MW). While individual resolvability will occur during the whole mission, the whole population will persistently contribute to the LISA data stream from the very beginning, piling up in a confusion noise. In this talk I will overview current understanding of the (MW) spatial distribution, discuss the implications on the confusion noise time variation, and present a first, fast and accurate inference, taking into account both. Employing a realistic data segmentation, I will present an Hamiltonian-Montecarlo-based scheme to (i) infer on the Galactic shape (ii) in the presence of data gaps, and quantify the evidence for the former while performing a Global fit.

Speaker: Riccardo Buscicchio (University of Milan Bicocca)

15:15 → 15:30 Discussion

15:30 → 16:00 Spectral density of astrophysical stochastic backgrounds

I will discuss a recent first-principle derivation of the spectral features of astrophysical stochastic backgrounds produced by populations of compact binary coalescences (CBCs). The treatment is based on the observation that, among the parameters characterizing a CBC, some of them (extrinsic) are distributed uniformly following symmetry principles, while other (intrinsic) carry the more complicated dependence on the astrophysical scenario. Averaging over the extrinsic parameters allows the extraction of simple structures in the spectral density, ultimately leading to an unpolarized background (which is also isotropic if the underlying distribution of sources is isotropic). However, shot noise effects, due to the specific realization of a finite number of events, produce anisotropies and give non-vanishing Stokes parameters for both circular and linear polarizations. In particular, it is remarkable that the Stokes parameters for linear polarization do not vanish even in the isotropic case. This fact, which can be justified in full generality, has been previously overlooked in the literature. Numerical results for populations of CBCs will be showed. Throughout the talk, deviations from stationarity due to the finite observation time will also be described.

Speaker: Enis Belgacem (Université de Genève)

16:00 → 17:00 Discussion