Cosmology and particle physics seminar
16.02.2026 – 31.12.2026
Organisé par
Faculté des sciencesDépartement de physique théorique
entrée libre
Classement
Catégorie: Séminaire
Sous-événements
Non-minimal coupling and the H0 and BAO tensions
20.02.2026 11:50 – 12:50
The Hubble tension is one of the central topics of cosmology, possibly pointing to new physics beyond LambdaCDM. Furthermore, recent baryon acoustic oscillation (BAO), combined with type Ia supernova, observation unveils a new “tension” indicating possible gaps in our understanding of the recent expansion history of our Universe. One way of explaining the Hubble tension is to introduce new physics in the early Universe during recombination while the BAO “tension” might be explained by evolving dark energy in the late Universe. I will present recent works that study the origin of the cosmological tensions by assuming either early- or late-Universe modification. Based on such analysis, I will discuss how a non-minimally coupled scalar field theory, dubbed thawing gravity, can unify the early- and late-Universe modifications and explain the cosmological tensions.
De Sitter momentum space
06.03.2026 11:50 – 12:50
Quantum field theory in de Sitter space is notoriously difficult. In this talk, I will introduce a new momentum space adapted to de Sitter isometries that provides a natural language and allows us to bypass several difficulties usually encountered. This construction is based on diagonalizing the Casimir operator together with spatial translations, effectively trading the usual (d+1)-dimensional Fourier space for what we call the Kontorovich–Lebedev–Fourier (KLF) space. I will show practical advantages of this description: the quadratic dynamics provides a simple propagator analogous to flat space, and nested time integrals appearing in the computation of cosmological correlators turn into frequency-space integrals over meromorphic functions. I will also show how this construction naturally accommodates the contributions from principal and complementary series in the Källén-Lehmann spectral decomposition of composite operators.
Computing the rate of hot baryon number violation in the early universe
13.03.2026 11:50 – 12:50
The baryon asymmetry in the early universe (BAU) is one of our best hints for the existence of new physics. While the standard model of particle phyiscs predicts efficient baryon-number violation at high temperatures, it lacks the amount of CP violation needed to explain the observed asymmetry. Nevertheless, the rate of baryon number violation in the standard model is an important input for any prediction of the BAU. In this talk, I will first give a primer on the physics of baryon number violation at high temperatures and its relation to the sphaleron rate and the dynamics of soft non-Abelian gauge fields. I will then outline the general strategy for computing the rate of baryon number violation in the standard model via the sphaleron rate and the role of higher-order corrections in this computation. Finally, I will present work published in 2510.20594, where I computed the leading QCD corrections to the sphaleron rate.
Constraint effective potential and stochastic theory in de Sitter spacetime
20.03.2026 11:50 – 12:50
The effective potential is a powerful and widely used tool in quantum field theory. However, in de Sitter spacetime, which is a good approximation for the inflationary era, it does not have a unique definition. I will discuss the constraint effective potential introduced by O'Raifeartaigh et al. in 1986, as an alternative to the usual textbook definition. I present the complete one-calculation of both potentials in scalar field theory, showing that the constraint effective potential does not suffer from the infrared problem that plagues perturbative calculations in de Sitter for light scalar fields. I further demonstrate that when the constraint effective potential is used in the Starobinsky-Yokoyama effective theory, it reproduces correctly the infrared behaviour of QFT correlation functions and the vacuum decay rate at one-loop order.
Gravitational wave tests of cosmology, gravity and astrophysics
27.03.2026 11:50 – 12:50
Gravitational-wave dark sirens provide a clean measurement of the luminosity distance to their sources, as this information is directly encoded in their waveform. However, their relatively poor angular resolution makes identifying the host galaxy challenging, limiting the ability to obtain spectroscopic redshifts needed to construct a Hubble diagram and constrain cosmological models. Furthermore, the standard line-of-sight method is very sensitive to incompleteness in galaxy catalogues, which is an important limitation. I will introduce a novel technique called Peak Sirens, based on three-dimensional cross-correlations between galaxies and gravitational-waves. Using this method, I will present the first measurement of the Hubble constant and new constraints on the gravitational-wave bias. This approach naturally incorporates large-scale structure information into dark siren analyses. In the second part of the talk, I will present a model-independent test of the graviton mass using multi-messenger lensing time-delays as well as a dark siren test of gravity, based on dispersion induced friction. Applying this method to selected events from the third observing run of LIGO–Virgo–KAGRA, I will show as an example, that we obtain competitive constraints on the graviton mass.
Probing Parity with Composite-Field Galaxy Correlators
17.04.2026 11:50 – 12:50
Detecting parity violation on cosmological scales would provide a striking clue to new physics in late or early Universe. Large-scale structure surveys offer the statistical power required to test such signatures. For scalar observables such as galaxy clustering, the leading parity-sensitive statistic is the trispectrum, whose high dimensionality makes direct measurement and noise estimation challenging. It is therefore desirable to construct lower-dimensional observables that retain sensitivity to parity-violating trispectrum signals. In this talk, I introduce a class of such observables, referred to as parity-odd kurto spectra, which arise as power spectra of composite fields constructed from the galaxy overdensity field. These statistics compress trispectrum information into one-dimensional pseudo–power spectra that can be measured using standard large-scale structure techniques. I present tests of the estimators on N-body simulations and discuss the dominant sources of noise affecting the measurements. Finally, I show results from analyses of BOSS and DESI galaxy samples using parity-odd kurto spectra and compare them with recent analyses of 4point correlation functions the same datasets.
Cosmological tests of gravity
24.04.2026 11:50 – 12:50
The discovery of the accelerated expansion of the Universe has come relatively late in our study of the cosmos, but in showing that gravity can act repulsively, it has opened up many new questions about the nature of gravity and what the Universe might contain. Is the acceleration being driven by dark energy? Or is general relativity itself in error, requiring a modification at large scales to account for the late acceleration? Structure formation in our Universe can be different even if the geometry of the homogeneous and isotropic universe is the same in these two classes of models, offering a possibility to distinguish between them observationally. I will discuss cosmological tests of dark energy and modified gravity models using the latest cosmological observations such as the recent results from DESI and conclude with prospects for upcoming results from Euclid.
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