Theoretical Cosmology meetings

To actively encourage the field of theoretical cosmology and to set an informal stage for the exchange of ideas, the Dutch theoretical cosmology community organizes Friday afternoon meetings approximately 6 times a year — usually on the first Friday of the month. The meetings typically start in the afternoon with a main speaker, followed by a short break to continue with another seminar or journal club discussion on some topic of current interest. We end the afternoon with drinks. The supporting institutes in Leiden, Amsterdam, Groningen, Utrecht where recently joined by the strings and cosmology group in Leuven and take turns in hosting the event.

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Groningen Winter 2023

March 10 @ 13:00 - 17:00

Groningen, March 10th.

13:30-14:30 CET: Talk by Jens Chluba (Manchester)

Title: Beyond the monopole: Spectral-spatial evolution of the CMB


Spectral distortions of the CMB have now been recognized as a unique probe of early universe and particle physics. In this context, we usually only think about distortions of the monopole. However, as I will explain in this talk once an average distortion is created, distortion anisotropies are also inevitable, very much like temperature anisotropies follow from the perturbations of the cosmic fluid. I will provide a formulation of the spectral-spatial evolution problem that allows describing the formation of distortion anisotropies for various scenarios. These signals can be targeted by future CMB imagers without the necessity to perform absolute measurements and thus provide a novel way to learn about the early universe.

14:30-14:45 CET: Break

14:45-15:45 CET: Talk by Thomas Flöss, “the kid” (Groningen)

Title: Primordial non-Gaussianity and non-Gaussian Covariance 


In the pursuit of primordial non-Gaussianities, we hope to access smaller scales across larger comoving volumes. At late times, the search for primordial non-Gaussianities is hindered by gravitational collapse, which induces secondary non-Gaussianity that swamps the weaker primordial signal. Additionally, it couples modes of different wavelength through (off-diagonal) covariance, reducing the amount of unique information available from the tracer field (e.g. galaxies or 21-cm). We assess the impact of such non-Gaussian covariance on the Fisher information of the primordial non-Gaussian amplitude f_{NL}, with a particular focus on higher redshifts, where this impact has previously been estimated to be negligible. We will comment on possible avenues to circumvent the impact of non-Gaussian covariance when estimating f_{NL} from future surveys.

15:45-16:00 CET: Break

16:00-17:00 CET: Talk by Aleksandr Chatrchyan (DESY, Hamburg)

Title: The Stochastic Relaxion


Light scalar fields, such as axions, can play an important role in cosmology. In this talk I will discuss the mechanism of cosmological relaxation of the electroweak scale, which provides a dynamical solution to the Higgs mass hierarchy problem. In the simplest model, the Higgs mass is scanned during inflation by a light field, the relaxion, whose slow-roll dynamics selects a naturally small Higgs vev. We revisit the original proposal and investigate the mechanism in a regime where the relaxion is subject to large fluctuations during inflation, including the “quantum-beats-classical” regime. The stochastic dynamics of the field is described by means of the Fokker-Planck formalism. We derive a new stopping condition for the relaxion taking into account the transitions between the local minima of its potential. We investigate the consequences both for the QCD relaxion and the strong CP problem, as well as for non-QCD models. We identify a new region of the parameter space where the stochastic misalignment of the relaxion from its local minimum due to fluctuations can naturally explain the observed dark matter density in the universe.

17:00 CET: Drinks at de Uurwerker


March 10
13:00 - 17:00
Event Category:


Daan Meerburg