Centre de Physique Théorique

Agenda

Mars 2018

Vendredi 9 mars 14:00-15:00, Amphi 5 du CPT

Mapping the Universe on the largest scales with the Sunyaev Zel’dovich effect

Matthew Johnson (Perimeter Institute)

The Sunyaev Zel’dovich (SZ) effect, temperature/polarization anisotropies and spectral distortions in the cosmic microwave background (CMB) induced by scattering from free electrons in the post-reionization Universe, is a primary target of current and future CMB instruments. Increasingly precise measurements of the SZ effect promise to greatly inform our understanding of the distribution of baryons in the cosmos. In addition, when combined with the next generation of galaxy surveys, the SZ effect has enormous potential to improve our understanding of fundamental cosmology. In this talk I will demonstrate how cross correlations between galaxy surveys and low-noise high-resolution observations of the CMB can be used to reconstruct the velocity and density fields on scales which are currently measured using the primary CMB anisotropies. I will then discuss how such a reconstruction could be used to resolve long-standing ‘anomalies’ in the primary CMB, constrain the properties of dark energy, and improve constraints on primordial non-gaussianity.

Lundi 12 mars 14:00-15:00, Amphi 5 du CPT

Topological susceptibility and axion mass at high temperature

Julien Frison (KEK Tsukuba)

Mercredi 21 mars 14:00-15:00, Amphi 5 du CPT

Graph spectral characterisation of the XY model on complex networks

Sarah de Nigris (IXXI, Inria, ENS Lyon)

Résumé

There is recent evidence that the XY spin model on complex networks can
display three different macroscopic states in response to the topology of
the network underpinning the interactions of the spins. In this seminar, I
shall introduce a different way to characterise the macroscopic states of
the XY spin model based on the spectral decomposition of time series using
topological information about the underlying networks.
In this work [1], we use three different classes of networks to generate
time series of the spins for the three possible macroscopic states. We then
apply the temporal Graph Signal Transform technique to decompose the time
series of the spins on the eigenbasis of the Laplacian.
From this decomposition, we produce spatial power spectra, which summarise
the activation of structural modes by the non-linear dynamics, and thus
coherent patterns of activity of the spins. Remarkably, these signatures of
the macroscopic states are independent of the underlying networks and can
thus be used as universal signatures for the macroscopic states.

[1] S.d.N, P. Expert, T. Takaguchi and R. Lambiotte, Phys. Rev. E,
96(1):012312, 2017

Mercredi 28 mars 14:00-15:00, Amphi 5 du CPT

Un théorème de fluctuation pour les mesures weak Gibbs

Noé Cuneo (Département de mathématique, Université de Cergy)

Résumé

L’irréversibilité de la dynamique, quantifiée par la production d’entropie (PE), est une propriété
inhérente aux systèmes hors équilibre. Le théorème de fluctuation, dans ses nombreuses variantes,
indique que la PE est positive pour une écrasante majorité des trajectoires. Je commencerai par
quelques définitions et exemples élémentaires. Je parlerai ensuite d’un théorème de fluctuation
de type Gallavotti-Cohen sur la PE dans le régime des grandes déviations. Le théorème s’applique
à toutes les mesures "weak Gibbs" sur des systèmes dynamiques faiblement chaotiques (en temps
discret sur des espaces métriques compacts). Notre approche consiste à passer par une relation
de fluctuation au niveau des mesures empiriques, ce qui permet de travailler sous des hypothèses
sensiblement plus générales que précédemment dans la littérature, et, en particulier, en présence
de transitions de phases (non unicité de l’état d’équilibre). *arXiv:1712.05167, avec V. Jaksic,
C.-A. Pillet et A. Shirikyan.

Agenda