The study of quantum field theory in de Sitter spacetime is notably motivated by the success of inflation, and the possibility to test fundamental physics through cosmological experiments. Going beyond the usual linearized description, one has to use nonperturbative methods to circumvent the infrared and secular divergences arising from the gravitational amplification of superhorizon modes.

We will first focus on the backreaction of an $O(N)$ quantum scalar theory on the background geometry, related to the cosmological constant problem, as such divergences may signal an instability of de Sitter space towards a Minkowski spacetime. Our study uses the functional renormalization group (FRG) and shows a finite backreaction in the presence of interactions, which saturates as a result of the nonperturbative generation of a mass.

The second topic of the talk is the effect of selfinteractions on scalar field correlators at separate spacetime points, in the infrared limit. The spacetime dependence gives access to several interesting physical quantities such as power spectra, relaxation time, or decoherence time. The stochastic formalism is considered, and we formulate the $1/N$ expansion of its Fokker-Planck formulation. This can be phrased as an equivalent, Schrödinger-like, eigenvalue problem. The corresponding spectrum is entirely computed at NLO, giving access to the spacetime dependence of any correlator.

Microscopic green algae are commonly found in mud, puddles or lakes, and show great diversity in structural complexity. The unicellular Chlamydomonas exhibits two flagella whose beating enables it to swim in a breast stroke towards light, a nowadays widely studied behaviour favouring photosynthesis. One also finds Gonium, an algae made of 16 Chlamydomonas-like cells. It is flat and squared, with all flagella on one side of the plate. This simple algae is a key organism to understand the evolution from unicellular to multicellular coherent behaviour.

In the absence of central nervous system, how can each cell adapt its individual photoresponse to efficiently reorient the whole algae? How crucial is the distinctive squared structure?
In this talk, I will present our experiments investigating the shape and the phototactic swimming of Gonium, using trajectory tracking and micro-pipette techniques. I will explain our model linking the individual flagella response to the 16-cell colony trajectory. This eventually emphasises the importance of biological noise to succeed swimming towards light.

Etude sur le carbone : de la cinétique de combustion à la spectroscopie Raman de nanotubes aplatis

Emmanuel Picheau

Université de Bordeaux - CNRS, Centre de Recherche Paul Pascal, UMR5031, 33600 Pessac, France.
emmanuel.picheau@crpp.cnrs.fr

Bien que l’ère du carbone soit prédite pour un temps futur, la vaste famille des matériaux carbonés a été le socle des plus grandes évolutions de l’humanité. La combustion du carbone notamment, a permis l’avènement des âges de bronze et de fer, de l’ère industrielle, ainsi que la maitrise de la métallurgie moderne.1 A l’échelle mondiale cette réaction représente encore aujourd’hui un moyen important de production d’énergie.2 Pourtant la réaction de combustion du carbone recèle toujours de nombreux mystères.
Dans cet exposé une étude portant sur la cinétique de combustion du carbone sera présentée. Une nouvelle méthode d’analyse thermogravimétrique, les isothermes successives (SIS), permettant l’étude cinétique de réactions solides-gaz sera exposée. Cette méthode permet notamment l’obtention de données cinétiques précises de manière rapide. Appliquée à la combustion du carbone, cette méthode a mis en évidence un changement continu du mécanisme réactionnel.3
A la suite, les principaux résultats d’une étude de spectroscopie Raman sur des nanotubes de carbone aplatis seront révélés. Il sera montré que la présence de cavités latérales sur ces objets induit la croissance d’une bande D intense et fine (voir illustration), indépendamment de la présence de défauts de structure. Ce résultat général, qui s’applique à tous types de matériaux graphéniques qui présentent des changements de courbure, permet de mettre en perspectives les points de vue différents entre physiciens et chimistes sur ce qu’est un défaut.4
1. Delhaes, P. Carbon-based Solids and Materials. Carbon-based Solids and Materials (John Wiley & Sons, Inc., 2013).
2. BP Statistical Review of World Energy Statistical Review of World, 68th edition. Ed. BP Stat. Rev. World Energy 1–69 (2019).
3. Picheau, E., Hof, F., Derré, A., Daffos, B. & Pénicaud, A. Thermal Oxidation of Carbonaceous Nanomaterials Revisited: Evidence of Mechanism Changes. Angew. Chemie Int. Ed. 58, 16013–16017 (2019).
4. Picheau, E. et al. Intense Raman D Band without Disorder in Flattened Carbon Nanotubes. Under revision. (2020).

The characterization of the genomic structural and functional state has been boosted by the concomitant development of different genomic techniques. Each of these experiments gives an extra layer of information from the localization on the genome sequence of histone modifications and transcription factor binding sites (Chip-seq) to the frequency of interactions of genomic loci (Hi-C). The interpretation of these data has been facilitated by complementary computational techniques. I contributed to these computational efforts by developing molecular dynamics approaches for modeling genome architecture and dynamics. After reviewing my doctoral and postdoctoral activities, I will discuss my research's latest developments concerning the generation of 3D models of the A. thaliana genome. Finally, I will briefly discuss how I plan to extend the genome-wide modeling approach in the next years.

References
Di Stefano M, Nützmann H-W, Marti-Renom MA, Jost D. Polymer modelling unveils the roles of heterochromatin and nucleolar organizing regions in shaping 3D genome organization in Arabidopsis thaliana. bioRxiv. 2020. p. 2020.05.15.098392. doi:10.1101/2020.05.15.098392

Andrea Parmeggiani's Zoom Meeting - Seminar Marco Di Stefano
Lunedì, 14 dicembre⋅14:55 – 16:55
Luogo:
https://umontpellier-fr.zoom.us/j/87126724144?pwd=cEp0UUFLN2hOMWJ0WlA0YUpoNE4yUT09
Descrizione:Andrea Parmeggiani vous invite à une réunion Zoom planifiée.

Participer à la réunion Zoom
https://umontpellier-fr.zoom.us/j/87126724144?pwd=cEp0UUFLN2hOMWJ0WlA0YUpoNE4yUT09

ID de réunion : 871 2672 4144
Code secret : 749330