Many natural systems remain far from thermodynamic equilibrium by exchanging matter, energy or information with their surroundings. As these transfers, or fluxes, break timereversal invariance, such processes are beyond the realm of traditional thermodynamics and their statistical fluctuations do not follow the principles of equilibrium statistical mechanics.

Though a fully general theory of non-equilibrium systems still remains to be constructed, a physical principle describing the macroscopic behaviour of diffusive systems far from equilibrium has been proposed by G. Jona-Lasinio and his collaborators in 2001: this is the Macroscopic Fluctuation Theory (MFT). In the MFT framework, macroscopic fluctuations far from equilibrium are determined by two coupled non-linear hydrodynamic equations. However, for a long time, the MFT equations have remained intractable.

In a recent work that has appeared in Physical Review Letters, K. Mallick (IPhT), H. Moriya and T. Sasamoto (Tokyo Tech.) have discovered an exact analytic solution for the time-dependent MFT equations for the symmetric exclusion process, a paradigmatic model of non-equilibrium statistical physics. The MFT was solved by using Inverse Scattering Theory, originally invented to study dispersive waves such as solitons in hydrodynamics or in optical guides. This classic method of non-linear physics is strikingly relevant to establish exact results in non-equilibrium thermodynamics and allows us to predict quantitatively the appearance of rare events and dynamical fluctuations. The application of soliton theory to non-equilibrium statistical mechanics might have far reaching consequences.

The IPhT is strongly involved in the preparation of the scientific exploitation of the Euclid mission. This participation is done at two levels. Francis Bernardeau is in charge of the coordination of the Science Performance Verification, an exercise that aims to ensure that the core objectives of the mission - constraints on the parameters of the dark energy equation of state - will be achieved given what is known about the general parameters of the mission. But more generally, the IPhT researchers are involved in the preparation of the tools necessary for the analysis of cosmological data. In particular, it is a question of developing and validating the tools allowing to predict the expected observables for a large variety of cosmological models. Thus, Filippo Vernizzi coordinates the efforts within the Euclid consortium to develop simulations and theoretical tools to study the non-linear development of modified gravity models.

The article "Implementing spectra response function approaches for fast calculation of power spectra and bispectra", Physical Review D, Volume 104, Issue 10, article id.103501, by Ken Osato, Takahiro Nishimichi, Atsushi Taruya (Kyoto University) and Francis Bernardeau explores such a methodology. As shown in the attached figure, this approach allows to compute the bispectrum of the density field by including the so-called one-loop corrections. This result opens the way to the implementation of fast calculations of the spectra in redshift space. Prof. Ken Osato, now at Chiba University, is currently visiting IPhT to pursue this goal.

The retreat of the Institute of Theoretical Physics was held in Autrans (Vercors) from May 23 to 25.

The approximately one hundred participants enjoyed the benefits of the altitude and the gorgeous environment surrounding the Escandille congress center, which hosted the event. This intense and joyful retreat provided an opportunity to strengthen the cohesion between members of our laboratory and to discuss among ourselves issues of science and the life of the Institute.

The IPhT is characterized by a broad spectrum of research topics. In order to highlight the skills and to pedagogically transmit the knowledge to other non-expert colleagues, 7 scientific presentations of IPhT researchers were organized, complemented by 2 presentations held by external speakers, Professors Luc Blanchet (Institut d'Astrophysique de Paris, long-term visitor at IPhT) and Emil Martinec (University of Chicago).

Short presentations allowed the new PhD students and post-doctoral fellows of the IPhT to be known by the other researchers. The IT and administrative support teams took the opportunity to present their work to everyone and gave us a lot of useful information. The traditional open discussion session "on the future of the laboratory" allowed the participants to exchange on multiple questions of the laboratory life.

A quiz game then tested the knowledge of the participants on various subjects: the CEA, the CNRS, the history of the IPhT, general physics or even on key publications of members of our lab. The random distribution of seats in the restaurant allowed to vary the guests, thus breaking the barriers of age, language, skills, etc. To keep fit and reinforce the team atmosphere, walks in the green nature were proposed to the participants, in addition to the sports facilities of the center (swimming pool, gym, sauna, ping-pong). Last but not least, the last evening was celebrated with a dance party that added a fun touch to this busy retreat!