Astrophysique | EPFL Graph Search

L’astrophysique (du grec astêr : étoile, astre et physis : science de la nature, physique) est une branche interdisciplinaire de l'astronomie qui concerne principalement la physique et l'étude des propriétés des objets de l'Univers (étoiles, planètes, galaxies, milieu interstellaire...), comme leur luminosité, leur densité, leur température et leur composition chimique. Au , les astronomes ont une formation en astrophysique et leurs observations sont généralement étudiées dans un contexte astrophysique, de sorte qu'il y a moins de distinction entre ces deux disciplines qu'auparavant. Disciplines de l'astrophysique L'astrophysique se compose de diverses disciplines :

cosmologie ;
planétologie ;
exobiologie ;
instrumentation ;
physique stellaire ;
héliosismologie et astérosismologie ;
physique du milieu interstellaire ;
plasmas astrophysiques ;
physique galactique.

L’astrophysique étant un sujet très vaste, les astrophysiciens utilisent généralement plus

Exploring beyond the Standard Model: the Heavy and the Dark

Kin Shanho Eliaou Mimouni

Physics beyond the Standard Model can appear as new particles too heavy to be produced in experiments (the energy frontier) or interacting too weakly to be seen in our measurements (the intensity frontier). In the first part of this thesis, we study two dark matter models of electroweak WIMP coming from two limiting cases in the MSSM: the Higgsino and the Wino model. The dark matter candidate is a particle too heavy to be directly seen in colliders but it could be seen indirectly by astrophysical observations. Dark matter particles in the galaxy can annihilate to Standard Model particles and be detected by satellites. We consider the charged neutral mass splitting as a free parameter in the theory and investigate its effect on the Sommerfeld enhancement which gives an important boost to the annihilation cross-section, the main observable of indirect detection. In the second part of this thesis, we present an idea of a fixed target experiment so search for dark sector models, a class of model with very weakly interacting particles. This experiment uses the experimental setup needed for a future muon collider but is independent from the muon collider program. We study the reach of our experiment for three benchmark models, the dark photon, the dark Higgs, the heavy neutral lepton and we compare its expected performances to current and future experiments. In the third part of this thesis, we consider a more model-independent approach to the search for new physics. If new states are heavy, they can be integrated out and their leading effects are encoded in effective operators made of Standard Model fields. Assuming baryon and lepton number conservation, one can classify the effective operators of dimension-6 which give the leading contribution and constrain the coefficient of these operators by looking at precision observables. We compile results from LEP-I and LEP-II experiments as well as neutrino scattering and other low-energy observables. We allow all operators to be present with an arbitrary flavour structure. Our result can then be used to translate these constaints to specific models of new physics.

EPFL2019

Towards Efficient and Accurate Numerical Simulations of Galaxies using Task-based Parallelism and Application to Dwarf Galaxies

Loïc Hausammann

Numerical simulations are of a tremendous help to understand the growth of non-linear cosmological structures and how they lead to the formation of galaxies. In recent years, with the goal of improving their prediction power, new hydrodynamical techniques and physical models have been developed. Beside those improvements, the community must be ready for what could be a major paradigm change, the exascale supercomputer that will allow for the simulation of higher resolution and/or larger cosmological volume. Being ready to fully benefit from those upcoming facilities requires an important effort in adapting existing codes.Among the domains that will benefit from the exascale computing are dwarf galaxies and their low mass end, the ultra faint dwarf galaxies (UFD). Since twenty years, dwarfs have become an important research topic in astrophysics due to recent observing facilities allowing a large number of unprecedented observations. Simulations of such galaxies require extremely high resolution down to the individual star level and, thus, require the adaptation of existing physical models.In the first part of my thesis, I extended a previous work performed with the code GEAR on understanding the formation of dwarf galaxies in a cosmological context. I studied the ram pressure stripping mechanism due to the hot halo of the Milky Way and how it modifies the observed properties of dwarf galaxies. I demonstrated that the thermal pressure due to the hot halo on the dwarf's gas can strongly impact the effect of ram pressure.To ensure the quality of our simulations, I have participated in the AGORA project that aims at comparing the predictions of different simulation codes and understanding the underlying differences. In the last AGORA paper, we have performed the first comparison of cosmological simulations using common models for the radiative cooling and star formation.In the second part of this thesis, an important effort has been dedicated to migrate our physical models from our code GEAR towards SWIFT. Thanks to the improved performance of SWIFT (speedup of 7.65x) and the successful migration, our code is now fast enough to simulate UFDs at the resolution of individual stars. On the physics side, our star formation and stellar feedback methods need to be further improved due to the high resolution required in UFDs. To solve this issue, I designed and started the implementation of a new star formation scheme based on a new type of particles called "sink particles".Going towards the exascale also requires a better design of the output system previously based on snapshots of the simulation saved at regular time intervals. In this purpose, I introduced the "Continuous Simulation Data Stream" (CSDS) specially designed for simulations where gravity is producing strong differences in timescale between particles or cells. The CSDS reduces the disk space used by a single simulation while improving the time resolution of the output and making the analysis easier and flexible.Finally, while the simulation code and the associated physical models have a tremendous impact on the quality of the simulations, the initial conditions need to be carefully designed as they can deeply impact the results. Using a Bayesian approach, I have developed my own code that generates constrained initial conditions that reproduce the Local Group.

EPFL2021

Probing non-Gaussian stochastic gravitational wave backgrounds with LISA

Valerie Fiona Domcke, Daniel Garcia Figueroa

The stochastic gravitational wave background (SGWB) contains a wealth of information on astrophysical and cosmological processes. A major challenge of upcoming years will be to extract the information contained in this background and to disentangle the contributions of different sources. In this paper we provide the formalism to extract, from the correlation of three signals in the Laser Interferometer Space Antenna (LISA), information about the tensor three-point function, which characterizes the non-Gaussian properties of the SGWB. This observable can be crucial to discriminate whether a SGWB has a primordial or astrophysical origin. Compared to the two-point function, the SGWB three-point function has a richer dependence on the gravitational wave momenta and chiralities. It can be used therefore as a powerful discriminator between different models. For the first time we provide the response functions of LISA to a general SGWB three-point function. As examples, we study in full detail the cases of an equilateral and squeezed SGWB bispectra, and provide the explicit form of the response functions, ready to be convoluted with any theoretical prediction of the bispectrum to obtain the observable signal. We further derive the optimal estimator to compute the signal-to-noise ratio. Our formalism covers general shapes of non-Gaussianity, and can be extended straightaway to other detector geometries. Finally, we provide a short overview of models of the early universe that can give rise to a non-Gaussian SGWB.