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Observations cosmologiques par LISA : limitations dues au bruit et aux signaux astrophysiques

Abstract : While waiting to measure the stochastic gravitational waves background with the Laser Interferometer Space Antenna (LISA), we propose to determine methods and strategies to understand and prepare for the cosmological observations by LISA. LISA is a space mission aiming at measuring gravitational waves between 1 x 10〖-5〗^ and 1 Hz. This mission, led by ESA, should be launched in 2034 or later. The subject of this PhD thesis is to understand and simulate the stochastic cosmological background produced at the beginning of our Universe in a context of other stochastic backgrounds of astrophysical or galactic origin, along with the LISA noise. The first one comes from unresolved binaries of black holes and neutron stars and is also present in the frequency range of LIGO/Virgo. The second one comes from the white dwarf binaries present in our galaxy. The gravitational wave signal of these binaries is not yet well understood, and we will have to simulate and study it. Indeed, some phenomena related to its properties, but also to its observation, make it difficult to study. We are looking for and have develop Bayesian and Fisher strategies to understand the spectral separation of the different stochastic backgrounds while evaluating the possible limits of detectability of the cosmological background.Moreover, LISA noise could have multiple sources of disturbance, such as correlated noise. To understand the issue of a non-perfect LISA noise we studied and then established some correlations from the LISA test mission, LISA PathFinder, of which we performed a detailed noise study. In order to understand the effects of possible correlated noise in LISA, we injected these correlations into a LISA noise level, and established new estimates of the limitation of the cosmological stochastic background measurement. We also sought to establish the limits of the current model with more complex noise models, i.e., correlating the noise of two optical banks on the same satellite, or independently increasing the noise of each optical system.Thus, we will be able, with our method, to detect and separate cosmological backgrounds of the order of Ω〖GW,cosmo〗_ ≃ 1 x 10〖-13〗^ to 1 x 10〖-12〗^. We have also studied the detectability limit of cosmic strings, for which we have established a tension limit Gμ_ ≃ 1 x 10〖-17〗^ to 1 x 10〖-16〗^.
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Guillaume Boileau. Observations cosmologiques par LISA : limitations dues au bruit et aux signaux astrophysiques. Astrophysique [astro-ph]. Université Côte d'Azur, 2021. Français. ⟨NNT : 2021COAZ4064⟩. ⟨tel-03438923⟩

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