<> "The repository administrator has not yet configured an RDF license."^^ . <> . . "The universe from a string-theoretic and cosmological perspective"^^ . "One major challenge of modern-day theoretical physics concerns the extension of our standard models for particle physics and cosmology. A motivation therefor arises from several problems and tensions that these models face and which suggest their modification. In this thesis we utilise distinct approaches to tackle some of these issues from different perspectives.\r\nIn a classical field-theoretic approach, we first consider an extension to the \\textLambda CDM model called coupled dark energy. Characteristic for this scalar-tensor theory, which has been shown to possess the ability to alleviate the (in-)famous Hubble tension, is an intrinsic coupling within the dark sector of the universe. We will demonstrate that under certain conditions this model can give rise to a novel transient regime of weak gravity. This may help to solve or at least alleviate the $\\sigma_8$ tension of the \\textLambda CDM model. We then shift to the paradigm of string theory, which presumably provides the required ultraviolet completion of gravity including the other fundamental forces, and assess its consequences on two extensions of standard-model physics. The first is constituted by the postulation of a new particle, the QCD axion, which arguably represents the most prominent way to solve the strong CP problem. In string theory, there are many candidates for this new particle and we investigate phenomenological consequences of such a stringy realisation of the axion. In particular, we find a novel way to seemingly solve the notorious issue of too much dark radiation, which is a generic prediction of these constructions, via a fast decay channel of the internal volume into standard-model Higgses. Even though we ascertain that eventually the dark radiation problem presumably re-appears due to the altered cosmological setting, we are confident that our results will prove to be helpful for future constructions. The second extension to standard-model physics we consider is a cosmological one, namely quintessence. With regard to the recently postulated de Sitter-swampland conjecture, we analyse the realisability of such a dynamical form of dark energy in a stringy context. Taking into account several phenomenological requirements, we identify two major challenges that need to be overcome: a so-called light-volume problem implying a very light internal-volume modulus, that would give rise to inadmissible fifth forces, and a novel F-term problem, which emerges from the fact that the required supersymmetry breaking scale raises the resulting scalar potential and hence the effective vacuum energy to a value that is parametrically above the observed one."^^ . "2022" . . . . . . . "Manuel"^^ . "Wittner"^^ . "Manuel Wittner"^^ . . . . . . "The universe from a string-theoretic and cosmological perspective (PDF)"^^ . . . "thesis_final_wittner.pdf"^^ . . . "The universe from a string-theoretic and cosmological perspective (Other)"^^ . . . . . . "indexcodes.txt"^^ . . . "The universe from a string-theoretic and cosmological perspective (Other)"^^ . . . . . . "lightbox.jpg"^^ . . . "The universe from a string-theoretic and cosmological perspective (Other)"^^ . . . . . . "preview.jpg"^^ . . . "The universe from a string-theoretic and cosmological perspective (Other)"^^ . . . . . . "medium.jpg"^^ . . . "The universe from a string-theoretic and cosmological perspective (Other)"^^ . . . . . . "small.jpg"^^ . . "HTML Summary of #32036 \n\nThe universe from a string-theoretic and cosmological perspective\n\n" . "text/html" . . . "500 Naturwissenschaften und Mathematik"@de . "500 Natural sciences and mathematics"@en . . . "530 Physik"@de . "530 Physics"@en . .