### Browsing by Author "Larena, Julien"

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- ItemOpen AccessA Lagrangian formulation of a theory of a scalar field superfluid dark matter(2022) Tema, Seturumane; Larena, Julien; Osano, BobIn this thesis we discuss the dynamics of the relativistic Lagrangian of the theory of dark matter superfluidity. The second and third chapters of the thesis are a review. In the fourth chapter we show that a complex scalar field whose dynamics are dictated by such a Lagrangian, models dust in the background universe on cosmological scale. Prior to our calculations, the theory was shown to model dust on cosmological scale and a superfluid on galactic scale in the non-relativistic case [1]. This project, extends the non-relativistic theory, to include the relativistic background. We continued, using the relativistic Lagrangian, to investigate how perturbations of such a theory grow in a perturbed universe, and found that the density contrast (of the theory) is constant when the complex scalar field is not coupled to baryons in the weak-field limit.
- ItemOpen AccessTesting General Relativity with the next generation of cosmological surveys(2019) Moloi, Teboho Abram; Larena, Julien; Clarkson, ChristopherThe late-time acceleration expansion of the Universe is conceptually considered the great burdensome issue in theoretical physics (cosmological problem) dubbed dark energy (DE) problem. In general relativity (GR) framework view point, there are two ways to explain where this acceleration might originate from; this riddle might either emerge from some unknown dark energy models or general relativity is a mistake on cosmological scale and dark energy is insubstantial. Innovative efforts have been carried out to comprehend the origin of the cosmic acceleration, involving surveys such as baryon acoustic oscillations (BAOs), Type Ia supernovae, weak gravitational lensing and the abundance of galaxy clusters. The next generation of cosmological surveys including LSST, DES, eBOSS, DESI, PFS, SKA and WFIRST; are aimed to provide percent-level or higher measurement of history of expansion and growth of structure over a volume which is sizable fraction of the whole observable Universe, these measurements provides strong constrains on DE. In this analysis, we investigate the Horndeski scalar-tensor theories and beyond which has been recently described in the generilized dark energy (DE) or scalar-tensor paradigm - dubbed unified dark energy (UDE). This applies the 3+1 Arnowitt-DeserMisner (ADM) formalism where a general action in unitary gauge depends on the lapse function and geometrical scalar quantities. This approach is convenient since it generates a unified framework of modified theories based on UDE or effective field theory (EFT) of linear cosmological perturbations on Friedmann-Lemaitre-Robertson-Walker (FLRW) background, this are generally characterized by five free time-dependent functions αi (αB,αH,αK,αM,αT ) each describing different properties of unified dark energy physical outcome. The evolution equations for the given UDE which assimilates beyond-Horndeski paradigms appear to correspond to a non-conservative DE scenario, in which the total energy-momentum tensor is not conserved. Furthermore, we evaluate the large-scale imprint of this UDE, by probing the two-point correlation function or power spectrum of galaxy number counts and the magnification of galaxies, on horizon scales; making sure to include the full relativistic corrections in the observed overdensity and convergence. This yield new observables which gives independent insights regarding the peculiar velocity of galaxies, the growth of structure of the Universe etc.
- ItemOpen AccessTopics in relativistic cosmology: Cosmology on the past lightcone and in modified gravitation(2018) Elmardi, Maye Y A; Clarkson, Chris; Larena, JulienThe lightcone gauge is a set of what are called the observational coordinates adapted to our past lightcone. We develop this gauge by producing a perturbed spacetime metric that describes the geometry of our past lightcone where observations are usually obtained. We then connect the produced observational metric to the perturbed Friedmann-Lemaître-Robertson-Walker metric in the standard general gauge or what is the so-called 1+3 gauge. We derive the relations between these perturbations of spacetime in the observational coordinates and those perturbations in the standard metric approach, as well as the dynamical equations for the perturbations in observational coordinates. We also calculate the observables in the lightcone gauge and rederive them in terms of Bardeen potentials to first order. A verification is made of the observables in the perturbed lightcone gauge with those in the standard gauge. The advantage of the method developed is that the observable relations are simpler than in the standard formalism. We use the perturbed lightcone gauge in galaxy surveys and galaxy number density contrast. The significance of the new gauge is that by considering the null-like light propagations, the calculations are much simpler since angular deviations are not considered. Standard cosmology based on General Relativity is generally believed to have serious shortcomings, such as the unexplained issues of dark matter and dark energy. As a remedy, many alternative theories of gravitation have been proposed over the years, one of which is ƒ(R) gravity. We explore classes of irrotational-fluid cosmological models in the context of ƒ(R) gravity in an attempt to put some theoretical and mathematical restrictions on the form of the ƒ(R) gravitational Lagrangian. In particular, we investigate the consistency of the linearised dust models for shear-free cases as well as in the limiting cases when either the gravito-magnetic or gravito-electric components of the Weyl tensor vanish. We also discuss the existence and consistency of classes of non-expanding irrotational spacetimes in ƒ(R)-gravity. Furthermore, we explore exact ƒ(R) gravity solutions that mimic Chaplygin-gas inspired ΛCDM cosmology. Starting with the original, generalized and modified Chaplygin gas equations of state, we reconstruct the forms of ƒ(R) Lagrangians. The resulting solutions are generally quadratic in the Ricci scalar, but have appropriate ΛCDM solutions in limiting cases. These solutions, given appropriate initial conditions, can be potential candidates for scalar field-driven early universe expansion (in ation) and dark energy-driven late-time cosmic acceleration.