Browsing by Author "Osano, Bob"

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    A Lagrangian formulation of a theory of a scalar field superfluid dark matter
    (2022) Tema, Seturumane; Larena, Julien; Osano, Bob
    In 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.
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    A study of relativistic fluids with applications to cosmology: A variational approach
    (2021) Oreta, Timothy; Osano, Bob
    This thesis examines relativistic fluids. We have used the variational approach to develop tools for studying the dynamics of relativistic fluids to apply this to cosmological modelling. Studies like these go beyond the standard model in cosmology. Researchers believe that such extensions to the standard cosmological model are pivotal to resolving some of the long-standing cosmological problems. An example of such problems is the origin, growth (from quantum electromagnetic fluctuations to large-scale magnetic fields during inflation) and evolution of cosmological magnetic fields that exhibit as large-scale (cosmological) magnetic fields in late time. One other example is the coincidence problem. The standard approach in such studies is to use modelling in the form of the single-fluid formalism. As an alternative one can consider the single-fluid and multi-fluid formalisms that incorporate aspects of electrodynamics and thermodynamics, respectively in the context of the variational approach. This might help us make progress in trying to either resolve some of these problems or at least open up new ways of addressing them. In this regard, we have extended the well-known M¨ueller-Israel-Stewart (hereafter MIS) formalism to allow us to examine the effect on fluid flow in which the components of the multi-species fluids interact thermodynamically. We use the extension to the MIS theory in the context of interacting species to study the growth of dark matter and dark energy, and find that either interaction or entrainment involving dark energy and dark matter suggests a mutual relative modulation of the growth behaviour of the two densities. This may aid in resolving the coincidence problem. Our examination of inflation-generated, large-scale magnetic fields reveals a super-adiabatically evolving mode from the beginning of the radiation-dominated epoch to either much later during the epoch or probably extending far into the era of matter domination which may account for late time, large-scale magnetic fields.
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    Cio-cio-san no Yūutsu: memoirs of magnetogenesis and Turbulent Dynamo Theory
    (2013) Adams, Patrick; Osano, Bob
    The origins of cosmic magnetic fields are not as yet well understood. In this dissertation we investigate, via direct numerical simulation, the temporal evolution and behaviour of magnetic fields that are generated from absolute zero initial conditions via a thermal battery term in the Induction Equations (i.e. the Magnetogenesis problem), whilst making use of the Ideal- and Chaplygin Gas equations of state, in turn, to model the relationship between pressure and density. The dependence of the onset of dynamo action on various values of the magnetic Reynolds- and Prandtl numbers for the cases of the Roberts Flow kinematic dynamo and a flow that, in turn, incorporates both a non-helical and helical forcing function that introduces turbulence into the system is also considered via direct numerical simulation. For the purposes of the simulation work conducted, we make use of the PENCIL CODE, which is a high-order finite-difference Magnetohydrodynamical code capable of performing simulation runs in parallel using the Message Passing Interface (MPI) system for parallel processing. Theoretical results relevant to the simulations conducted are partially recovered and discussed in detail. These include, and are not limited to, the emergence of the thermal battery term in the General Ohm's Law as a consequence of the two-fluid approximation of a plasma, derivation of the Induction Equations incorporating the aforementioned battery term, introduction and discussion of the Chaplygin Gas and its place in the field of Cosmology, energetics governing the flow of kinetic- and magnetic energy during the dynamo process, the Zel'dovich stretch-twist-fold dynamo as an example of both a fast dynamo and a cornerstone underlying the operation of all dynamos and, finally, the Kazantsev Theory for small-scale, turbulent dynamos. For our magnetogenesis simulations, it is found that the magnetic fields produced undergo two distinct growth phases (the first, classified as an initial """"upshoot"""" that is possibly due to the battery term and the second, classified as an exponential growth phase), as well as two distinct phases of decay in strength, which is attributed to the effects of magnetic diffusion. This behaviour is observed for fields generated using both the Ideal- and Chaplygin Gas equations of state in turn and it is noted that the Chaplygin Gas equation of state produces magnetic fields that are of comparable strength to those produced by the Ideal Gas equation of state. Dynamo action simulations confirm the existence of a critical magnetic Reynolds number, beyond which, an initial prescribed magnetic field will grow exponentially in strength. In the case of the forced turbulence simulations, it is noted that the use of a helical forcing function greatly lowers the value of the critical magnetic Reynolds number required for the onset of guaranteed dynamo action and also produces stronger magnetic fields when compared to University of Cape Town the cases that used a non-helical forcing function. In both cases of the forced turbulence, the magnetic field is observed to saturate when its kinematic (i.e. exponential growth) phase is complete, provided that the magnetic Reynolds number is above the aforementioned critical threshold. Results of the magnetogenesis simulations are also investigated for dynamo action, and it is concluded that a type of """"kinematic dynamo"""" phase was most probably present when these fields underwent the observed phase of exponential growth.
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    Tool for evaluation of building sustainability
    (2013) Sanya, Tom; Osano, Bob
    SoftArch is a developed for evaluating building sustainability on social, economic and environmental aspects in developing country contexts. This tool is an interactive simulation that evaluates the environmental, economic and social impact of different building models.