Dissertations / Theses on the topic 'Scalar field cosmology'
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1
Castro, Fábio Chibana de. "Tachyon Scalar Field Cosmology." Universidade de São Paulo, 2017. http://www.teses.usp.br/teses/disponiveis/43/43134/tde-17052017-063702/.
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In this work we test a cosmological model with an interaction between dark energy and dark matter, where a tachyon scalar field plays the role of dark energy. With that in mind, we developed a numerical code that solves the background equations and extracts the cosmological parameters and we compared the results of the interacting tachyon model with those of other dark energy candidates. Our results show that the model indeed explains the observational data and has interesting cosmological properties, but might face challenges when compared to other dark energy candidates.Neste trabalho testamos um modelo cosmológico com uma interação entre energia escura e matéria escura, onde um campo escalar taquiônico desempenha o papel da energia escura. Para isso, desenvolvemos um código computacional que resolve as equações numericamente e vincula os parâmetros cosmológicos e, assim, comparamos os resultados do modelo taquiônico interagente com os de outros candidatos à energia escura. Nossas análises mostram que o modelo, de fato, consegue explicar os dados observacionais, além de possuir propriedades cosmológicas interessantes, mas apresenta dificuldades quando comparado a outros modelos de energia escura.
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Westmoreland, Shawn. "Energy conditions and scalar field cosmology." Kansas State University, 2013. http://hdl.handle.net/2097/15811.
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Master of ScienceDepartment of Physics
Bharat Ratra
In this report, we discuss the four standard energy conditions of General Relativity (null, weak, dominant, and strong) and investigate their cosmological consequences. We note that these energy conditions can be compatible with cosmic acceleration provided that a repulsive cosmological constant exists and the acceleration stays within certain bounds. Scalar fields and dark energy, and their relationships to the energy conditions, are also discussed. Special attention is paid to the 1988 Ratra-Peebles scalar field model, which is notable in that it provides a physical self-consistent framework for the phenomenology of dark energy. Appendix B, which is part of joint-research with Anatoly Pavlov, Khaled Saaidi, and Bharat Ratra, reports on the existence of the Ratra-Peebles scalar field tracker solution in a curvature-dominated universe, and discusses the problem of investigating the evolution of long-wavelength inhomogeneities in this solution while taking into account the gravitational back-reaction (in the linear perturbative approximation).
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Kujat, Jens. "Scalar fields in cosmology." Columbus, Ohio : Ohio State University, 2006. http://rave.ohiolink.edu/etdc/view?acc%5Fnum=osu1142978764.
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Laycock, Andrew Mark. "Aspects of non-minimally coupled scalar field cosmology." Thesis, University of Sussex, 1995. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.282083.
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Graham, Alexander Alan Hewetson. "Scalar fields in cosmology and black holes." Thesis, University of Cambridge, 2016. https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.709524.
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Parsons, Paul. "Scalar-field models of the early universe." Thesis, University of Sussex, 1997. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.390077.
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Leith, Ben Maitland. "Scalar Fields and Alternatives in Cosmology and Black Holes." Thesis, University of Canterbury. Physics and Astronomy, 2007. http://hdl.handle.net/10092/1444.
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Extensions to general relativity are often considered as possibilities in the quest for a quantum theory of gravity on one hand, or to resolve anomalies within cosmology on the other. Scalar fields, found in many areas of physics, are frequently studied in this context. This is partly due to their manifestation in the effective four dimensional theory of a number of underlying fundamental theories, most notably string theory. This thesis is concerned with the effects of scalar fields on cosmological and black hole solutions. By comparison, an analysis of an inhomogeneous cosmological model which requires no extensions to general relativity is also undertaken. In chapter three, examples of numerical solutions to black hole solutions, which have previously been shown to be linearly stable, are found. The model includes at least two scalar fields, non-minimally coupled to electromagnetism and hence possesses non-trivial contingent primary hair. We show that the extremal solutions have finite temperature for an arbitrary coupling constant. Chapter four investigates the effects of higher order curvature corrections and scalar fields on the late-time cosmological evolution. We find solutions which mimic many of the phenomenological features seen in the post-inflation Universe. The effects due to non-minimal scalar couplings to matter are also shown to be negligible in this context. Such solutions can be shown to be stable under homogeneous perturbations. Some restrictions on the value of the slope of the scalar coupling to the Gauss-Bonnet term are found to be necessary to avoid late-time superluminal behaviour and dominant energy condition violation. A number of observational tests are carried out in chapter five on a new approach to averaging the inhomogeneous Universe. In this "Fractal Bubble model" cosmic acceleration is realised as an apparent effect, due to quasilocal gravitational energy gradients. We show that a good fit can be found to three separate observations, the type Ia supernovae, the baryon acoustic oscillation scale and the angular scale of the sound horizon at last scattering. The best fit to the supernovae data is χ² ≃ 0:9 per degree of freedom, with a Hubble parameter at the present epoch of H0 = 61:7+1:4 -1:3 km sec⁻¹ Mpc⁻¹ , and a present epoch volume void fraction of 0:76 ± 0:05.
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Bertacca, Daniele. "Unified Dark Matter in Scalar Field Cosmologies." Doctoral thesis, Università degli studi di Padova, 2008. http://hdl.handle.net/11577/3425159.
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In this thesis I have investigated the possibility that the
dynamics of a single scalar field can account for a unified description of the Dark Matter and Dark Energy sectors: Unified Dark Matter (UDM).
In particular considering the general Lagrangian of k-essence models, I study and classify them through variables
connected to the fluid equation of state parameter w_kappa. This allows to find solutions around which the scalar field describes a mixture of dark matter and cosmological constant-like dark energy (UDM) (Bertacca, Matarrese, Pietroni 2007).
Subsequently I also perform an analytical study of the Integrated Sachs-Wolfe (ISW) effect within the framework of Unified Dark Matter models based on a scalar field which aim at a unified description of dark energy and dark matter. Computing the temperature power spectrum of the Cosmic Microwave Background anisotropies I am able to isolate
those contributions that can potentially lead to strong deviations from the usual ISW effect occurring in a Lambda CDM Universe.
This helps to highlight the crucial role played by the sound speed in the unified dark matter models. This treatment is completely general in that all the results depend only on the speed of sound of the dark component and thus it can be applied to a variety of unified models, including those which are not described by a scalar field but relies on a single dark fluid (Bertacca and Bartolo 2007).
Finally I also investigated the static and spherically symmetric
solutions of Einstein's equations for a scalar field with non-canonical kinetic term (Bertacca, Bartolo, Matarrese 2007).
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Foster, Scott. "Singularity structure of scalar field cosmologies /." Title page, contents and abstract only, 1996. http://web4.library.adelaide.edu.au/theses/09PH/09phf757.pdf.
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Rossi, Massimo. "Dark energy as a scalar field non-minimally coupled to gravity." Master's thesis, Alma Mater Studiorum - Università di Bologna, 2017. http://amslaurea.unibo.it/12825/.
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The cosmological constant is not the only possibility in order to describe the accelerated expansion of the Universe. A different approach is to modify the gravitational sector of the Einstein equations. In scalar-tensor theories the gravitational interaction is affected by both a scalar and a tensor field. The dependence of gravity from the scalar field is obtained through a non-minimal coupling function which multiplies the Ricci scalar in the Lagrangian. In this thesis we consider a specific shape of the coupling function that reduces to the minimal coupling case and to the induced gravity case for specific choices of the parameters.
We consider two shapes for the potential: one leads to an effectively massless Klein-Gordon equation while the other is motivated by the fact that it is a viable potential for the chaotic inflation in superconformal theory. For the former we consider also the conformal coupling case, which is the required coupling in order to obtain a conformally invariant theory.
We derive the fundamental equation at the background and linear perturbations level and then we recover the initial condition for the perturbations.
In order to study the evolution for the background and linear fluctuations within non-minimally coupling we modified the publicly available Einstein-Boltzmann code CLASS.
The evolution of the dark energy density parameter and the equation of state are shown. Furthermore we pay attention to the actual value of the post-Newtonian parameters in order to see which choices of the parameters satisfies the Solar System constraints.
We present the results obtained for CMB anisotropies, linear matter power spectrum, metric and scalar field perturbations. As for the background we confront them with the ΛCDM model for both the potential considered.
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Samushia, Lado. "Constraining scalar field dark energy with cosmological observations." Diss., Manhattan, Kan. : Kansas State University, 2009. http://hdl.handle.net/2097/1524.
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Gordon, Christopher. "Adiabatic and entropy perturbations in cosmology." Thesis, University of Portsmouth, 2001. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.369474.
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Lyons, Glenn. "Time asymmetry." Thesis, University of Cambridge, 1993. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.309076.
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GENGO, MASSIMO MANUEL. "INTEGRABLE MULTIDIMENSIONAL COSMOLOGIES WITH MATTER AND A SCALAR FIELD." Doctoral thesis, Università degli Studi di Milano, 2019. http://hdl.handle.net/2434/613446.
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The thesis discusses some integrable (i.e., explicitly solvable) cosmologies à la Robertson
-Walker, with possibly non zero spatial curvature. In these models, the
content of the universe consists of matter and of a scalar field. The expression
"matter" is used in a broad sense, and includes as a special case the radiation; the
considered matter has pressure p_m and density ϱ_m, related by an equation of state
p_m = w ϱ_m, with w an arbitrary constant (w = 0 in the case of dust; w = 1/d
in the case of a radiation gas). The scalar field is used as a model for the dark
energy content of the universe, refining the standard cosmological constant model.
(We recall the denomination of "quintessence", often used for a scalar field with this
rôle.)
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Metcalf, Thomas Patrick. "Dissipative effects in the Early Universe." Thesis, University of Edinburgh, 2015. http://hdl.handle.net/1842/15863.
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Inflationary cosmology is the leading candidate for explaining the homogeneity, isotropy and spatial flatness of the universe whilst also providing the mechanism for the seeding of large scale structure. The central theme of inflationary dynamics involves the evolution of a scalar field, called the inflaton, such that its potential drives an accelerated expansion. Warm inflation is the dynamical realization in which interactions between the inflaton and other fields can lead to dissipation of inflaton energy to other dynamical degrees of freedom. Heavy fields coupled to the inflaton mediate the transfer of inflaton energy to light degrees of freedom which thermalize and heat the universe. This damps the inflaton’s motion and allows for the potential formation of a thermal bath during the inflationary period. Hybrid inflation models are a natural way in which warm inflation can be realized, with dissipation of inflaton energy mediated by the waterfall fields to fields in the light sector. In this thesis I outline the dynamics and observational predictions of supersymmetric hybrid inflation driven by radiative corrections in the warm regime. As in the standard cold inflationary scenario inflation ends when the effective mass squared of the waterfall field becomes negative, with the tachyonic instability driving the system to a global minimum in a process called the waterfall transition. I present the effect of including thermal mass corrections to the waterfall fields, and SUSY mass splittings on the quantum effective potential and the resulting dissipation coefficient. I show that including dissipative effects can significantly prolong the inflationary period to produce 50-60 e-folds of inflation with an observationally consistent primordial spectrum. Inflation still requires a microphysical description within a fundamental theory of quantum gravity. This has prompted the search for inflaton candidates within the superabundance of scalar fields present in string theory compactifications, with brane-antibrane inflation in particular emerging as a concrete implementation of SUSY hybrid inflation in a UV complete particle physics model. Inflation proceeds in a brane-antibrane system through the movement of a stack of branes towards a stack of antibranes, with the inflaton field being the interbrane distance. Warm inflation can be implemented in a brane-antibrane system with dissipation of inflaton energy mediated by fields corresponding to strings stretched between the brane and antibrane stacks. It has been shown that this dissipation of inflaton energy in warm inflation can greatly alleviate the η-problem in brane-antibrane scenarios. Whilst these strings mediating dissipation have end points fixed on to both the D3 and D3 stacks, the compact nature of the geometry within which the system is constructed allows these strings to have different winding modes. We investigated how strings with increasing winding number can provide an enhancement to the dissipation coefficient, allowing a significant reduction in the number of branes and antibranes in the warm inflation system, whilst also modifying the inflationary dynamics by reducing the speed at which the system evolves. This may go some way to alleviating the η-problem associated with some constructions of brane-antibrane inflation whilst also potentially providing the best way to motivate the large field multiplicities associated with warm inflation models.
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Wilhelm, Söderkvist Vermelin. "3+1 Approach to Cosmological Perturbations : Deriving the First Order Scalar Perturbations of the Einstein Field Equations." Thesis, Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), 2016. http://urn.kb.se/resolve?urn=urn:nbn:se:kau:diva-43257.
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Experimental data suggest that the universe is homogeneous and isotropic on sufficiently large scales. An exact solution of the Einstein field equations exists for a homogeneous and isotropic universe, also known as a Friedmann-Lemaître-Robertson-Walker (FLRW) universe. However, this model is only a first approximation since we know that, locally, the universe has anisotropic and inhomogeneous structures such as galaxies and clusters of galaxies. In order to successfully introduce inhomogeneities and anisotropies to the model one uses perturbative methods. In cosmological perturbations the FLRW universe is considered the zeroth order term in a perturbation expansion and perturbation theory is used to derive higher order terms which one tries to match with observations. In this thesis I present a review of the main concepts of general relativity, discuss the 3+1 formalism which gives us the Einstein field equations in a useful form for the perturbative analysis, and lastly, I derive the first order scalar perturbations of the Einstein field equations.
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Ashcroft, P. R. "Cosmology with scalar fields." Thesis, University of Cambridge, 2004. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.596177.
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In this thesis we investigate a number of roles that scalar fields can play in cosmology. In particular it is generally believed that the early Universe underwent a period of very rapid expansion. We call this epoch inflation. Initially we investigate the evolution of two slow-rolling scalar fields with potentials of the form V = Voφ-a exp(-bφm). By considering different values of the parameters a, b and m, we drive several new inflationary solutions in which one field just evolves in the background and is not important for the inflationary dynamics. In addition, we find new solutions where both fields are dynamically important during inflation. Moreover, we discuss the evolution of perturbations in both the scalar fields and the spacetime metric, concentrating on the production of entropy perturbations. We find that for a large region in parameter space and initial conditions, tensor modes are negligible, and that adiabatic and isocurvature perturbations are essentially uncorrelated. We move on to consider perturbations in the Randall-Sundrum braneworld scenario. At energies higher than the brane tension, the dynamics of a scalar field rolling down a potential are modified compared with the predictions of General Relativity. These modifications imply, among other things, that steeper potentials can be used to drive an epoch of slow-roll inflation. The evolution of entropy and adiabatic modes during inflation driven by two scalar fields confined to the brane is studied. We show that the amount of entropy perturbation produced during inflation is suppressed compared to the predictions made by General Relativity. As a consequence, the initial conditions do not matter in multiple field inflation in braneworlds if inflation is driven at energies much higher than the brane tension. Following this, we study the evolution of slow-roll inflation in a five-dimensional braneworld model with two boundary branes and a bulk scalar field. Assuming that the inflationary scale is below the brane tension, we can employ the moduli space approximation to study the dynamics of the system. Detuning the brane tension results in a potential for the moduli fields which we show will not support a period of slow-roll inflation. We then study an inflation field confined to the positive tension brane, to which the moduli fields are non-minimally coupled. We discuss in detail the two cases of V (χ) = ½m2 χ 2 and V (χ) = l χ 4 and demonstrate that increasing the coupling results in spectra which are further away from scale-invariance. Furthermore, there is an increase in the tensor mode production, while entropy perturbations are subdominant. Finally, we point out that the five-dimensional spacetime is unstable during inflation because the negative tension brane collapses.
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Escobal, Anderson Almeida. "Matéria escura como campo escalar : aspectos teóricos e observacionais /." Guaratinguetá, 2020. http://hdl.handle.net/11449/192424.
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Orientador: José Fernando de JesusResumo: Estudamos o campo escalar real como um possível candidato para explicar a matéria escura no universo. No contexto de um campo escalar livre com potencial quadrático, após encontrar as equações dinâmicas do modelo usamos os dados observacionais para limitar os parâmetros livres e assim encontrar um limite inferior para o valor da massa que foi da ordem de $10^{-34}$eV, esse valor está próximo ao encontrado por alguns autores. Não foi possível encontrar um limite superior para a massa da matéria escura do campo escalar combinando os dados de $H(z)$, SN Ia. Como verificado neste trabalho e observado em outros estudos, a matéria escura pode ser descrita por um campo escalar real. Em outra linha de pesquisa, usando um método estatístico não-paramétrico envolvendo os chamados Processos Gaussianos, obtivemos um valor do redshift de transição, $z_t$, de $z_t = 0.59^{+0.12}_{-0.11}$ para dados de $H(z)$ e $z_t= 0.683^{+0.11}_{-0.082}$ para dados de SNs Ia.
Abstract: We studied the real scalar field as a possible candidate to explain the dark matter in the universe. In the context of a free scalar field with quadratic potential, after finding the dynamic equations of the model we used the observational data to limit the free parameters and thus find a lower limit for the mass value that was in the order of 10−34 eV , this value is close to that found by some authors. It was not possible to find an upper limit for the mass of dark matter in the scalar field by combining the H(z) + SNe Ia data. As verified in this work and observed in other studies, dark matter can be described by a real scalar field. In another line of research, using a non-parametric statistical method involving the so-called Gaussian Processes, we obtained a value of the transition redshift, zt , of zt = 0.59+0.12 −0.11 for H(z) data and zt = 0.683+0.11 −0.082 for SNs Ia data.
Mestre
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Gundlach, C. "Classical and quantum scalar fields in cosmology." Thesis, University of Cambridge, 1992. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.240140.
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Easther, Richard John Maddock. "The evolution of scalar fields and inflationary cosmology." Thesis, University of Canterbury. Physics, 1993. http://hdl.handle.net/10092/8058.
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The thermal restoration of symmetry in Grand Unified Field Theories with Coleman-Weinberg symmetry breaking is considered. A concise rederivation of the high temperature approximation to the one loop effective potential is given and the critical temperature for the transition is then calculated exactly, within the one loop effective potential.
A number of new exact scalar field cosmologies are derived from effective potentials that are composed of two or more exponential terms. These solutions are considered from the standpoint of inflationary cosmology. In particular, solutions are presented that combine eras of exponential and power-law growth and which have a transition between inflationary and non-inflationary expansion. For some parameter choices, the density perturbation spectra produced by these models are significantly tilted with a minimal component of tensor perturbations.
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Nunes, Nelson. "Attractor solutions in cosmology and particle physics." Thesis, University of Sussex, 2002. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.247958.
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Perrotta, Francesca. "Cosmologies with a Dynamical Vacuum Energy." Doctoral thesis, SISSA, 2000. http://hdl.handle.net/20.500.11767/4331.
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The possibility of a nonzero cosmological constant has been invoked several times in the past, both
for theoretical and observational motivations. It has been often discarded by particle physicists, due
to the huge difficulties in justifying a value of vacuum energy tiny enough to allow the universe to
survive more than 10-41 s after the Big Bang. At present, the cosmological constant problem is still,
probably, the most ununderstood issue of the physics.
However, in recent times, it has again come into vogue, and again as a consequence of a number of
observational evidences. Despite their apparent simplicity, the results of observations of more than 40
distant type lA supernovae seem to converge on the astonishing evidence that the present expansion
of the universe is accelerating; this fact, together with many other experimental evidences of a lowdensity
universe, and with the most recent CMB data indicating that the universe is very near to
the flatness, has opened again the difficult question of what is the unobserved energy component that
would balance a low-density universe with a flat one.
At the same time, a new branch of cosmology has been opened, involving scalar fields as candidates for
such "missing" energy. Motivated by the difficulties of cosmological constant models and by the most
recent observational case of an accelerating universe, many alternative scenarios have been proposed:
amongst them, the Quintessence scenarios, which are the subject of this thesis.
In these models, the "missing energy" should reside in a dynamical scalar field rather than in a pure
vacuum state; the dynamics of the field plays an important role, since the field energy density can
adjust in a way that it comes to dominate at late times.
The most important and distinctive feature of a scalar field vs. a cosmological constant, is that a field
will develop fluctuations, that interact gravitationally with those of matter. To obtain the correct
predictions of their impact on the Cosmic Microwave Background radiation and on the evolution of
perturbations, the formalism of linear perturbation theory must be widely used.
In this thesis we will focus on some basic issues connected with the attempt to build predictions on
the cosmological impact of such scalar fields, following the results discussed in refs. [173, 174, 10, 15].
The first chapter is introductory and aims at giving an overview of the current observational evidences
from which the case of a positive non-zero vacuum energy arises, motivating the consideration of the
cosmological constant problem. In Chapter 2, the gauge-invariant formalism of linear cosmological
perturbation theory is described, with particular attention to quantities such as the gravitational
potentials, entropy and curvature perturbations, which are used in the following of the thesis.
In Chapter 3 we recall some basic ideas on the mechanisms that generated the observed Cosmic
Microwave Background of radiation, whose small but detectable anisotropies contain a large amount
of information on the history of the Universe. In particular, CMB anisotropies turned out to be a
very rich ground of investigation for discriminating between Quintessence and cosmological constant
scenarios.
These chapters set the framework for the results that will be presented in the following chapters,
containing the original work of the thesis.
In Chapter 4, focusing on scalar-type perturbations, we settled the analytical initial conditions that
must be imposed on the components of cosmic fluid involving a minimally-coupled scalar field, in order
to produce purely adiabatic or purely isocurvature initial conditions on super-horizon scales. Thus,
an interesting comparison with the "standard" pure CDM flat model is performed. The distinctive
imprints of Quintessence on large scale structure and on CMB anisotropies, both of polarization and
temperature, are extensively analyzed.
Chapter 5 extends the concept of Quintessence to a larger class of scalar fields, having an explicit
coupling with the Ricci scalar in the Lagrangian. These more general models, here named "Extended
Quintessence", are shown to enrich the phenomenology with respect to the simple minimally-coupled Quintessence. In fact, the predictions for the CMB anisotropies show new distinctive features, directly
related to the presence of a non-zero coupling of the field with the gravitational sector of the Lagrangian
and, ultimately, with the time-variation of the gravitational constant.
A problem of "fine tuning" is however inherent both to cosmological constant and quintessence models:
in order to have today an amount of vacuum energy comparable with that of matter, the vacuum energy
density should have been initially vanishingly small.
A way out to such fine tuning problem is possible in Quintessence scenarios, where one can select a
subclass of models which admit "tracking solutions". This means that the present value of scalar field
energy density, once fixed, can be determined starting from a very wide set of initial conditions, even
though the tracking solutions are not perfect attractors and do not solve the problem of why the field
energy density should have this value just today.
Chapter 6 considers tracking behaviors in Extended Quintessence scenarios and presents a description
of the rich phenomenology that arises from the corresponding dynamics; in particular, we show that
the coupling with the Ricci scalar can act initially as an effective potential pushing the field in the
tracking trajectory ( "R-boost"). The dependence of the phenomenology on the sign of the coupling
constant is also described.
Finally, Chapter 7 presents the conclusions and faces the future observational perspectives, on the
light of the most recent data from MAXIMA and BOOMERANG-98 balloon experiments and of the
future satellite missions MAP and Planck.
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Bartrum, Sam John Richard. "Scalar fields : fluctuating and dissipating in the early Universe." Thesis, University of Edinburgh, 2015. http://hdl.handle.net/1842/11760.
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It is likely that the early Universe was pervaded by a whole host of scalar fields which are ubiquitous in particle physics models and are employed everywhere from driving periods of accelerated expansion to the spontaneous breaking of gauge symmetries. Just as these scalar fields are important from a particle physics point of view, they can also have serious implications for the evolution of the Universe. In particular in extreme cases their dynamical evolution can lead to the failure of the synthesis of light elements or to exceed the dark matter bound in contrast to observation. These scalar fields are not however isolated systems and interact with the degrees of freedom which comprise their environment. As such two interrelated effects may arise; fluctuations and dissipation. These effects, which are enhanced at finite temperature, give rise to energy transfer between the scalar field and its environment and as such should be taken into account for a complete description of their dynamical evolution. In this thesis we will look at these effects within the inflationary era in a scenario termed warm inflation where amongst other effects, thermal fluctuations can now act as a source of primordial density perturbations. In particular we will show how a model of warm inflation based on a simple quartic potential can be brought back into agreement with Planck data through renormalizable interactions, whilst it is strongly disfavoured in the absence of such effects. Moving beyond inflation, we will consider the effect of fluctuation-dissipation dynamics on other cosmological scalar fields, deriving dissipation coefficients within common particle physics models. We also investigate how dissipation can affect cosmological phase transitions, potentially leading to late time periods of accelerated expansion, as well as presenting a novel model of dissipative leptogenesis.
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SANTOS, Maria Aparecida dos. "Modelos de campos escalares no estudo da cosmologia inflacionária." Universidade Federal de Campina Grande, 2014. http://dspace.sti.ufcg.edu.br:8080/jspui/handle/riufcg/2021.
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Capes
Considerando as diferentes abordagens possíveis referentes ao Universo, este trabalho está voltado para o estudo da Cosmologia Padrão e Inflacionária utilizando campos escalares para descrever a fase de expansão acelerada do Universo. Assim, através da Teoria da Gravitação proposta pela Relatividade Geral é possível determinar as equações de Friedmann e utilizando a Teoria de Campos em Cosmologia podemos obter uma equação de movimento que descreve a evolução temporal de um campo escalar chamado ínflaton, responsável pela inflação. Nesse sentido, propomos como alternativa a utilização de alguns modelos de potenciais já existentes, dentre os quais: V ( ) =12m2 2 (quadr atico), V ( ) = C cos2 (tipo cosseno), V ( ) = C sin2 (tipo seno), V ( ) = (t) 4 e o potencial constante V = V0. Buscando dessa forma descrever a evolução temporal do fator de escala a(t) e o comportamento do parâmetro de desaceleração q(t) com o objetivo de analisar a fase inflacionária, identi cando regiões de aceleração e desaceleração do Universo nos cenários dos espaços plano e curvo.
Taking into consideration the set of di erent approaches to the Universe existent today this work focuses on standard cosmology and in ationary expansion of the said using scalar elds to describe the expansion acceleration rate. Therefore, through a gravitation theory proposed by General Relativity is possible to set Friedmann`s equations and using Field Theory applied to Cosmology to obtain an equation of motion which describes the temporal evolution of a scalar eld called in action, which is responsible for the in ationary process. In this sense, we propose as alternative some models whose potentials are already established, among them: V ( ) = 12m2 2 (quadratic), V ( ) = C cos2 (cosinelike) , V ( ) = C sin2 (sinelike), V ( ) = (t) 4 and the constant potential V = V0 . We seek with this to describe the temporal evolution of the scale factor a(t) and how the decelerating parameter behaves and then analyze the in ationary faze, indentifying periods when the Universe was accelerating or decelerating given curve or plane space scenarios.
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Radermacher, Katharina Maria. "Strong Cosmic Censorship and Cosmic No-Hair in spacetimes with symmetries." Doctoral thesis, KTH, Matematik (Avd.), 2017. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-220400.
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This thesis consists of three articles investigating the asymptotic behaviour of cosmological spacetimes with symmetries arising in Mathematical General Relativity. In Paper A and B, we consider spacetimes with Bianchi symmetry and where the matter model is that of a perfect fluid. We investigate the behaviour of such spacetimes close to the initial singularity ('Big Bang'). In Paper A, we prove that the Strong Cosmic Censorship conjecture holds in non-exceptional Bianchi class B spacetimes. Using expansion-normalised variables, we further show detailed asymptotic estimates. In Paper B, we prove similar estimates in the case of stiff fluids. In Paper C, we consider T2-symmetric spacetimes satisfying the Einstein equations for a non-linear scalar field. To given initial data, we show global existence and uniqueness of solutions to the corresponding differential equations for all future times. In the special case of a constant potential, a setting which is equivalent to a linear scalar field on a background with a positive cosmological constant, we investigate in detail the asymptotic behaviour towards the future. We prove that the Cosmic No-Hair conjecture holds for solutions satisfying an additional a priori estimate, an estimate which we show to hold in T3-Gowdy symmetry.Denna avhandling består av tre artiklar som undersöker det asymptotiska beteendet hos kosmologiska rumstider med symmetrier som uppstår i Matematisk Allmän Relativitetsteori. I Artikel A och B studerar vi rumstider med Bianchi symmetri och där materiemodellen är en ideal fluid. Vi undersöker beteendet av sådana rumstider nära ursprungssingulariteten ('Big Bang'). I Artikel A bevisar vi att den Starka Kosmiska Censur-förmodan håller för icke-exceptionella Bianchi klass B-rumstider. Med hjälp av expansions-normaliserade variabler visar vi detaljerade asymptotiska uppskattningar. I Artikel B visar vi liknande uppskattningar för stela fluider. I Artikel C betraktar vi T2-symmetriska rumstider som uppfyller Einsteins ekvationer för ett icke-linjärt skalärfält. För givna begynnelsedata visar vi global existens och entydighet av lösningar till motsvarande differentialekvationer för all framtid. I det speciella fallet med en konstant potential, en situation som motsvarar ett linjärt skalärfält på en bakgrund med en positiv kosmologisk konstant, undersöker vi i detalj det asymptotiska beteendet mot framtiden. Vi visar att den Kosmiska Inget-Hår-förmodan håller för lösningar som uppfyller en ytterligare a priori uppskattning, en uppskattning som vi visar gäller i T3-Gowdy-symmetri.
QC 20171220
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Ma, Yin-Zhe. "Cosmology with CMB and large scale structure." Thesis, University of Cambridge, 2011. https://www.repository.cam.ac.uk/handle/1810/239394.
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Cosmology has become a precision science due to a wealth of new precise data from various astronomical observations. It is therefore important, from a methodological point of view, to develop new statistical and numerical tools to study the Cosmic Microwave Background (CMB) radiation and Large Scale Structure (LSS), in order to test different models of the Universe. This is the main aim of this thesis. The standard inflationary -dominated Cold Dark Matter ( CDM) model is based on the premise that the Universe is statistically isotropic and homogeneous. This premise needs to be rigorously tested observationally. We study the angular correlation function C(θ) of the CMB sky using the WMAP 5-year data, and find that the low-multipoles can be reconstructed from the data outside the sky cut. We apply a Bayesian analysis and find that S1/2 statistic (S1/2 = R [C(θ)]2d cos θ, used by various investigators as a measure of correlations at large angular scales) cannot exclude the predictions of the CDM model. We clarify some issues concerning estimation of correlations on large angular scales and their interpretation. To test for deviation from statistical isotropy, we develop a quadratic maximum likelihood estimator which we apply to simulated Planck maps. We show that the temperature maps from Planck mission should be able to constrain the amplitude of any spherical multipole of a scaleinvariant quadrupole asymmetry at the 1% level (2σ). In addition, polarization maps are also precise enough to provide complimentary constraints. We also develop a method to search for the direction of asymmetry, if any, in Planck maps. B-mode polarisation of the CMB provides another important test of models of the early Universe. Different classes of models, such as single-field inflation, loop quantum cosmology and cosmic strings give speculative but testable predictions. We find that the current ground-based experiments such as BICEP, already provided fairly tight constraints on these models. We investigate how these constraints might be improved with future observations (e.g. Planck, Spider). In addition to the CMB related research, this thesis investigates how peculiar velocity fields can be used to constrain theoretical models of LSS. It has been argued that there are large bulk flows on scales of & 50 Mpc/h. If true, these results are in tension with the predictions of the CDM model. We investigate a possible explanation for this result: the unsubtracted intrinsic dipole on the CMB sky may source this apparent flow, leading to the illusion of the tilted Universe. Under the assumption of a superhorizon isocurvature fluctuation, the constraints on the tilted velocity require that inflation lasts at least 6 e-folds longer (at the 95% confidence interval) than that required to solve the horizon problem. Finally, we investigate Cosmic Mach Number (CMN), which quantifies the ratio between the mean velocity and the velocity dispersion of galaxies. We find that CMN is highly sensitive to the growth of structure on scales (10, 150) Mpc/h, and can therefore be used to test modified gravity models and neutrino masses. With future CMN data, it should be possible to constrain the growth factor of linear perturbation, as well as the sum of the neutrino mass to high accuracy.
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Valente, Ema Filipa dos Santos. "A simple model of exotic compact objects : interaction with a scalar field." Master's thesis, Universidade de Aveiro, 2017. http://hdl.handle.net/10773/23655.
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Mestrado em FísicaModelos de objetos compactos exóticos (OCEs) foram propostos nas últimas décadas como alternativas aos buracos negros. Esses modelos visam reproduzir a fenomenologia que caracteriza os (candidatos a) buracos negros observados. No entanto, para superar os problemas associados ao horizonte de eventos (e à consequente singularidade de curvatura, de acordo com o teorema de Penrose), estes OCEs não possuem horizonte de eventos. Nesta dissertação, exploramos um modelo simples de um OCE, descrito pela métrica de Kerr-Newman no exterior de uma superfície com condições de fronteira reflectivas, localizada fora do horizonte de eventos de Kerr-Newman. Nesta geometria, estudamos OCEs que podem estar em equilíbrio com con figurações estáticas de um campo escalar. Consideramos um campo escalar sem massa, tanto no caso eletricamente não carregado como no caso carregado, e obtemos, através de métodos analíticos, um conjunto discreto de raios críticos da superfície do OCE que podem suportar con figurações estáticas não triviais do campo escalar. Dentro deste conjunto discreto, o OCE com maior raio crítico separa os OCEs estáveis e instáveis relativamente à instabilidade superradiante, induzida por um campo escalar. O conjunto discreto de raios críticos da superfície do OCE foi construído para os três regimes diferentes da métrica de Kerr-Newman: regime sub-extremo, regime extremo e regime super-extremo. Estes espectros de ressonância dependem dos parâmetros físicos {a, Q, q, l,m }
Models of exotic compact objects (ECOs) have been proposed in the past decades as alternatives to black holes. These models aim at reproducing the phenomenology that characterises the observed black hole (candidates). However, to overcome the problems associated to the event horizon (and the consequent curvature singularity, following from Penrose's singularity theorem), these ECOs do not possess an event horizon. In this thesis, we explore a simple ECO model, described by the Kerr-Newman metric in the exterior of a surface wherein re ective boundary conditions are imposed, placed outside the event horizon of the Kerr-Newman geometry. We then study, on this geometry, ECOs that may be in equilibrium with static scalar eld con- gurations. We consider both electrically charged and uncharged massless scalar elds, and, using analytical methods, we obtain a discrete set of critical ECO surface radii that can support static scalar eld con gurations. Within this discrete set, the ECO with the largest critical surface radius separates stable and unstable Kerr-Newman-type ECOs against the superradiant instability induced by a scalar eld. The discrete set of ECO critical surface radii was constructed for three di erent regimes of the Kerr-Newman metric: sub-extremal regime, extremal regime and super-extremal regime. These resonance spectra are dependent on the physical parameters {a,Q,q,l,m}
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Feix, Martin. "Extragalactic and cosmological tests of gravity theories with additional scalar or vector fields." Thesis, University of St Andrews, 2011. http://hdl.handle.net/10023/1901.
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Despite the many successes of the current standard model of cosmology on the largest physical scales, it relies on two phenomenologically motivated constituents, cold dark matter and dark energy, which account for approximately 95% of the energy-matter content of the universe. From a more fundamental point of view, however, the introduction of a dark energy (DE) component is theoretically challenging and extremely fine-tuned, despite the many proposals for its dynamics. On the other hand, the concept of cold dark matter (CDM) also suffers from several issues such as the lack of direct experimental detection, the question of its cosmological abundance and problems related to the formation of structure on small scales. A perhaps more natural solution might be that the gravitational interaction genuinely differs from that of general relativity, which expresses itself as either one or even both of the above dark components. Here we consider different possibilities on how to constrain hypothetical modifications to the gravitational sector, focusing on the subset of tensor-vector-scalar (TeVeS) theory as an alternative to CDM on galactic scales and a particular class of chameleon models which aim at explaining the coincidences of DE. Developing an analytic model for nonspherical lenses, we begin our analysis with testing TeVeS against observations of multiple-image systems. We then approach the role of low-density objects such as cosmic filaments in this framework and discuss potentially observable signatures. Along these lines, we also consider the possibility of massive neutrinos in TeVeS theory and outline a general approach for constraining this hypothesis with the help of cluster lenses. This approach is then demonstrated using the cluster lens A2390 with its remarkable straight arc. Presenting a general framework to explore the nonlinear clustering of density perturbations in coupled scalar field models, we then consider a particular chameleon model and highlight the possibility of measurable effects on intermediate scales, i.e. those relevant for galaxy clusters. Finally, we discuss the prospects of applying similar methods in the context of TeVeS and present an ansatz which allows to cast the linear perturbation equations into a more convenient form.
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Rubira, Henrique. "Melhorias na predição da estrutura de larga escala do universo por meio de teorias efetivas de campo." Universidade de São Paulo, 2018. http://www.teses.usp.br/teses/disponiveis/43/43134/tde-08102018-145202/.
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Com os próximos grandes projetos the observação do Universo, a cosmologia entrará em uma era de alta precisão de medidas. Novos dados trarão um novo entendimento da evolução do Universo, seus principais componentes e do comportamento da gravi- dade. Sendo assim, é fundamental também ter uma boa predição teórica para a formação de estrutura de larga escala em regime não-linear. A melhor maneira de resolver as equações hidrodinâmicas que descrevem o nosso universo é por meio de simulações cosmológicas na rede. Entretando, estas contém desafios, como a correta inclusão de física bariônica e a diminuição do alto tempo computacional. Uma outra abordagem muito usada é o cálculo das funções de cor- relação por meio de métodos perturbativos (em inglês, Standard Perturbation Theory, ou SPT). Entretanto, esta contém problemas variados: pode não convergir para algu- mas cosmologias e, caso convirja, não há certeza de convergência para o resultado correto. Além disso, há uma escala privilegiada nos limites integrais que envolvem o método perturbativo. Nós calculamos o resultado por esse método até terceira ordem e mostramos que o termo de terceira ordem é ainda maior que o de 2-loops e 3-loops. Isso evidencia alguns problemas descritos com o método perturbativo. O método de Teorias Efetivas de Campo aplicado ao estudo de LSS busca corrigir os problemas da SPT e, desta forma, complementar os resultados de simulações na rede. Em outras áreas da física, como a Cromodinâmica Quântica de baixas energias, EFTs também são usadas como um complemento a essas simulações na rede. EFTs melhoram a predição do espectro de potência da matéria por meio da inclusão dos chamados contra-termos, que precisam ser fitados em simulações. Estes contratermos, que são parâmetros livres, contém importante informação sobre como a física em pequenas escalas afeta a física nas escalas de interesse. Explicaremos os resultados para a predição em 3-loops de EFT, trabalho inédito. É possível usar as EFTs também no problema de conectar a campo de matéria com outros traçadores, como os halos e as galáxias, chamado de bias. Com as EFTs podemos construir uma base completa de operadores para parametrizar o bias. Será explicado como utilizar esses operadores para melhorar a predição do bias em escalas não-lineares. Serão calculados esses termos de EFT em simulações. Também será mostrado como renormalizar o bias em coordenadas de Lagrange. Por fim, será explicada outra importante aplicação das EFTs em cosmologia, mais especificamente em teorias de inflação. EFTs parametrizam desvios nas teorias de um campo único no chamado regime de slow-roll.With future cosmological surveys, cosmology will enter in the precision era. New data will improve the constraints on the standard cosmological model enhancing our knowledge about the universe history, its components and the behavior of gravity. In this context, it is vital to come up with precise theoretical predictions for the formation of large-scale structure beyond the linear regime. The best way of solving the fluid equations that describe the large-scale universe is through lattice simulations, which faces difficulties in the inclusion of accurate baryonic physics and is very computationally costly. Another approach is the theoreti- cal calculation of the correlation statistics through the perturbative approach, called Standard Perturbation Theory (SPT). However, SPT has several problems: for some cosmologies, it may not converge and even when it converges, we cannot be sure it converges to the right result. Also, it contains a special scale that is the loop momenta upper-bound in the integral. In this work, we show results for the 3-loop calculation. The term of third order is larger than the terms of 2-loops and 3-loops, making explicit SPT problems. In this work, we describe the recent usage of Effective Field Theories (EFTs) on Large Scale Structure problems to correct SPT issues and complement cosmological simulations. EFTs are used in other areas of physics, such as low energy QCD, serving as a complement to lattice calculations. EFT improves the predictions for the matter power spectrum and bispectrum by adding counterterms that need to be fitted. The free parameters, instead of being a problem, bring relevant information about how the small-scale physics affects the scales for which we are trying to make statistical predictions. We show the calculation of the 3-loop EFT counterterms. EFTs are also used to explain main points connecting the matter density field with tracers like galaxies and halos. EFTs highlighted how to construct a complete basis of operators that parametrize the bias. We explain how we can use EFT to improve the bias prediction to non-linear scales. We compute the non-linear halo-bias by fitting the bias parameters in simulations. We also show the EFT renormalization in Lagrangian coordinates. Finally, we explain another critical EFT application to cosmology: in primordial physics. It can be used to parametrize deviations to the slow-roll theory within the inflationary paradigm.
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Santos, José Jamilton Rodrigues dos. "Tópicos em cosmologia com campos escalares." Universidade Federal da Paraíba, 2011. http://tede.biblioteca.ufpb.br:8080/handle/tede/5695.
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Cosmological models involving scalar fields allow the description of a phase of accelerated cosmic expansion and thus appear as a promising alternative for the study of the cosmic inflation and dark energy. We are interested here in analyzing these cosmological models. In particular, we will explore cosmological solutions based on the first order formalism. The inclusion of this method favors the search for analytic solutions with scalar fields in cosmology, and this is particularly important when we consider the component of nonrelativistic matter (dust) in the presence of dark energy, in order to construct a cosmological model capable of explaining, in good agreement with observational data, the current phase of cosmic acceleration. Considering a regime of Lorentz violation, the use of this method allowed us to verify that new considerations must be implemented so that the inflationary regime can now solve the problem of initial conditions. Another question of interest, which can be addressed with the aid of the first order formalism, takes into account the possibility of the dark energy equation of state parameter to be a constant other than −1 and in this case we get that a lot of fine-tuning is needed, which should be interpreted as strong evidence in favor of a dynamic model of dark energy. We also introduce the so-called deformation method on the slow-roll inflationary models, and we explore this framework in applications of current interest to this branch of research.
Modelos cosmológicos envolvendo campos escalares permitem a descrição de uma fase de expansão cósmica acelerada e, portanto, se apresentam como uma alternativa promissora no estudo da inflação cósmica e da energia escura. Estamos aqui interessados em analisar esses modelos cosmológicos; em especial, vamos explorar soluções cosmológicas baseadas no formalismo de primeira ordem. A inclusão desse método favorece a busca por soluções analíticas na cosmologia com campos escalares e isso é particularmente interessante no caso em que consideramos o componente de matéria não relativística (poeira) na presença da energia escura, afim de construir um modelo cosmológico capaz de explicar, em bom acordo com os dados observacionais, a atual fase de aceleração cósmica. Considerando um regime de violação de Lorentz, a utilização desse método nos permitiu verificar que novas considerações devem ser implementadas, para que o regime inflacionário possa resolver o problema das condições iniciais. Outra questão de interesse, que pode ser analisada com auxílio do formalismo de primeira ordem, leva em conta a possibilidade da equação de estado da energia escura ser um constante qualquer diferente de −1 e, nesse caso, obtemos que uma grande quantidade de ajuste fino é necessária, o que deve ser interpretado como uma forte evidência em favor de um modelo dinâmico de energia escura. Também introduzimos o chamado método de deformação a modelos inflacionários sob o regime de rolagem lenta e exploramos essa ferramenta em aplicações de corrente interesse na literatura.
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MANCARELLA, MICHELE. "An effective description of dark energy: from theory to phenomenology (titolo originale: Consistency tests of the Universe and cosmic relics)." Doctoral thesis, Université Paris-Saclay, 2017. https://hdl.handle.net/10281/400827.
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Martineau, Killian. "Quelques aspects de cosmologie et de physique des trous noirs en gravitation quantique à boucles Detailed investigation of the duration of inflation in loop quantum cosmology for a Bianchi I universe with different inflaton potentials and initial conditions Some clarifications on the duration of inflation in loop quantum cosmology A first step towards the inflationary trans-Planckian problem treatment in loop quantum cosmology Scalar spectra of primordial perturbations in loop quantum cosmology Phenomenology of quantum reduced loop gravity in the isotropic cosmological sector Primordial Power Spectra from an Emergent Universe: Basic Results and Clarifications Fast radio bursts and the stochastic lifetime of black holes in quantum gravity Quantum fields in the background spacetime of a polymeric loop black hole Quasinormal modes of black holes in a toy-model for cumulative quantum gravity Seeing through the cosmological bounce: Footprints of the contracting phase and luminosity distance in bouncing models Dark matter as Planck relics without too exotic hypotheses A Status Report on the Phenomenology of Black Holes in Loop Quantum Gravity: Evaporation, Tunneling to White Holes, Dark Matter and Gravitational Waves." Thesis, Université Grenoble Alpes (ComUE), 2019. http://www.theses.fr/2019GREAY044.
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Cantonnée à la physique mathématique depuis des décennies, la gravitation quantique entre désormais dans le giron de la science expérimentale. Suivant cette mouvance nous considérons dans cette thèse trois cadres d’application de la gravitation quantique à boucles (LQG) : le système Univers, les trous noirs et les astroparticules. Le troisième n’est qu’esquissé tandis que les deux premiers sont présentés plus en détails.Le secteur cosmologique étant l’un des domaines les plus prometteurs pour tester et contraindre des théories de gravité quantique, le développement de différents modèles tentant d’appliquer les idées de la LQG à l’Univers primordial ne s’est pas fait attendre. Les travaux que nous présentons portent sur la phénoménologie associée à ces modèles; tant dans le secteur homogène (où nous nous focalisons notamment sur la durée de la phase d’inflation), que dans le secteur inhomogène (nous étudions ce coup-ci le devenir des spectres de puissance primordiaux). Ces études combinées nous permettent alors de préciser dans quelle mesure des effets de gravité quantique (à boucles) peuvent être observés dans les anisotropies du fond diffus cosmologique.D’autre part les trous noirs, non contents de faire partie des objets les plus étranges et les plus fascinants de l’Univers, constituent également des sondes privilégiées pour tester des théories de gravitation. Nous développons la phénoménologie associée à différents traitements des trous noirs en gravitation quantique à boucles. Celle-ci intervient sur une grande variété de fronts : de l’évaporation de Hawking aux ondes gravitationnelles, en passant par la matière noire. C’est sans nul doute un domaine riche et vaste.Finalement, l’existence d’une échelle de longueur minimale, prédite par la majorité des théories de gravité quantique, suggère une généralisation du principe d’incertitude de Heisenberg. Partant de ce constat nous présentons également dans ce manuscrit une méthodologie permettant de calculer une nouvelle relation de dispersion de la lumière à partir du principe d’incertitude généralisé le plus couramment répanduAfter decades of being confined to mathematical physics, quantum gravity now enters the field of experimental science. Following this trend, we consider throughout this thesis three implementation frameworks of Loop Quantum Gravity (LQG): the Universe as a system, black holes and astroparticles. The last one is only outlined while the first two are presented in more detail.Since the cosmological sector is one of the most promising areas for testing and constraining quantum gravity theories, it was not long before the development of different models attempting to apply the ideas of the LQG to the primordial Universe. The work we present deals with the phenomenology associated with these models; both in the homogeneous sector (where we focus particularly on the duration of the inflation phase), as in the inhomogeneous sector (where this time, we study the fate of the primordial power spectra). These combined studies then allow us to specify to what extent effects of (loop) quantum gravity can be observed in the anisotropies of the cosmic microwave background.On the other hand black holes, not content to be among the strangest and most fascinating objects of the Universe, are also prominent probes to test the theories of gravitation. We develop the phenomenology associated with different treatments of black holes in the loop quantum gravity framework, which intervenes on multiple levels: from the evaporation of Hawking to gravitational waves, including dark matter. This is undoubtedly a rich and vast area.Finally, the existence of a minimal length scale, predicted by the majority of quantum gravity theories, suggests a generalization of the Heisenberg uncertainty principle. On the basis of this observation, we also present in this manuscript a methodology to derive a new relation dispersion of light from the most widely used generalized uncertainty principle
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Di, Nella Hélène. "Structure et cinématique de l'univers local." Université Joseph Fourier (Grenoble), 1995. http://www.theses.fr/1995GRE10220.
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Partie a, nous examinons les diverses procedures permettant de s'assurer de la completude d'un echantillon. Les echantillons extraits de la base de donnees leda nous permettent de re-identifier le plan hypergalactique qui regroupe 50% des galaxies de l'univers local dont les principaux superamas. Deux redshift surveys avec le spectro multifibre flair sont presentes. Nous trouvons une extension sud de la chaine de persee-poissons atteignant maintenant 150mpc et posant la question de la signification physique de la longueur de correlation de 5mpc des galaxies. Nous montrons que la longueur de correlation mesuree par la fonction de correlation a 2 points croit avec le rayon de l'echantillon. Nous expliquons ces resultats avec de nouveaux outils statistiques nous permettant de deriver une dimension fractale de 2 pour la distribution observee des galaxies. Partie b, nous proposons deux nouvelles methodes de calibration de l'echelle des distances des elliptiques. Les observations permettant de deriver le champ des vitesses propres sont presentees. Nous examinons ce champ de vitesses dans le plan hypergalactique. Partie c, nous presentons des simulations de la formation des grandes structures avec des conditions initiales homogenes ou fractales. Nous montrons qu'une distribution primordiale homogene pourrait evoluer vers une distribution fractale de dimension d=2
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Wallisch, Benjamin. "Cosmological probes of light relics." Thesis, University of Cambridge, 2018. https://www.repository.cam.ac.uk/handle/1810/283003.
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One of the primary targets of current and especially future cosmological observations are light thermal relics of the hot big bang. Within the Standard Model of particle physics, an important thermal relic are cosmic neutrinos, while many interesting extensions of the Standard Model predict new light particles which are even more weakly coupled to ordinary matter and therefore hard to detect in terrestrial experiments. On the other hand, these elusive particles may be produced efficiently in the early universe and their gravitational influence could be detectable in cosmological observables. In this thesis, we describe how measurements of the cosmic microwave background (CMB) and the large-scale structure (LSS) of the universe can shed new light on the properties of neutrinos and on the possible existence of other light relics. These cosmological observations are remarkably sensitive to the amount of radiation in the early universe, partly because free-streaming species such as neutrinos imprint a small phase shift in the baryon acoustic oscillations (BAO) which we study in detail in the CMB and LSS power spectra. Building on this analytic understanding, we provide further evidence for the cosmic neutrino background by independently confirming its free-streaming nature in different, currently available datasets. In particular, we propose and establish a new analysis of the BAO spectrum beyond its use as a standard ruler, resulting in the first measurement of this imprint of neutrinos in the clustering of galaxies. Future cosmological surveys, such as the next generation of CMB experiments (CMB-S4), have the potential to measure the energy density of relativistic species at the sub-percent level and will therefore be capable of probing physics beyond the Standard Model. We demonstrate how this improvement in sensitivity can indeed be achieved and present an observational target which would allow the detection of any extra light particle that has ever been in thermal equilibrium. Interestingly, even the absence of a detection would result in new insights by providing constraints on the couplings to the Standard Model. As an example, we show that existing bounds on additional scalar particles, such as axions, may be surpassed by orders of magnitude.
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Hrycyna, Orest. "Regular and chaotic dynamics in scalar field cosmology." Praca doktorska, 2011. https://ruj.uj.edu.pl/xmlui/handle/item/53490.
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This dissertation is devoted to investigations of dynamics of Friedmann-Robertson-Walker cosmological models with a non-minimally coupled scalar field and a barotropic matter content. Using dynamical systems methods two types of evolution: the complex behaviour and the regular expansion of the universe were studied in details. Different manifestations of the coexistence of chaotic and regular behaviour in the conformally coupled scalar field cosmology were demonstrated. Also, the fragility of cosmological evolution with respect to the value of non-minimal coupling parameter was presented. The full characteristic of the dynamical evolution of a non-minimally coupled scalar field cosmological models was performed. It was shown that the inclusion of both types of matter opens an enormous dynamical complexity of possible evolutional paths of the universe, which are not present in standard cosmological models or in the models filled with a scalar field only. The generic evolutional path, which does not depend on the form of the scalar field potential function, was found. It leads to new and natural possibility of unified description of the cosmological evolution. Within one framework of a non-minimally coupled scalar field cosmology all the major epochs in the history of the universe emerge as critical points of the corresponding dynamical system (a finite scale factor singularity, an inflation (slow-roll and fast-roll), a radiation era, a barotropic matter domination era and finally the present accelerated expansion epoch).
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Li, Bohua. "Cosmology with Bose-Einstein-condensed scalar field dark matter." 2013. http://hdl.handle.net/2152/21308.
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Despite the great successes of the Cold Dark Matter (CDM) model in
explaining a wide range of observations of the global evolution and the formation of galaxies and large-scale structure in the universe, the origin and microscopic nature of this dark matter is still unknown. The most common form of CDM considered to-date is that of Weakly Interacting Massive Particles (WIMPs), but some of the cosmological predictions for this kind of CDM are in apparent conflict with observations (e.g. cuspy-cored halos and an overabundance of satellite dwarf galaxies). For these reasons, it is important to consider the consequences of different forms
of CDM. We focus here on the hypothesis that the dark matter is comprised, instead, of ultralight bosons that form a Bose-Einstein Condensate (BEC),
described by a complex scalar field.
We start from the Klein-Gordon and Einstein field equations to describe
the evolution of the Friedmann-Robertson-Walker (FRW) universe in the presence of this kind of dark matter. We find that, in addition to the
phases of radiation-domination (RD), matter-domination (MD) and
Lambda-domination (LD) familiar from the standard CDM model, there is an
earlier phase of scalar-field-domination (SFD) which is special to this model. In addition, while WIMP CDM is non-relativistic at all times after
it decouples, the equation of state of BEC-SFDM is found to be relativistic at early times, evolving from incompressible ($\bar{p} = \bar{\rho}$)
to radiation-like ($\bar{p} = \bar{\rho}/3$), before it becomes non-relativistic and CDM-like at late times. The timing of the transitions between these
phases and regimes is shown to yield fundamental constraints on the particle
mass and self-interaction coupling strength. We also discuss progress
on the description of structure formation in this model, which includes
additional constraints on these parameters.text
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Foster, Scott. "Singularity structure of scalar field cosmologies / Scott Foster." Thesis, 1996. http://hdl.handle.net/2440/18744.
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Errata inserted opposite p.177.Bibliography: p. 173-177.
x, 177 p. : ill. ; 30 cm.
The classical dynamical structure of cosomological models in which the matter content of the universe consists of a scalar field with arbitrary non-negative potential is analyzed in full. (abstract)
Thesis (Ph.D.)--University of Adelaide, Dept. of Physics and Mathematical Physics, 1996?
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Ahmed, Aqeel. "Scalar fields within warped extra dimension." Doctoral thesis, 2015. https://depotuw.ceon.pl/handle/item/1277.
Full textAbstract:
In this thesis, we explored three different implications of scalar fields in warped extra-dimension.
First, scalar fields were employed to dynamically generate singular branes in Randall-Sundrum (RS)-like models by appropriate profiles --- the smooth/thick-branes. In the context of thick-branes, we constructed four different setups: (i) a smooth generalization of RS2 where a scalar field dynamically generates a singular brane allowing symmetric or asymmetric warped geometries on either side of the brane; (ii) a double thick-brane scenario which mimics two positive tension branes and allows to address the hierarchy problem; (iii) a Z_2 symmetric triple thick-brane; and (iv) a dilatonic thick-brane scenario. The stability of background solution is verified in all the above mentioned setups.
Second, we considered a thick-brane cosmological model with warped fifth-dimension where the dynamics of the 4D universe is driven by the time-dependent 5D background. Different scenarios were found for which the cosmic scale factor a(t,y) and the scalar field \phi(t,y) depend non-trivially on the time t and the 5th-dimension y.
Third, we discussed a symmetric 5D model with three D3-branes (IR--UV--IR) where the Higgs doublet and the other Standard Model (SM) fields are embedded in the bulk. The Z_2 geometric symmetry led to the warped KK-parity for all the bulk fields. Within this setup we
investigated the low-energy effective theory for the bulk SM bosonic sector.
It turned out that the zero-mode scalar sector contains an even scalar which mimics the SM Higgs boson and a second, stable odd scalar particle which is a dark matter candidate. The model that resulted from the Z_2-symmetric background geometry resembles the Inert Two Higgs Doublet Model. Implications for dark matter were discussed within this model.
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Chatwin-Davies, Aidan. "A Covariant Natural Ultraviolet Cutoff in Inflationary Cosmology." Thesis, 2013. http://hdl.handle.net/10012/7759.
Full textAbstract:
In the field of quantum gravity, it is widely expected that some form of a minimum length scale, or ultraviolet cutoff, exists in nature. Recently, a new natural ultraviolet cutoff that is fully covariant was proposed. In the literature, most studies of ultraviolet cutoffs are concerned with Lorentz-violating ultraviolet cutoffs. The difficulty in making a minimum length cutoff covariant is rooted in the fact that any given length scale can be further Lorentz contracted. It was shown that this problem is avoided by the proposed covariant cutoff by allowing field modes with arbitrarily small wavelengths to still exist, albeit with exceedingly small, covariantly-determined bandwidths. In other words, the degrees of freedom of sub-Planckian modes in time are highly suppressed.
The effects of this covariant ultraviolet cutoff on the kinematics of a scalar quantum field are well understood. There is much to learn, however, about the effects on a field’s dynamics. These effects are of great interest, as their presence may have direct observational consequences in cosmology. As such, this covariant ultraviolet cutoff offers the tantalizing prospect of experimental access to physics at the Planck scale.
In cosmology, the energy scales that are probed by measurements of cosmic microwave background (CMB) statistics are the closest that we can get to the Planck scale. In particular, the statistics of the CMB encodes information about the quantum fluctuations of the scalar inflaton field. A measure of the strength of a field’s quantum fluctuations is in turn given by the magnitude of the field’s Feynman propagator. To this end, in this thesis I study how this covariant ultraviolet cutoff modifies the Feynman propagator of a scalar quantum field.
In this work, I first calculate the cutoff Feynman propagator for a scalar field in flat spacetime, and then I address the cutoff Feynman propagator of a scalar field in curved spacetime. My studies culminate with an explicit calculation for the case of a power-law Friedmann-Lemaître-Robertson-Walker (FLRW) spacetime. This last calculation is cosmologically significant, as power-law FLRW spacetime is a prototypical and realistic model for early-universe inflation.
In preparation for studying the covariant cutoff on curved spacetime, I will review the necessary back- ground material as well as the kinematic influence of the covariant cutoff. I will also discuss several side results that I have obtained on scalar quantum field theories in spacetimes which possess a finite start time.