Doctor of Philosophy / Department of Physics / Bharat Ratra / High precision cosmological observations in last decade suggest that about 70% of our universe's energy density is in so called "Dark Energy" (DE). Observations show that DE has negative effective pressure and therefore unlike conventional energy sources accelerates the cosmic expansion instead of decelerating it. DE is highly uniform and has become a dominant component only recently. The simplest candidate for DE is the time-independent cosmological constant. Although successful in fitting available data, the cosmological constant model has a number of theoretical shortcomings and because of that alternative models of DE are considered. In one such scenario a cosmological scalar field that slowly rolls down its potential acts like a time-dependent cosmological constant. I have used different independent cosmological data sets to constrain the time dependence of DE's energy density in the framework of the slowly-rolling cosmological scalar field model. Present data favors a time-independent cosmological constant, but the time-dependent DE can not be ruled out at high confidence level. Ongoing and planned cosmological probes and surveys will provide more and better quality data over the next decade. When the new data sets are available we will be able to either detect the time dependence of DE or constrain it to a very small physically uninteresting value.

Made available in DSpace on 2014-12-17T15:14:50Z (GMT). No. of bitstreams: 1 FranciscoES.pdf: 1953348 bytes, checksum: d700754d09d0b7966901ab6cbebb4085 (MD5) Previous issue date: 2008-11-24 / Coordena??o de Aperfei?oamento de Pessoal de N?vel Superior / Recent astronomical observations (involving supernovae type Ia, cosmic background radiation anisotropy and galaxy clusters probes) have provided strong evidence that the observed universe is described by an accelerating, flat model whose space-time properties can be represented by the FriedmannRobertsonWalker (FRW) metric. However, the nature of the substance or mechanism behind the current cosmic acceleration remains unknown and its determination constitutes a challenging problem for modern cosmology. In the general relativistic description, an accelerat ing regime is usually obtained by assuming the existence of an exotic energy component endowed with negative pressure, called dark energy, which is usually represented by a cosmological constant ? associated to the vacuum energy density. All observational data available so far are in good agreement with the concordance cosmic ?CDM model. Nevertheless, such models are plagued with several problems thereby inspiring many authors to propose alternative candidates in the relativistic context. In this thesis, a new kind of accelerating flat model with no dark energy and fully dominated by cold dark matter (CDM) is proposed. The number of CDM particles is not conserved and the present accelerating stage is a consequence of the negative pressure describing the irreversible process of gravitational particle creation. In order to have a transition from a decelerating to an accelerating regime at low redshifts, the matter creation rate proposed here depends on 2 parameters (y and ??): the first one identifies a constant term of the order of H0 and the second one describes a time variation proportional to he Hubble parameter H(t). In this scenario, H0 does not need to be small in order to solve the age problem and the transition happens even if there is no matter creation during the radiation and part of the matter dominated phase (when the ? term is negligible). Like in flat ACDM scenarios, the dimming of distant type Ia supernovae can be fitted with just one free parameter, and the coincidence problem plaguing the models driven by the cosmological constant. ACDM is absent. The limits endowed with with the existence of the quasar APM 08279+5255, located at z = 3:91 and with an estimated ages between 2 and 3 Gyr are also investigated. In the simplest case (? = 0), the model is compatible with the existence of the quasar for y > 0:56 whether the age of the quasar is 2.0 Gyr. For 3 Gyr the limit derived is y > 0:72. New limits for the formation redshift of the quasar are also established / Observa?c~oes astron?micas recentes (envolvendo supernovas do tipo Ia, anisotropias da radia??o c?smica de fundo e aglomerados de gal?xias) sugerem fortemente que o Universo observado ? descrito por um modelo cosmol?gico plano e acelerado, cujas propriedades do espa?o-tempo podem ser representadas pela m?trica de Friedmann-Robertson-Walker (FRW). Entretanto, a natureza ou mecanismo respons?vel pela acelera??o permanece desconhecida e sua determina??o constitui o problema mais candente da Cosmologia moderna. Em cosmologias relativ?sticas, um regime acelerado ? usualmente obtido supondo a exist?ncia de uma componente ex?tica de energia com press?o negativa, denominada energia escura, cuja representa??o te?rica mais simples ? uma constante cosmol?gica ?, usualmente associada com a densidade de energia do v?cuo. Todas as observa??es conhecidas est?o de acordo com o chamado modelo de concordancia c?smica (ACDM). No entanto, tais modelos apresentam v?rios problemas te?ricos e tem inspirado muitos autores a proporem candidatos alternativos para representar a energia escura no contexto relativ?stico. Nesta tese, propomos um novo tipo de modelo plano, acelerado e sem energia escura, que ? completamente dominado pela mat?ria escura fria (CDM). O n?mero de part?culas de mat?ria escura n?o ? conservado e o atual est?gio acelerado ? uma consequ?ncia da press?o negativa descrevendo o processo irrevers?vel de cria??o gravitacional de mat?ria. Para ocorrer uma transi??o de um regime desacelerado para outro acelerado em baixos redshifts, a taxa de cria??o de mat?ria proposta aqui depende de 2 par?metros (y e ?): o primeiro deles identifca um termo constante da ordem de H0 enquanto o segundo especifica uma varia??o proporcional ao parametro de Hubble H(t). Neste cen?rio, H0 n?o precisa ser pequeno para resolver o problema da idade e a transi??o ocorre mesmo quando n?o existe cria??o de mat?ria durante a era da radia??o e parte da era da mat?ria (quando o termo ? ? desprez?vel). Tal como nos modelos ?CDM planos, os dados de supernovas tipo Ia distantes podem ser ajustados com um ?nico par?mero livre. Al?m disso, neste cen?rio n?o h? o problema da coincid?ncia c?smica existente nos modelos dirigidos pela constante cosmol?gica. Os limites oriundos da exist?ncia do quasar APM 08279+5255, localizado em z = 3:91, e com idade estimada entre 2 e 3 bilh?es de anos s?o tamb?m investigados. No caso mais simples (? = 0), o modelo ? compat?vel com a exist?ncia do quasar para y > 0; 56 se a idade do quasar for 2 bilh?es de anos. Para 3 bilh?es de anos o limite obtido ? y > 0; 72. Novos limites para o redshift de forma??o do quasar s?o tamb?m estabelecidos