20 research outputs found
Gravitational waves in a stringlike fluid cosmology
The coupling of a stringlike fluid with ordinary matter and gravity may lead
to a closed Universe with the dynamic of an open one. This can provide an
alternative solution for the age and horizon problems. A study of density
perturbations of the stringlike fluid indicates the existence of instabilities
in the small wavelength limit when it is employed a hydrodynamic approach.
Here, we extend this study to gravitational waves, where the hydrodynamical
approach plays a less important role, and we argue that traces of the existence
of this fluid must be present in the anisotropies of the cosmic background
radiation.Comment: Latex, 8 page
Amoroso Costa and the first brazilian book on general relativity
Em 1922, o fĂsico-matemĂĄtico brasileiro Amoroso Costa publicou um livro de introdução Ă Teoria da Relatividade. Este livro, um dos primeiros textos sobre o assunto no mundo, surpreende ainda hoje pela sua limpidez, precisĂŁo e concisĂŁo. Fazemos uma anĂĄlise do texto de Amoroso Costa, situando-o no contexto cientĂfico mundial e brasileiro.ABSTRACT In 1922, the Brazilian physicist and mathematician Amoroso Costa published an introduction to the Theory of Relativity. This book, one of the first on the subject in the world, is until now an impressive example of precision, clarity and concision in the treatement of this difficult subject. We present an analysis and evaluation of the text of Amoroso Costa in the scientific context of that period
Perturbative analysis of generalized Einstein's theories
The hypothesis that the energy-momentum tensor of ordinary matter is not
conserved separately, leads to a non-adiabatic expansion and, in many cases, to
an Universe older than usual. This may provide a solution for the entropy and
age problems of the Standard Cosmological Model. We consider two different
theories of this type, and we perform a perturbative analysis, leading to
analytical expressions for the evolution of gravitational waves, rotational
modes and density perturbations. One of these theories exhibits satisfactory
properties at this level, while the other one should be discarded.Comment: 14 pages, Latex fil
Density perturbations in Kaluza--Klein theories during a de Sitter phase
In the context of Kaluza-Klein theories, we consider a model in which the
universe is filled with a perfect fluid described by a barotropic equation of
state. An analysis of density perturbations employing the synchronous gauge
shows that there are cases where these perturbations have an exponential growth
during a de Sitter phase evolution in the external space.Comment: LaTex file, 10 pages. To be published in Classical and Quantum
Gravit
Gravitational waves : a 100-year tool applied to the dark energy problem
Recent observations from type Ia Supernovae and from cosmic microwave background (CMB) anisotropies have revealed that most of the matter of the Universe interacts in a repulsive manner, composing the so-called dark energy constituent of the Universe. Determining the properties of dark energy is one of the most important tasks of modern cosmology and this is the main motivation for this work. The analysis of cosmic gravitational waves (GW) represents, besides the CMB temperature and polarization anisotropies, an additional approach in the determination of parameters that may constrain the dark energy models and their consistence. In recent work, a generalized Chaplygin gas model was considered in a flat universe and the corresponding spectrum of gravitational waves was obtained. In the present work we have added a massless gas component to that model and the new spectrum has been compared to the previous one. The Chaplygin gas is also used to simulate a L-CDM model by means of a particular combination of parameters so that the Chaplygin gas and the L-CDM models can be easily distinguished in the theoretical scenarios here established. We find that the models are strongly degenerated in the range of frequencies studied. This degeneracy is in part expected since the models must converge to each other when some particular combinations of parameters are considered