Cosmologists are interested in topological defects as a possible source for the primordial density perturbations which seeded structure formation through gravitational instability. In this thesis, the gravitational properties of various topological defects are studied in the context of low-energy string theory, a likely modification of Einstein gravity at the high energy scales prevalent in the early universe. We consider in turn global monopole, local monopole, global cosmic string and global texture defects, allowing for an arbitrary coupling of defects to the string theory dilaton. For global defects we find the following behaviour. If the dilaton is massless, this modification to general relativity generically destroys the global good behaviour of the monopole and cosmic string, making their spacetimes singular. For the texture non-singular spacetimes exist, but only for certain values of the matter-dilaton coupling, dependent on the gravitational strength of the defect; in addition, this non-singular behaviour exists only in a certain frame. In the case of a massive dilaton, the metric behaviour of these defects is similar to that found in Einstein gravity, though we find they generically induce a long-range dilaton cloud. For the local monopole, which we study only in the presence of a massless dilaton, a rich variety of behaviour is found. For particular parameter values the local monopole spacetime approximates that of an extremal dilaton black hole.

Research on topological defects has been of increasing interest in recent years, since no laboratory-produced crystal is perfect, that is, it will always present a remarkable number of defects in its structure. Topological defects have different names that depend particularly on the symmetry breaking of the system. Thus, in this work we investigate the acoustic scattering by rigid scatter included in media with topological defects such as declination and global monopole. For these media the acoustic scattering by solid scatter and isotropic included in liquid crystals in the nematic phase with disclination as well as global monopole defects were investigated. In these systems, the amplitude and the scattering patterns were computed as well as the cross section. The results revealed that for low frequencies and < 1 the backscattered wave is dominant, however for values of > 1 the spread wave becomes dominant, where is the default parameter. For high frequencies, when the value of approaches to 0.1 we found that the backscattered wave becomes dominant. We conclude that both the disclination and the global monopoles media have a great advantage in the use in devices that make use of acoustic wave scattering because of the large amplitudes that can be achieved using these cylindrical and spherical structures included in defective fluids. / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / As pesquisas em defeitos topológicos têm apresentado um interesse crescente nos últimos anos, visto que nenhum cristal produzido em laboratório é perfeito, ou seja, ele sempre apresentará um notável número de defeitos em sua estrutura. Os defeitos topológicos apresentam diferentes nomes que dependem particularmente da quebra de simetria do sistema. Assim, neste trabalho investigamos o espalhamento acústico por espalhadores rígidos inclusos em meios com defeitos topológicos tais como desclinação e monopolo global. Para estes meios foram investigados o espalhamento acústico por espalhadores sólidos e isotrópicos inclusos em cristais líquidos na fase nemática com desclinação bem como defeitos de monopolo. Nestas sistemas, foram computados a amplitude e os padrões de espalhamento bem como a seção de choque. Os resultados reveleram que para baixas frequências e < 1 a onda retroespalhada é dominante, porém para valores de > 1 a onda espalhada se torna dominante, onde é o parâmetro de defeito. Para altas frequências, quando o valor de se aproxima de 0.1 verificamos que a onda retroespalhada passa a ser dominante. Concluímos que tanto os meios com desclinação quanto os monopolos globais apresentam uma grande vantagem na utilização em dispositivos que fazem uso de espalhamento de ondas acústicas em razão das grandes amplitudes que se pode alcançar utilizando essas estruturas cilíndricas e esféricas inclusas em fluidos com defeitos.