String theories in modified formulations.

Abstract

This thesis deals with string theories in modified formulations. First, we study the open bosonic string with a dust field, this model gives the possibility of construction of a non-critical string. In the same context, we give the string stretching between two parallel Dp-branes where we care about the possibility of non-critical string with no tachyons and no-ghosts. Second, we study bosonic string theories with deformed relativistic momentum energy relations which were developed by Joào Magueijo and Lee Smolin. We show the ability of such theories to describe a string with non deformed constraints in a non-commutative space-time . We study also the paraquantum extension. Third, we study modified fermionic string theories with deformed dispersion relations. We use the square roots of the bosonic string deformed constraints to obtain the whole constraints of these theories, which verify energy dependent closed algebra. We quantize these theories and we find that the characteristics of the spectrum change with respect to the total energy functions. In a subset of these models, the ordinary fermionic string results remain possible, including theories with no ghost, with space-time supersymmetry, and without tachyons (after the GSO projection).