Aspects of finite temperature corrections in string theory

PhDIn this thesis some aspects of temperature corrections in string theory are analyzed: in particular, we study the thermal contributions to the 4 dimensional effective potential arising from string theory compactifications. String theory predicts that the spacetime has more than 4 dimensions; in particular, supersymmetric string theories are consistent only if the spacetime has 10 dimensions, 1 time plus 9 space directions. In order to describe the physics of our Universe with string theory we make 6 spatial directions very small, namely, we curl them into a 6-dimensional space. The resulting 4-dimensional theory depends on a large number of parameters which are massless scalar fields called moduli. The different values that the moduli can take represent both the possible deformations of the 6- dimensional compact space and the values of the coupling constants and masses in the 4-dimensional spacetime. Allowing them to have arbitrary values leads to a lack of predictability of various 4D physical quantities, to a huge vacuum degeneracy and to unobserved long range fifth forces. In the thesis we review some methods established in the literature in order to fix the moduli values and hence to fix a particular geometry and we investigate how the inclusion of temperature corrections alter their values and affect the geometry of the compact space. The analysis seems to suggest that at least in the specific compactification scenarios considered in this thesis, temperature corrections do not alter substantially the zero temperature results. In the final part of this work, we analyze instead an example in which the inclusion of temperature corrections alters dramatically the picture at zero temperature. In particular, we study an unstable system constituted by a pair of Dirichlet (D) and anti-D brane that, although being unstable at zero temperature, it can become stable once finite temperature corrections are switched on