Spacetime aspects of non-supersymmetric strings

The consistency of string theory as a theory of gravity does not rely on the presence of spacetime supersymmetry; however, its absence leads to the emergence of tadpole diagrams in the on-shell worldsheet formulation, thus undermining our control over the physics. Two-dimensional conformal invariance, a pillar of string theory, requires tadpole subtraction through the Fischler-Susskind mechanism, whose spacetime consequence is the emergence of scalar tadpole potentials. In this thesis, I explore several implications of the absence of supersymmetry in string-derived scenarios. I focus on the three ten-dimensional tachyon-free string theories in which spacetime supersymmetry is absent or broken, studying the gravitational consequences of tadpole potentials. First, I address vacuum solutions in terms of singular codimension-one vacua, explore their generalizations, and discuss the properties of their singular boundaries. I present an alternative perspective on these vacua based on the formalism of fake supersymmetry, in which the second-order equations of motion are replaced by supersymmetry-like first-order equations. I then turn to the topic of engineering vacua in the non-supersymmetric string models by constructing solutions that are supported by electric and magnetic fluxes and by providing evidence for their instability. Finally, I investigate brane-like solutions in the presence of tadpole potentials that yield both flux vacua with the isometries of supersymmetric branes and gravitational profiles that capture the backreaction of localized sources.