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`String Vacua with Massive boson-fermion Degeneracy & Non-Singular Cosmology'

Abstract: Understanding the phases of string theory in the strong 
curvature and high temperature regime, which is inaccessible to the 
field theory approximation, may provide insights about the physics of 
the very early Universe. Cosmological solutions can be described at the 
perturbative string level, arising as quantum or thermal instabilities 
of an initially flat background. Two major obstacles that typically 
prevent a perturbative treatment of the backreaction are the 
Hagedorn/tachyonic divergences that occur in such strong curvature 
and/or high temperature regions and the initial gravitational 
singularity ("Big Bang"), that always appears in the field theory 
approach. In this talk, I will present recent progress in tackling 
these problems within the framework of perturbative string theory. 
In particular, I will consider a special toy model whose high degree of 
symmetry may help uncover the stringy mechanism that protects the 
cosmological evolution from Hagedorn or gravitational type singularities.
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