  The recent search for a stochastic background of gravitational waves with
LIGO interferometers has produced a new upper bound on the amplitude of this
background in the 100 Hz region. We investigate the implications of the current
and future LIGO results on pre-Big-Bang models of the early Universe,
determining the exclusion regions in the parameter space of the minimal pre-Big
Bang scenario. Although the current LIGO reach is still weaker than the
indirect bound from Big-Bang nucleosynthesis, future runs by LIGO, in the
coming year, and by Advanced LIGO (~2009) should further constrain the
parameter space, and in some parts surpass the Big-Bang nucleosynthesis bound.
It will be more diffcult to constrain the parameter space in non-minimal
pre-Big-Bang models, which are characterized by multiple cosmological phases in
the yet not well understood stringy phase, and where the higher-order curvature
and/or quantum-loop corrections in the string effective action should be
included.
