How well do we understand the thermal history of the Universe?
  Implications of the recent BICEP2 data
  Applying Grand Unified Theories in the context of the early Universe, cosmic
strings are an unavoidable partner of the inflaton field. To render such models
compatible with the CMB temperature anisotropies, the inflationary scale has to
be lower than the 2x10^{16} GeV value deduced from the tensor-to-scalar ratio
announced from the BICEP2 collaboration. Some questions are consequently raised
about our understanding of the thermal history of the Universe.
