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 cosmic microwave background temperature anisotropies, the inflationary scale has to be less than ${10}^{15}\mathrm{GeV}$, a value which is inconsistent with the high inflationary scale deduced from the tensor-to-scalar ratio announced from the BICEP2 collaboration. Some questions may be consequently raised about our understanding of the thermal history of the Universe.