Maximal-entropy initial state of the Universe as a microscopic description of inflation:

We propose that the initial state of the Universe was an isotropic state of maximal entropy. Such a state can be described in terms of a state of closed, interacting, fundamental strings in their high-temperature Hagedorn phase, which constitutes a novel microscopic model for the state of the Universe when it is at the highest sustainable temperature. This state resolves the big-bang singularity by replacing the past of the hot big-bang Universe and sets inflationary initial conditions for the subsequent evolution of the thermal radiation and the semiclassical cosmological geometry. The entropy density in this state is equal to the square root of the energy density in Planck units, while the pressure is positive and equal to the energy density