  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. 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. These relations imply a maximally large entropy density and,
therefore, a state that cannot be described by a semiclassical spacetime
geometry. If one nevertheless insists on an effective semiclassical description
of this state, she can do so by ignoring the entropy. This leads to a partially
equivalent description in which the pressure appears to be negative and equal
in magnitude to the energy density, as if the energy-momentum tensor was that
of a cosmological constant. From this effective perspective, the state
describes a period of string-scale inflation. The bound state of strings
ultimately decays, possibly by a process akin to Hawking radiation, and
undergoes a transition into a phase of hot radiation. But, from the effective
perspective, the same decay corresponds to the heating of the Universe at the
end of inflation. Small quantum mechanical fluctuations in the initial state
lead to scale-invariant temperature anisotropies in the hot radiation. The
temperature anisotropies are interpreted in the effective description as
arising from quantum fluctuations of the curvature and an effective inflaton
field. The stringy microscopic description determines the parameters of the
model of inflation, as well as the cosmological observables, in terms of the
string length scale and coupling strength. Our framework is similar,
conceptually, to a recent description of black holes in terms of a maximal
entropy state of strings in the Hagedorn phase.
