budget is dominated by photons and relativistic neutrinos. For consistency, the RD epoch must begin therefore before the onset of BBN. With the period of inflation ending at an energy scale $\lesssim 10^{16}\text{ GeV}$ and the onset of RD occurring at least above $\sim 1 \text{ MeV}$, we are left with an unknown intermediary period which may span up to $\sim 19$ orders of magnitude in energy scale.

During inflation, tensor metric perturbations generated from quantum fluctuations are spatially stretched to scales exponentially larger than the inflationary Hubble radius. This process results in a (quasi-)scale invariant tensor power spectrum at superhorizon scales [@Grishchuk:1974ny; @Starobinsky:1979ty; @Rubakov:1982df; @Fabbri:1983us], with a very small red tilt. After inflation, the horizon begins to grow faster than the redshifting of length scales, and the tensor modes re-enter the horizon successively as the Hubble radius catches up with each wavelength. Once a tensor mode crosses inside the horizon, it becomes part of the stochastic background of gravitational waves (GWs). Different tensor modes re-enter the horizon at different times, and hence propagate through different periods of the evolution of the Universe after they have become sub-horizon. For tensor modes crossing during RD, the resulting present GW energy density 