Fluid phonons, protoinflationary dynamics and large-scale gravitational fluctuations

We explore what can be said on the effective temperature and sound speed of a statistical ensemble of fluid phonons present at the onset of a conventional inflationary phase. The phonons are the actual normal modes of the gravitating and irrotational fluid that dominates the protoinflationary dynamics. The bounds on the tensor to scalar ratio result in a class of novel constraints involving the slow roll parameter, the sound speed of the phonons and the temperature of the plasma prior to the onset of inflation. If the current size of the Hubble radius coincides with the inflationary event horizon redshifted down to the present epoch, the sound speed of the phonons can be assessed from independent measurements of the tensor to scalar ratio and of the tensor spectral index.