A safe way to reheat the universe , in models of natural and quadratic inflation , is through shift symmetric couplings between the inflaton \phi and the Standard Model ( SM ) , since they do not generate loop corrections to the potential V ( \phi ) .
We consider such a coupling to SM gauge fields , of the form \phi F \tilde { F } / f , with sub-Planckian f .
In this case gauge fields can be exponentially produced already during inflation and thermalize via interactions with charged particles , as pointed out in previous work .
This can lead to a plasma of temperature T during inflation and the thermal masses gT of the gauge bosons can equilibrate the system .
In addition , inflaton perturbations \delta \phi can also have a thermal spectrum if they have sufficiently large cross sections with the plasma .
In this case inflationary predictions are strongly modified : ( 1 ) scalar perturbations are thermal , and so enhanced over the vacuum , leading to a generic way to suppress the tensor-to-scalar ratio r ; ( 2 ) the spectral index is n _ { s } -1 = \eta - 4 \epsilon .
After presenting the relevant conditions for thermalization , we show that thermalized natural and monomial models of inflation agree with present observations and have r \approx 10 ^ { -3 } -10 ^ { -2 } , which is within reach of next generation CMB experiments .