f(R)$f(R)$‐Gravity Generated Post‐Inflationary Eras and Their Effect on Primordial Gravitational Waves

In this work we shall consider the effects of a geometrically generated post-inflationary era on energy spectrum primordial gravitational waves. Specifically, constant equation state era, by synergistic effect $f(R)$ gravity and radiation matter perfect fluids. Two cases interest be studied, one with parameter $w=-1/3$, in which case Universe neither accelerates nor decelerates, $w=0$ so an early domination era. For evaluation inflationary observational indices is relevant for calculation waves spectrum, also took into account $e$-foldings number. both $w=-1/3$ cases, amplified, but case, stronger.