Might the radiation era extend back to the Big Bang? On dark matter production and the relic graviton background in quadratic gravity

Abstract: Naive estimates based on Einstein gravity with Lagrangian $\frac{1}{2}M_{pl}<sup>{2}(R-2Λ)$ suggest that, if the radiation era extended back to the Big Bang (i.e., as far back as the classical spacetime background makes sense), this would result in an over-production of dark matter and a relic cosmic background of thermal gravitons. Here we revisit these conclusions in quadratic gravity, the minimal renormalizable completion of Einstein gravity, whose Lagrangian also includes the terms $$\frac{1}{6}f_0<sup>{-2}R<sup>2-\frac{1}{2}f_{2}<sup>{-2}C<sup>{2}$$. Assuming the radiation era does extend back to the bang, we find a novel relation between the coefficient $f_{2}$ and the dark matter mass $m_{dm}$, needed to obtain the correct dark matter abundance. This yields a new gravitational production mechanism for dark matter (e.g., stable right-handed neutrinos). Moreover, the presence or absence of the relic graviton background (detectable by forthcoming CMB experiments via its small imprint on $N_{\rm eff}$) will place new constraints on $f_{0}, f_{2}$.