A new study explores whether the radiation era could have extended back to the Big Bang, a scenario that, under Einstein gravity, predicts an overproduction of dark matter and a cosmic background of thermal gravitons. Researchers have re-examined these implications using quadratic gravity, a minimal renormalizable extension of Einstein gravity that includes additional terms in its Lagrangian, such as R² and C² (where R is the scalar curvature and C is the Weyl tensor). This approach aims to offer a more complete description of gravity at high energies, relevant for the earliest moments of the universe.

Assuming the radiation era extends back to the Big Bang, the study reveals a crucial new relationship between the coefficient f₂ of quadratic gravity and the dark matter mass (m_dm) needed to obtain the observed abundance of dark matter. This suggests a novel gravitational production mechanism for dark matter, which could explain the existence of particles such as stable right-handed neutrinos. This finding is significant because it offers an alternative to dark matter production mechanisms based on non-gravitational interactions, which are the most studied to date.

Furthermore, the research highlights that the presence or absence of a relic graviton background, detectable through its subtle imprint on the N_eff parameter (effective number of neutrino species) in forthcoming cosmic microwave background (CMB) experiments, will place new constraints on the f₀ and f₂ coefficients of quadratic gravity. These future observations could validate or refute the proposed model, providing a unique window to test theories of gravity beyond Einstein in the early universe and shed light on the nature of dark matter and the origin of the cosmos.