Dark matter in scale-invariant gravity with hidden-sector condensation
The origin of the electroweak scale, cosmic inflation, and dark matter are often treated as independent problems beyond the Standard Models of particle physics and cosmology. In this work, we explore the possibility that they instead arise from a common underlying framework based on classically scale-invariant quadratic gravity coupled to a strongly interacting hidden sector. The $R^2$ term naturally realizes Starobinsky inflation, while confinement in the hidden sector dynamically generates the Planck scale and triggers electroweak symmetry breaking through a gravitationally induced Higgs mass generation mechanism. The scalar degree of freedom associated with the $R^2$ term subsequently reheats both the visible and hidden sectors through universal couplings to the energy-momentum tensor, leading to the gravitational freeze-in production of hidden-sector states. We investigate three representative realizations of the hidden sector in which the dark matter candidate is either a hidden $η'$ meson, a hidden vector boson, or charged hidden pions, and derive the corresponding dark matter and dark radiation relic abundances.