Systematic study of background cosmology in unitary Poincaré gauge theories with application to emergent dark radiation and H0 tension

We propose a one-parameter extension to Λ CDM, expected to strongly affect cosmological tensions. An effective dark radiation component in the early Universe redshifts away as hot dark matter, then quintessence, tracking the dominant equation-of-state parameter and leaving a falsifiable torsion field in the current epoch. This picture results from a new Poincaré gauge theory (PGT), one of the most promising among the latest batch of 58 PGTs found to be both power-counting renormalizable and free from ghosts and tachyons. We systematically categorize the cosmologies of 33 of these PGTs, as special cases of the most general parity-preserving, Ostrogradsky-stable PGT with a purely Yang-Mills action. The theory we consider contains two propagating massless gravitons, which may be JP = 2+ (long-range gravitation and gravitational waves). A conspiracy among the coupling constants eliminates the spatial curvature k ∈ {±1, 0} from the field equations. We show that this " k-screening" is not restricted to conformal gravity theories. The flat Friedmann equations are then emergent, with potentially tension-resolving freedom at the early scale-invariant epoch that reliably gives way to an attractorlike state of modern Λ CDM evolution. We compare with related theories and promising special cases, such as k -screened theories with negative-definite effective k , and more traditional theories with effective Λ and a JP = 0− massive graviton (dark matter candidate). As a bonus, we analyze similarly constrained actions in the new extended Weyl gauge theory (eWGT). We show that in cosmology, PGT and eWGT span exactly the same classical phenomenology up to a linear map between their coupling constants, hinting at a deeper relationship between the two.