Systematic study of background cosmology in unitary Poincare gauge theories with application to emergent dark radiation and H-0 tension

We propose a one-parameter extension to Lambda 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 Poincare 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 J(P) = 2(+) (long-range gravitation and gravitational waves). A conspiracy among the coupling constants eliminates the spatial curvature k is an element of {+/- 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 Lambda 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. and a J(P) = 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.