Addressing $H_0$ tension with emergent dark radiation in unitary gravity
  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 Poincar\'{e} gauge
theory (PGT), one of the most promising among the latest batch of 58 PGTs found
to be both power-counting renormalisable and free from ghosts and tachyons. We
systematically categorise 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 $k\in\{\pm 1,0\}$ 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
attractor-like 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
$\Lambda$ and a $J^P=0^-$ massive graviton (dark matter candidate). As a bonus,
we analyse 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.
