Key paper is T. W. B. Kibble's "Lorentz Invariance and the Gravitational
Field" JMP 1961 reproduced in "Gauge Theories in the Twentieth Century"
John C Taylor, Imperial College Press (2001), p. 119.

R. Utiyama Phys. Rev. 101, 1597 (1956) applied the local gauge force
principle (e.g. Yang-Mills for weak and strong forces Phys. Rev 96, 191
(1954) anticipating modern QCD and standard U(1)SU(2)SU(3) model with 17
- 25 free parameters) not to the 4-parameter translation subgroup of the
Poincare group, but to the 6-parameter Lorentz group of rigid
space-time rotations. Kibble points out problems with Utiyama's
approach and instead includes the 4-parameter translation subgroup, i.e.
the entire Poincare group leaving out the 5 additional parameters
(dilation and hyperbolic uniform acceleration) of the zero rest mass
conformal group used by Roger Penrose in twistor theory.

Kibble finds that there is a small torsion field coupling only when
Tuv(Matter) =/= 0 and that Einstein's

Ruv = 0

is obeyed in "vacuum". This was before the recent discovery of "dark
energy" that Mike Turner says may well be w = pressure/energy density =
-1 exotic vacuum with dominating negative pressure causing the Universe
to accelerate its expansion rather than slowing down. This issue is
still controversial due to recent NEWTON data as interpreted by some.
It's too early to know.