  Any reasonable form of quantum gravity can explain (by phase interference)
why on a large scale, inertial frames seem not to rotate relative to the
average matter distribution in the universe without the need for absolute
space, finely tuned initial conditions, or without giving up independent
degrees of freedom for the gravitational field. A simple saddlepoint
approximation to a path-integral calculation for a perfect fluid cosmology
shows that only cosmologies with an average present relative rotation rate
smaller than about $T^*H^2 \approx 10^{-71}$ radians per year could contribute
significantly to a measurement of relative rotation rate in our universe, where
$T^*\approx 10^{-51}$ years is the Planck time and $H \approx 10^{-10}$
yr$^{-1}$ is the present value of the Hubble parameter. A more detailed
calculation (taking into account that with vorticity, flow lines are not normal
to surfaces of constant global time, and approximating the action to second
order in the mean square vorticity) shows that the saddlepoint at zero
vorticity is isolated and that only cosmologies with an average present
relative rotation rate smaller than about $T^*H^2 a_1^{1/2} \approx 10^{-73}$
radians per year could contribute significantly to a measurement of relative
rotation rate in our universe, where $a_1 \approx 10^{-4}$ is the value of the
cosmological scale factor at the time when matter became more significant than
radiation in the cosmological expansion. This is consistent with measurements
indicating a present relative rotation rate less than about $10^{-20}$ radians
per year. The observed lack of relative rotation may be evidence for the
existence of quantum gravity.
