# Loop quantum gravity without the Hamiltonian constraint

March 29, 2012

We show that under certain technical assumptions, including the existence of
a constant mean curvature (CMC) slice and strict positivity of the scalar
field, general relativity conformally coupled to a scalar field can be
quantised on a partially reduced phase space, meaning reduced only with respect
to the Hamiltonian constraint and a proper gauge fixing. More precisely, we
introduce, in close analogy to shape dynamics, the generator of a local
conformal transformation acting on both, the metric and the scalar field, which
coincides with the CMC gauge condition. A new metric, which is invariant under
this transformation, is constructed and used to define connection variables
which can be quantised by standard loop quantum gravity methods. While it is
hard to address dynamical problems in this framework (due to the complicated
'time' function), it seems, due to good accessibility properties of the CMC
gauge, to be well suited for problems such as the computation of black hole
entropy, where actual physical states can be counted and the dynamics is only
of indirect importance. The corresponding calculation yields the surprising
result that the usual prescription of fixing the Barbero-Immirzi parameter beta
to a constant value in order to obtain the well-known formula S = a(Phi) A/(4G)
does not work for the black holes under consideration, while a recently
proposed prescription involving an analytic continuation of beta to the case of
a self-dual space-time connection yields the correct result. Also, the
interpretation of the geometric operators gets an interesting twist, which
exemplifies the deep relationship between observables and the choice of a time
function and has consequences for loop quantum cosmology.

Keywords:

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