Vacuum Condensate Picture of Quantum Gravity
  In quantum gravity perturbation theory in Newton's constant G is known to be
badly divergent, and as a result not very useful. Nevertheless some of the most
interesting phenomena in physics are often associated with non-analytic
behavior in the coupling constant and the existence of nontrivial quantum
condensates. It is therefore possible that pathologies encountered in the case
of gravity are more likely the result of inadequate analytical treatment, and
not necessarily a reflection of some intrinsic insurmountable problem. The
nonperturbative treatment of quantum gravity via the Regge-Wheeler lattice path
integral formulation reveals the existence of a new phase involving a
nontrivial gravitational vacuum condensate, and a new set of scaling exponents
characterizing both the running of G and the long-distance behavior of
invariant correlation functions. The appearance of such a gravitational
condensate is viewed as analogous to the (equally nonperturbative) gluon and
chiral condensates known to describe the physical vacuum of QCD. The resulting
quantum theory of gravity is highly constrained, and its physical predictions
are found to depend only on one adjustable parameter, a genuinely
nonperturbative scale xi in many ways analogous to the scaling violation
parameter Lambda MSbar of QCD. Recent results point to significant deviations
from classical gravity on distance scales approaching the effective infrared
cutoff set by the observed cosmological constant. Such subtle quantum effects
are expected to be initially small on current cosmological scales, but could
become detectable in future high precision satellite experiments.
