Realistic Observable in Background-Free Quantum Gravity: the Planck-Star
  Tunnelling-Time
  A gravitationally collapsed object can bounce-out from its horizon via a
tunnelling process that violates the classical equations in a finite region.
Since tunnelling is a non-perturbative phenomenon, it cannot be described in
terms of quantum fluctuations around a classical solution and a background-free
formulation of quantum gravity is needed to analyze it. Here we use Loop
Quantum Gravity to compute the amplitude for this process, in a first
approximation. The amplitude determines the tunnelling time as a function of
the mass. This is the key information to evaluate the astrophysical relevance
of this process. The calculation offers a template and a concrete example of
how a background-free quantum theory of gravity can be used to compute a
realistical observable quantity.
