  In approaches to quantum gravity, where smooth spacetime is an emergent
approximation of a discrete Planckian fundamental structure, any effective
smooth field theoretical description would miss part of the fundamental degrees
of freedom and thus break unitarity. This is applicable also to trivial
gravitational field (low energy) idealizations realized by the use of the
Minkowski background geometry which, as any other spacetime geometry,
corresponds, in the fundamental description, to infinitely many different and
closely degenerate discrete microstates. The existence of such microstates
provides a large reservoir for information to be coded at the end of black hole
evaporation and thus opens the way to a natural resolution of the black hole
evaporation information puzzle.
  In this paper we show that these expectations can be made precise in a simple
quantum gravity model for cosmology motivated by loop quantum gravity.
Concretely, even when the model is fundamentally unitary, when microscopic
degrees of freedom irrelevant to low-energy cosmological observers are suitably
ignored, pure states in the effective description evolve into mixed states due
to decoherence with the Planckian microscopic structure. Moreover, in the
relevant physical regime these hidden degrees freedom do not carry any `energy'
and thus realize in a fully quantum gravitational context the idea (emphasized
before by Unruh and Wald) that decoherence can take place without dissipation,
now in a concrete gravitational model strongly motivated by quantum gravity.
All this strengthen the perspective of a quite conservative and natural
resolution of the black hole evaporation puzzle where information is not
destroyed but simply degraded (made unavailable to low energy observers) into
correlations with the microscopic structure of the quantum geometry at the
Planck scale.
