  We first present open questions related to the foundations of thermodynamics
and statistical physics. We then argue that in principle one can not have
"closed systems", and that a universal background should exist. We propose that
the gravitational field plays this role, due to its vanishing energy-momentum
tensor. This leads to a new possible picture, in which entropy and
irreversibility in macroscopic systems emerge from their coupling to the
background gravitational field.
