  We consider a cosmological horizon, named thermo-horizon, to which are
associated a temperature and an entropy of Bekenstein-Hawking and which obeys
the first law for an energy flow calculated through the corresponding limit
surface. We point out a contradiction between the first law and the definition
of the total energy contained inside the horizon. This contradiction is removed
when the first law is replaced by a Gibbs' equation for a vacuum-like component
associated to the event horizon.
