  The nonexponential relaxation and aging inherent to complex dynamics
manifested in a wide variety of dissipative systems is analyzed through a model
of diffusion in phase space in the presence of a nonconservative force. The
action of this force establishes a heat flow which maintains the system away
from equilibrium. The inability of the system to find its equilibrium state
becomes apparent through the presence of an effective temperature field. This
is the temperature of the stationary nonequilibrium state reached by the system
satisfying a generalyzed version of the fluctuation-dissipation theorem. The
presence of a nonequilibrium temperature leads to a hierarchy of relaxation
times responsible for the aging phenomena and to a relation similar to the
Vogel-Fulcher-Tammann law.
