  In analyzing the nature of thermal radiance experienced by an accelerated
observer (Unruh effect), an eternal black hole (Hawking effect) and in certain
types of cosmological expansion, one of us proposed a unifying viewpoint that
these can be understood as arising from the vacuum fluctuations of the quantum
field being subjected to an exponential scale transformation. This viewpoint,
together with our recently developed stochastic theory of particle-field
interaction understood as quantum open systems described by the influence
functional formalism, can be used to address situations where the spacetime
possesses an event horizon only asymptotically, or none at all. Examples
studied here include detectors moving at uniform acceleration only
asymptotically or for a finite time, a moving mirror, and a collapsing mass. We
show that in such systems radiance indeed is observed, albeit not in a precise
Planckian spectrum. The deviation therefrom is determined by a parameter which
measures the departure from uniform acceleration or from exact exponential
expansion. These results are expected to be useful for the investigation of
non-equilibrium black hole thermodynamics and the linear response regime of
backreaction problems in semiclassical gravity.
