Abstract
The vacuum expectation value of the renormalized stress tensor for a conformally coupled massless scalar field propagating in an arbitrary hyperbolic Robertson-Walker spacetime is calculated. The vacuum state used is that obtained by conformal transformation to the static hyperbolic universe. The result differs from calculations performed using the vacuum defined by conformal transformation to Minkowski spacetime, and is also different from earlier results obtained using the conformally static vacuum which are shown to be incorrect.