An Euclidean approach for investigating quantum aspects of a scalar field living on a class of D-dimensional static black hole space-times, including the extremal ones, is reviewed. The method makes use of a near horizon approximation of the metric and ζ-function formalism for evaluating the partition function and the expectation value of the field fluctuations φ 2 (x) . After a review of the non-extreme black hole case, the extreme one is considered in some details. In this case, there is no conical singularity, but the finite imaginary time compactification introduces a cusp singularity. It is found that the ζ-function regularized partition function can be defined, and the quantum fluctuations are finite on the horizon, as soon as the cusp singularity is absent, and the corresponding temperature is T = 0. 04.70.Dy,04.20.Cv,04.60.Kz
I. INTRODUCTION.The issue of determining the equilibrium (Unruh-Hawking) temperature of a black hole, is important. In fact, one can extract thermodynamical informations from its knowledge: for example the Bekenstein-Hawking entropy (i.e. the tree-level contribution to the entropy) can be defined as the response of the free energy of the black hole to the change of this equilibrium temperature. Furthermore, it defines the admissible temperatures of thermal states of free scalar fields in a static and globally hyperbolic space-time region with horizons.As is well known, there exist several methods for evaluating the possible equilibrium temperature of a stationary black hole. Whithin the simplest of these methods, one has to make a Wick rotation of the time coordinate (passing in this way to the Euclidean time τ = it), and eliminate all the metric (conical) singularities connected to the horizon by an opportune choice of the time periodicity β M [1]. Then, one has to impose the KMS condition for thermal states [2,3], i.e. to impose the periodicity condition on the imaginary time dependence of the thermal Wightman functions, and interprete the common period β T as the inverse of the temperature T of the state. Although this procedure determines the correct Unruh-Hawking temperature in the case of a non-extreme black hole, it does not apply to the extreme case (for example to the case of an extreme Reissner-Nordström black hole), since one is unable to determine the time periodicity of the manifold β M .Later, a more sophisticated Lorentzian method was introduced in [4] and successively developed in [5] and [6,8]. Without entering in the details of this approach, we only recall that the method is connected to the well known Hadamard expansion of the two-point Green functions in a curved background and in the limit of coincidence of the arguments. Basically in [4] was proved that assuming fairly standard axioms of quantum (quasi-free) field theory (such as local definitness and local stability in a stationary space-time region), then, when the distance between the two arguments vanishes, the thermal Wightman functions in the interior of this region will transform into non-thermal and mas...