We present an approximate time-dependent metric in ingoing Eddington-Finkelstein coordinates for an evaporating nonrotating black hole as a first-order perturbation of the Schwarzschild metric, using the linearized back reaction from a realistic approximation to the stress-energy tensor for the Hawking radiation in the Unruh quantum state.
I. INTRODUCTIONThe physics of black holes is an abundant field in which the convergence of gravitation, quantum theory, and thermodynamics takes place. The original derivation of Hawking radiation [1] from black holes is based on semiclassical effective field theory. Normally, quantum fields are considered test fields in the curved spacetime of a classical background geometry. A quantum field theory constructed on a curved background spacetime experiences gravitationally induced vacuum polarization and/or particle creation. These effects induce a nonzero expectation value for the stress-energy tensor. The renormalized expectation value of the complete quantum stress-energy tensor outside the classical event horizon has been calculated by various authors [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21], usually using a framework established by Christensen and Fulling [3]. In this framework, the assumptions are that the stress-energy tensor is time independent, satisfies local stress-energy conservation, and has a trace determined solely by the conformal anomaly, both for the fields that are classically conformally invariant (such as a massless scalar field and the electromagnetic field) and for the gravitational field. The quantum state considered is usually either the Hartle-Hawking state [22] or the Unruh state [23,24]. (For discussion of the various black hole vacuum states, see [25]). In the Hartle-Hawking state, one has thermal equilibrium, and zero net energy flux, with the outgoing Hawking radiation balanced by incoming radiation from an external heat bath at the Hawking temperature. In [4] a fairly good closed-form approximation for the energy density and stresses of a conformal scalar field in the Hartle-Hawking state everywhere outside a static black hole can be found.In the Unruh state, there is the absence of incoming radiation at both past null infinity and the past horizon, plus regularity of the stress-energy tensor on the future event horizon in the frame of a freely falling observer, representing a black hole formed from gravitational collapse, with nothing falling into the black hole thereafter. There have been many calculations of the quantum stress-energy tensor in the Unruh state in the Schwarzschild spacetime, both for a massless scalar field and for the electromagnetic field [5-7, 9-13, 15-17]. A method for computing the stress-energy tensor for the quantized massless spin-1/2 field in a general static spherically symmetric spacetime was presented in [18][19][20]. The canonical quantization of the electromagnetic field has also been investigated in the Kerr metric [21].One of the important questions that one wants to answer concerns...