Let us first consider the intrinsic metric fluctuations,
Using the metric decomposition (148) we may compute the linearized Einstein tensor . It is found that the vectorial part of the metric perturbation gives no contribution to this tensor, and the scalar and tensorial components give rise, respectively, to scalar and tensorial components: and . Thus, let us now write the Fourier transform of the homogeneous Einstein–Langevin equation (121), which is equivalent to the linearized semiclassical Einstein equation,
where and are given by Equations (123), and and denote, respectively, the Fourier-transformed scalar and tensorial parts of the linearized Einstein tensor. To simplify the problem and to illustrate, in particular, how the runaway solutions arise, we will consider the case of a massless and conformally coupled field (see  for the massless case with arbitrary coupling and [10, 259] for the general massive case). Thus substituting and into the functions and , and using Equation (117), the above equations become
For the scalar component when the only solution is . When the solutions for the scalar component exhibit an oscillatory behavior in spacetime coordinates, which corresponds to a massive scalar field with ; for the solutions correspond to a tachyonic field with . In spacetime coordinates they exhibit an exponential behavior in time, growing or decreasing, for wavelengths larger than and an oscillatory behavior for wavelengths smaller than . On the other hand, the solution is completely trivial since any scalar metric perturbation giving rise to a vanishing linearized Einstein tensor can be eliminated by a gauge transformation.
For the tensorial component, when , where is the Planck length (), the first factor in Equation (154) vanishes for four complex values of of the form and , where is some complex value. This means that, in the corresponding propagator, there are two poles on the upper half-plane of the complex plane and two poles in the lower half-plane. We will consider here the case in which ; a detailed description of the situation for can be found in Appendix A of . The two zeros on the upper half of the complex plane correspond to solutions in spacetime coordinates, which exponentially grow in time, whereas the two on the lower half correspond to solutions exponentially decreasing in time. Strictly speaking, these solutions only exist in spacetime coordinates, since their Fourier transform is not well-defined. They are commonly referred to as runaway solutions and for they grow exponentially in time scales comparable to the Planck time.
Consequently, in addition to the solutions with , there are other solutions that in Fourier space take the form for some particular values of , but all of them exhibit exponential instabilities with characteristic Planckian time scales. In order to deal with those unstable solutions, one possibility is to make use of the order-reduction prescription , which we will briefly summarize in Section 6.5.3. Note that the terms in Equations (153) and (154) come from two spacetime derivatives of the Einstein tensor, moreover, the term comes from the nonlocal term of the expectation value of the stress tensor. The order-reduction prescription amounts here to neglecting these higher derivative terms. Thus, neglecting the terms proportional to in Equations (153) and (154), we are left with only the solutions, which satisfy . The result for the metric perturbation in the gauge introduced above can be obtained by solving for the Einstein tensor, which in the Lorentz gauge of Equation (149) reads:
A second possibility, proposed by Hawking et al. [161, 162], is to impose boundary conditions, which discard the runaway solutions that grow unbounded in time. These boundary conditions correspond to a special prescription for the integration contour when Fourier transforming back to spacetime coordinates. As we will discuss in more detail in Section 6.5.2, this prescription reduces here to integrating along the real axis in the complex plane. Following that procedure we get, for example, that for a massless conformally-coupled matter field with the intrinsic contribution to the symmetrized quantum correlation function coincides with that of free gravitons plus an extra contribution for the scalar part of the metric perturbations. This extra-massive scalar renders Minkowski spacetime stable, but also plays a crucial role in providing a graceful exit in inflationary models driven by the vacuum polarization of a large number of conformal fields. Such a massive scalar field would not be in conflict with present observations because, for the range of parameters considered, the mass would be far too large to have observational consequences .
Induced metric fluctuations are described by the second term in Equation (17). They are dependent on the noise kernel that describes the stress-tensor fluctuations of the matter fields,
As we have seen in Section 6.4, following , the Einstein–Langevin equation can be entirely written in terms of the linearized Einstein tensor. The equation involves second spacetime derivatives of that tensor and, in terms of its Fourier components, is given in Equation (121) as
Following the steps after Equation (133), the Fourier transform of the two-point correlation for the linearized Einstein tensor can be written in our case as,
We may also use the order-reduction prescription, which amounts in this case to neglecting terms in the propagator, which are proportional to , corresponding to two spacetime derivatives of the Einstein tensor. The propagator then becomes a constant, and we have
Let us now write the two-point metric correlation function in spacetime coordinates for the massless and conformally coupled fields. In order to avoid runaway solutions we use the prescription that the propagator should have a well-defined Fourier transform by integrating along the real axis in the complex plane. This was, in fact, done in Section 6.4.3 and we may now write Equation (146) as
To estimate the above integral let us follow Section 6.4.3 and consider spacelike separated points and introduce the Planck length . For space separations we have that the two-point correlation (161) goes as and for we have that it goes as Since these metric fluctuations are induced by the matter stress fluctuations we infer that the effect of the matter fields is to suppress metric fluctuations at small scales. On the other hand, at large scales the induced metric fluctuations are small compared to the free graviton propagator, which goes like .
We thus conclude that, once the instabilities giving rise to the unphysical runaway solutions have been discarded, the fluctuations of the metric perturbations around the Minkowski spacetime induced by the interaction with quantum scalar fields are indeed stable (instabilities lead to divergent results when Fourier transforming back to spacetime coordinates). We have found that, indeed, both the intrinsic and the induced contributions to the quantum correlation functions of metric perturbations are stable, and consequently Minkowski spacetime is stable.
Runaway solutions are a typical feature of equations describing backreaction effects, such as in classical electrodynamics, and are due to higher than two time derivatives in the dynamical equations. Here we will give a qualitative analysis of this problem in semiclassical gravity. In a very schematic way the semiclassical Einstein equations have the form
Semiclassical gravity is expected to provide reliable results as long as the characteristic length scales under consideration, say , satisfy that . This can be qualitatively argued by estimating the magnitude of the different contributions to the effective action for the gravitational field, considering the relevant Feynman diagrams and using dimensional arguments. Let us write the effective gravitational action, again in a very schematic way, as
However, if we have a large number of matter fields, the estimates for the different terms change in a remarkable way. This is interesting because the large expansion seems, as we have argued in Section 3.3.1, the best justification for semiclassical gravity. In fact, now the vacuum-polarization terms involving loops of matter are of order . For this reason, the contribution of the graviton loops, which is just of order as is any other loop of matter, can be neglected in front of the matter loops; this justifies the semiclassical limit. Similarly, higher-order corrections are of order . Now there is a regime, when , where the Einstein–Hilbert term is comparable to the vacuum polarization of matter fields, , and yet the higher correction terms can be neglected because we still have , provided . This is the kind of situation considered in trace anomaly driven inflationary models , such as that originally proposed by Starobinsky , see also , where exponential inflation is driven by a large number of massless conformal fields. The order-reduction prescription would completely discard the effect from the vacuum polarization of the matter fields even though it is comparable to the Einstein–Hilbert term. In contrast, the procedure proposed by Hawking et al. keeps the contribution from the matter fields. Note that here the actual physical Planck length is considered, not the rescaled one, , which is related to by .
An analysis of the stability of any solution of semiclassical gravity with respect to small quantum perturbations should include not only the evolution of the expectation value of the metric perturbations around that solution, but also their fluctuations encoded in the quantum correlation functions. Making use of the equivalence (to leading order in , where is the number of matter fields) between the stochastic correlation functions obtained in stochastic semiclassical gravity and the quantum correlation functions for metric perturbations around a solution of semiclassical gravity, the symmetrized two-point quantum correlation function for the metric perturbations can be decomposed into two different parts: the intrinsic metric fluctuations due to the fluctuations of the initial state of the metric perturbations itself, and the fluctuations induced by their interaction with the matter fields. From the linearized perturbations of the semiclassical Einstein equation, information on the intrinsic metric fluctuations can be retrieved. On the other hand, the information on the induced metric fluctuations naturally follows from the solutions of the Einstein–Langevin equation.
We have analyzed the symmetrized two-point quantum correlation function for the metric perturbations around the Minkowski spacetime interacting with scalar fields initially in the Minkowski vacuum state. Once the instabilities that arise in semiclassical gravity, which are commonly regarded as unphysical, have been properly dealt with by using the order-reduction prescription or the procedure proposed by Hawking et al. [161, 162], both the intrinsic and the induced contributions to the quantum correlation function for the metric perturbations are found to be stable . Thus, we conclude that Minkowski spacetime is a valid solution of semiclassical gravity.
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