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It is well known that standard perturbative methods of quantum field theory does not lead to a consistent (renormalizable) quantum theory of gravity in four dimensions. However, the issue of quantization of gravity can also be approached from an alternative perspective, namely, using nonperturbative techniques. Here we discuss one such method based on the physics of instantons.
We begin with simple examples in particle mechanics to elucidate that nonperturbative quantum phenomena like tunneling can be interpreted as instanton effects. Next, we elaborate on a recently discovered instanton in gravity theory. We show that these gravitational instantons lead to effects similar to tunneling (in quantum gravity). As a result, the true ground state of quantum gravity is modified and acquires a nonperturbative structure. This new vacuum and its energy depends on a `quantum' coupling constant, which is exactly similar to the famous theta parameter of gauge theories (QCD). This also leads to observable effects.
Our analysis provides a clear demonstration of the fact that using instanton methods, one can unravel important features of nonperturbative quantum gravity which cannot be captured otherwise. We discuss a few important phenomenological implications of such a rich vacuum structure in quantum gravity, namely, in the context of particle physics and cosmology. These effects, if observationally significant, might bring quantum gravity to the realm of experimental tests. |