Spacetime Instability and the Problems with Quantum Gravity. (arXiv:1901.08785v1 [hep-th])
<a href="http://arxiv.org/find/hep-th/1/au:+Matsui_H/0/1/0/all/0/1">Hiroki Matsui</a>

In this paper we discuss spacetime instability problems in effective field
theories of the quantum gravity (QG). The effective action of the gravity
requires higher-derivative curvature terms $R^{2}, { R }_{ mu nu }{ R }^{ mu
nu }, R_{munukappalambda} R^{munukappalambda}dots$ and they are
leading quantum gravitational corrections. Although these higher-curvature
terms are indispensable for the construction of the semiclassical or quantum
gravity theories, they lead to many pathologies. We clearly show that even if
they are Planck-suppressed operators they lead to serious consequences of the
spacetime stability and, de Sitter spacetime or radiation dominated Universe
are highly unstable for the Hubble perturbation. Furthermore, these curvature
terms also violate the null energy condition (NEC) and provides undesired
consequences. Thus, the standard effective field theory of the gravity might
have failed to explain the Universe.

In this paper we discuss spacetime instability problems in effective field
theories of the quantum gravity (QG). The effective action of the gravity
requires higher-derivative curvature terms $R^{2}, { R }_{ mu nu }{ R }^{ mu
nu }, R_{munukappalambda} R^{munukappalambda}dots$ and they are
leading quantum gravitational corrections. Although these higher-curvature
terms are indispensable for the construction of the semiclassical or quantum
gravity theories, they lead to many pathologies. We clearly show that even if
they are Planck-suppressed operators they lead to serious consequences of the
spacetime stability and, de Sitter spacetime or radiation dominated Universe
are highly unstable for the Hubble perturbation. Furthermore, these curvature
terms also violate the null energy condition (NEC) and provides undesired
consequences. Thus, the standard effective field theory of the gravity might
have failed to explain the Universe.

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