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Universe 2018, 4(9), 91;

Recent Progress in Fighting Ghosts in Quantum Gravity

Centro Brasileiro de Pesquisas Físicas, URCA, Rio de Janeiro 22290-180, RJ, Brazil
Departamento de Física, ICE, Universidade Federal de Juiz de Fora, Juiz de Fora 36036-900, MG, Brazil
Faculty of Physics and Mathematics, Tomsk State Pedagogical University, Tomsk 634041, Russia
Faculty of Physics, Tomsk State University, Tomsk 634050, Russia
Based on the invited talk given by I.L.S. at the International Conference on Quantum Gravity, Shenzhen, China, 26–28 March 2018.
Author to whom correspondence should be addressed.
Received: 30 June 2018 / Revised: 10 August 2018 / Accepted: 22 August 2018 / Published: 26 August 2018
(This article belongs to the Special Issue Gravity, Black Holes and Cosmology XXI)
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We review some of the recent results which can be useful for better understanding of the problem of stability of vacuum and in general classical solutions in higher derivative quantum gravity. The fourth derivative terms in the purely gravitational vacuum sector are requested by renormalizability already in both semiclassical and complete quantum gravity theories. However, because of these terms, the spectrum of the theory has unphysical ghost states which jeopardize the stability of classical solutions. At the quantum level, ghosts violate unitarity, and thus ghosts look incompatible with the consistency of the theory. The “dominating” or “standard” approach is to treat higher derivative terms as small perturbations at low energies. Such an effective theory is supposed to glue with an unknown fundamental theory in the high energy limit. We argue that the perspectives for such a scenario are not clear, to say the least. On the other hand, recently, there was certain progress in understanding physical conditions which can make ghosts not offensive. We survey these results and discuss the properties of the unknown fundamental theory which can provide these conditions satisfied. View Full-Text
Keywords: higher derivatives; massive ghosts; stability; cosmological solutions; gravitational waves higher derivatives; massive ghosts; stability; cosmological solutions; gravitational waves

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de O. Salles, F.; Shapiro, I.L. Recent Progress in Fighting Ghosts in Quantum Gravity. Universe 2018, 4, 91.

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