https://doi.org/10.1140/epjp/s13360-024-05125-y
Regular Article
Higher-order gravity, finite action, and a safe beginning for the universe
1
Max–Planck–Institute for Gravitational Physics (Albert–Einstein–Institute), 14476, Potsdam, Germany
2
The Blackett Laboratory, Imperial College London, Prince Consort Road, SW7 2AZ, London, UK
Received:
3
January
2024
Accepted:
19
March
2024
Published online:
3
May
2024
General relativity allows for inhomogeneous and anisotropic universes with finite action. By contrast, in quadratic gravity such solutions obtain infinite action and are thus eliminated. What remains are homogeneous and isotropic solutions undergoing accelerated expansion, thereby automatically inducing an early inflationary phase. In this manner, semi-classical consistency may explain some of the basic, coarse-grained features of the early universe. This includes suitable initial conditions for the second law of thermodynamics, in the spirit of the Weyl curvature hypothesis. We note that quadratic gravity is a renormalisable theory and may admit an asymptotically safe regime at high energies, rendering the theory trustworthy to high energies. We also comment on theories containing curvature terms up to infinite derivative order, and on the contrast with no-boundary initial conditions.
© The Author(s) 2024
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