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Linearized propagation equations for metric fluctuations in a general (non-vacuum) background geometry

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TLDR
The linearized dynamical equation for metric perturbations in a fully general, non-vacuum, background geometry is obtained from the Hamilton variational principle applied to the action up to second order.
Abstract
The linearized dynamical equation for metric perturbations in a fully general, non-vacuum, background geometry is obtained from the Hamilton variational principle applied to the action up to second order. We specialize our results to the case of traceless and transverse metric fluctuations, and we discuss how the intrinsic properties of the matter stress tensor can affect (and modify) the process of gravity wave propagation even in most conventional geometric scenarios, like (for instance) those described by a FLRW metric background. We provide explicit examples for fluid, scalar field and electromagnetic field sources.

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Microphysical manifestations of viscosity and consequences for anisotropies in the very early universe

- 31 Jan 2022 - 
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References
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Journal ArticleDOI

Action Integrals and Partition Functions in Quantum Gravity

TL;DR: In this article, the authors derived finite, purely imaginary values for the actions of the Kerr-Newman solutions and de Sitter space, which they used to evaluate the entropy of these metrics and find that it is always equal to one quarter the area of the event horizon in fundamental units.
Book

Theory of Cosmological Perturbations

TL;DR: In this paper, the authors present in a manifestly gauge-invariant form the theory of classical linear gravitational perturbations in part I, and a quantum theory of cosmological perturbation in part II.
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Role of conformal three-geometry in the dynamics of gravitation.

TL;DR: In this article, the unconstrained dynamical degrees of freedom of the gravitational field are identified with the conformally invariant three-geometries of spacelike hypersurfaces.
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Cosmological Backgrounds of Gravitational Waves

TL;DR: In this article, the authors review early universe sources that can lead to cosmological backgrounds of GWs and discuss the basic characteristics of present and future GW detectors, including advanced LIGO, advanced Virgo, the Einstein telescope, KAGRA, and LISA.
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Viscous Inflationary Universe Models

TL;DR: In this article, the authors reviewed and carried further in inflationary cosmological models of Bianchi type-I with shear, bulk, and nonlinear viscosity.
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