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Helmoltz problem for the Riccati equation from an analogous Friedmann equation
Valerio Faraoni
We report a solution of the inverse Lagrangian problem for the first order Riccati differential equation by means of an analogy with the Friedmann equation of a suitable Friedmann-…
A new approach to the thermodynamics of scalar-tensor gravity
Valerio Faraoni, Andrea Giusti, Andrea Mentrelli
We discuss and expand a new approach to the thermodynamics of scalar-tensor gravity and its diffusion toward general relativity (seen as an equilibrium state) proposed in a previou…
The curious case of the Buchdahl-Land-Sultana-Wyman-Ibañez-Sanz spacetime
Valerio Faraoni, Sonia Jose, Alexandre Leblanc
We revisit Wyman's "other" scalar field solution of the Einstein equations and its Sultana generalization to positive cosmological constant, which has a finite 3-space and correspo…
The Life and Science of Thanu Padmanabhan
Jasjeet Singh Bagla, Krishnakanta Bhattacharya, Sumanta Chakraborty +21
Thanu Padmanabhan was a renowned Indian theoretical physicist known for his research in general relativity, cosmology, and quantum gravity. In an extraordinary career spanning fort…
Area-law versus Rényi and Tsallis black hole entropies
Shin'ichi Nojiri, Sergei D. Odintsov, Valerio Faraoni
The Rényi and Tsallis entropies are discussed as possible alternatives to the Bekenstein-Hawking area-law entropy. It is pointed out how replacing the entropy notion, but not the H…
First-order thermodynamics of Horndeski gravity
Andrea Giusti, Stefan Zentarra, Lavinia Heisenberg +1
We extend to the Horndeski realm the irreversible thermodynamics description of gravity previously studied in "first generation" scalar-tensor theories. We identify a subclass of H…
Revisiting geodesic observers in cosmology
Geneviève Vachon, Robert Vanderwee, Valerio Faraoni
Geodesic observers in cosmology are revisited. The coordinates based on freely falling observers introduced by Gautreau in de Sitter and Einstein-de Sitter spaces (and, previously,…
Disformal mappings of spherical DHOST geometries
Valerio Faraoni, Alexandre Leblanc
New solutions of DHOST theories can be generated by applying a disformal tranformation to a known seed solution. We examine the nature of spherically symmetric solutions of DHOST g…
Multi-fluid cosmology in Einstein gravity: analytical solutions
Valerio Faraoni, Sonia Jose, Steve Dussault
We review analytical solutions of the Einstein equations which are expressed in terms of elementary functions and describe Friedmann-Lemaître-Robertson-Walker universes sourced by…
Maximum force and cosmic censorship
Valerio Faraoni
Although the idea that there is a maximum force in nature seems untenable, we explore whether this concept can make sense in the restricted context of black holes. We discuss unifo…
Analogies between logistic equation and relativistic cosmology
Steve Dussault, Valerio Faraoni, Andrea Giusti
We develop several formal analogies between the logistic equation and the spatially homogeneous and isotropic relativistic cosmology described by the Einstein-Friedmann equations.…
Thermodynamics of scalar-tensor gravity
Valerio Faraoni, Andrea Giusti
Previously, the Einstein equation has been described as an equation of state, general relativity as the equilibrium state of gravity, and gravity as a non-equilibrium…
Generalized Fibonacci numbers, cosmological analogies, and an invariant
Valerio Faraoni, Farah Atieh
Continuous generalizations of the Fibonacci sequence satisfy ODEs that are formal analogues of the Friedmann equation describing spatially homogeneous and isotropic cosmology in ge…
Spherical inhomogeneous solutions of Einstein and scalar-tensor gravity: a map of the land
Valerio Faraoni, Andrea Giusti, Bardia H. Fahim
We review spherical and inhomogeneous analytic solutions of the field equations of Einstein and of scalar-tensor gravity, including Brans-Dicke theory, non-minimally (possibly conf…