Article
Astronomy & Astrophysics
Andreu Maso-Ferrando, Nicolas Sanchis-Gual, Jose A. Font, Gonzalo J. Olmo
Summary: In the Palatini formulation of f (R) gravity, equilibrium solutions of spherically symmetric boson stars were explored. The study compared solutions of the quadratic theory f (R) = R + xi R-2 with those in general relativity, showing similar existence curves but significant differences for negative coupling parameters leading to repulsive gravitational components. Introducing self-interactions further emphasized the similarities between f (R) and general relativity, making observational discrimination between the two theories challenging.
CLASSICAL AND QUANTUM GRAVITY
(2021)
Article
Astronomy & Astrophysics
Hai-Feng Ding, Xiang-Hua Zhai
Summary: In this paper, we generalize the off-shell Abbott-Deser-Tekin (ADT) conserved charge formalism to Palatini theory of gravity with torsion and non-metricity, where the independent dynamic fields are the metric and the affine connection. For a general Palatini theory of gravity, we obtain the most general expression for off-shell ADT potential.
CLASSICAL AND QUANTUM GRAVITY
(2021)
Article
Astronomy & Astrophysics
J. McDonald
Summary: In comparison to the conventional metric formulation, reconsideration of unitarity violation in the electroweak vacuum under the Palatini formulation reveals that Higgs Inflation model does not violate unitarity at low energies. This suggests that Palatini Higgs Inflation may have a significant advantage over metric Higgs Inflation in terms of conservation of unitarity.
JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS
(2021)
Article
Astronomy & Astrophysics
Konstantinos Dimopoulos, Alexandros Karam, Samuel Sanchez Lopez, Eemeli Tomberg
Summary: We propose a model of quintessential inflation in Palatini R2 gravity, where a scalar field with a simple exponential potential is coupled to gravity with a running non-minimal coupling. By studying the combination of the scalar sector and an ideal fluid, we investigate the cosmological evolution of the model from inflation to dark energy domination. Numerical scan over the parameter space reveals points that align with observations for both the inflationary CMB data and the late-time behavior.
JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS
(2022)
Article
Astronomy & Astrophysics
Minxi He, Yusuke Mikura, Yuichiro Tada
Summary: We extend the Higgs inflation to include non-minimal couplings between Higgs and both the metric-type and the Palatini-type Ricci scalars in hybrid metric-Palatini gravity. Our model realizes slow-roll inflation in the large-field regime with observationally favored predictions. We also explore the unitarity property of our model and discuss possible UV-extended theories to achieve a weakly-coupled effective field theory.
JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS
(2023)
Article
Astronomy & Astrophysics
Angelos Lykkas, Kyriakos Tamvakis
Summary: The study explores an extension of the Palatini-R-2 model by introducing field dependencies to the R-2 coefficient in the form of logarithmic corrections, finding that the predicted values lie in the favored region of observations. The assumed scalar field dependence tends to decrease the value of the spectral index n(s) and potentially increase the tensor-to-scalar ratio r, deviating from the highly suppressed values commonly found in similar models within the same framework.
JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS
(2021)
Article
Astronomy & Astrophysics
Dhruba Jyoti Gogoi, Umananda Dev Goswami
Summary: This study investigates the behavior of dark energy f(R) gravity models along with two other cosmological models under the Palatini formalism. The results show that the new model behaves similarly to the other models in the early stages of the universe but has a unique prediction for the future history of the universe. All models show good agreement with observational data within a constrained range of parameters, with the new model showing particularly satisfactory agreement throughout its allowed parameter range.
INTERNATIONAL JOURNAL OF MODERN PHYSICS D
(2022)
Article
Astronomy & Astrophysics
Yusuke Mikura, Yuichiro Tada
Summary: In this study, we investigate the UV-completion of Higgs inflation in both the metric and the Palatini formalisms. We find that the cutoff scales for these inflation models need to be much smaller than the Planck scale in order to be consistent with observations. By embedding the curved field-space into a higher dimensional flat space, we are able to successfully UV-complete the Higgs inflation in the metric formalism. However, this approach does not work in the Palatini formalism, where the cutoff cannot be uplifted to the Planck scale.
JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS
(2022)
Article
Astronomy & Astrophysics
Jaakko Annala, Syksy Rasanen
Summary: We investigate the stability of gravitational theories with arbitrary algebraic dependence on three traces of the Riemann tensor: the Ricci tensor, the co-Ricci tensor, and the homothetic curvature tensor, which we collectively refer to as Ricci-type tensors. We consider both unconstrained connections and cases where either torsion or non-metricity is assumed to vanish. We determine the combinations of Ricci-type tensors that introduce new degrees of freedom around Minkowski and FLRW space and identify the presence of ghosts. We find that only theories with zero torsion and dependence solely on the Ricci tensor have healthy new degrees of freedom, and projective invariance is not sufficient to ensure a ghost-free theory.
JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS
(2023)
Article
Astronomy & Astrophysics
Jaakko Annala, Syksy Rasanen
Summary: In this study, we investigate inflation using the most general non-degenerate gravitational action, involving the symmetric part of the Ricci tensor coupled to a scalar field in the Palatini formulation of gravity. By employing field redefinitions, we manage to shift the impact of the Ricci terms from gravity to the scalar field, specifically focusing on slow-roll inflation. As exemplified by cases of actions quadratic and cubic in the Ricci tensor, we find that in the quadratic scenario, the tensor-to-scalar ratio r can be suppressed with no effect on the scalar spectrum, while in the cubic scenario, r can be suppressed by at most 2/9, with significant changes in the scalar spectral index n(s).
JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS
(2021)
Article
Astronomy & Astrophysics
Diego Saez-Chillon Gomez
Summary: In this study, a general f(R) gravitational theory is examined within the Palatini formalism. It is shown that an additional surface term in the gravitational action plays a fundamental role in calculating the entropy of black holes.
Article
Astronomy & Astrophysics
Mio Kubota, Kin-ya Oda, Keigo Shimada, Masahide Yamaguchi
Summary: This study investigates cosmological perturbations ofscalar-tensor theories in Palatini formalism. By introducing actions and computing perturbations in different frames, it is shown that the sound speed of gravitational waves is unity for certain models. Additionally, the presence of ghosts and potential deviations in sound speed in Palatini formalism are explored, with the conclusion that ghost-free terms can still serve as dark energy and inflaton.
JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS
(2021)
Article
Astronomy & Astrophysics
Joao Luis Rosa, Francisco S. N. Lobo, Diego Rubiera-Garcia
Summary: This study explores cosmological sudden singularities in the dynamically equivalent scalar-tensor representation of generalized hybrid metric-Palatini gravity. The research shows that sudden singularities may appear in the fourth-order time derivative of the scale factor under certain assumptions, providing constraints on the occurrence of sudden singularities in our universe as measured from the Big Bang.
JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS
(2021)
Article
Astronomy & Astrophysics
A. B. Lahanas
Summary: In this study, we investigate R2 inflation in Palatini gravity with scalar fields coupled to gravity. We explore the similarities between these theories, in the Einstein frame, and K-inflation models. Additionally, we examine single-field inflationary models with monomial potentials V similar to hn, where n is a positive even integer. The analysis also includes the study of the Higgs model with nonminimal coupling to gravity. Through constraints from cosmological data, we find bounds on the inflationary scale Ms similar to 1/√(alpha) and the reheating temperature of the Universe.
Article
Astronomy & Astrophysics
Alexandros Karam, Sotirios Karamitsos, Margus Saal
Summary: Attractor inflation is a robust framework for developing inflationary models that are insensitive to potential details. This study classifies the strong coupling limit of inflationary models in both the metric and Palatini approaches using the fl-function formalism, determining the range of non-minimal coupling values that lead to indistinguishable theories within current accuracy. The reconstruction of the Jordan frame potential for-attractors demonstrates the impact of metric or Palatini choice on inflationary observables.
JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS
(2021)
Article
Physics, Particles & Fields
Valerio Faraoni
Summary: By analogizing with the Friedmann equation in cosmology, we have solved the inverse Lagrangian problem for the first-order Riccati differential equation, offering new insights into various physical systems described by the Riccati equation.
EUROPEAN PHYSICAL JOURNAL C
(2022)
Article
Physics, Mathematical
Shin'ichi Nojiri, Sergei D. Odintsov, Valerio Faraoni
Summary: The article explores the consistency between alternative entropy notions and the traditional Bekenstein-Hawking entropy in certain classes of modified gravity black holes, and provides examples to support this.
INTERNATIONAL JOURNAL OF GEOMETRIC METHODS IN MODERN PHYSICS
(2022)
Article
Astronomy & Astrophysics
Duncan Farrah, Kevin S. Croker, Michael Zevin, Gregory Tarle, Valerio Faraoni, Sara Petty, Jose Afonso, Nicolas Fernandez, Kurtis A. Nishimura, Chris Pearson, Lingyu Wang, David L. Clements, Andreas Efstathiou, Evanthia Hatziminaoglou, Mark Lacy, Conor McPartland, Lura K. Pitchford, Nobuyuki Sakai, Joel Weiner
Summary: Observations have found black holes of varying masses throughout cosmic history, but the Kerr black hole solution is incomplete. Black hole models with realistic behavior predict that black holes can increase in mass with the expansion of the universe, independently of accretion or mergers. By studying supermassive black holes in elliptical galaxies, evidence is found for cosmologically coupled mass growth among these black holes, suggesting that black holes contribute as vacuum energy to the cosmological energy density.
ASTROPHYSICAL JOURNAL LETTERS
(2023)
Article
Physics, Mathematical
Shin'ichi Nojiri, Sergei D. D. Odintsov, Valerio Faraoni
Summary: The Bekenstein-Hawking entropy of a black hole is constant in two-dimensional spacetime dimensions due to the degeneration of the horizon into two points. This is consistent with the topological nature of Einstein's gravity in two dimensions. In F(R) gravity and Jackiw-Teitelboim gravity, which are equivalent to two-dimensional F(R) gravity, the entropy also remains constant. However, in two-dimensional dilaton gravity, the entropies are not always constant and can depend on the mass, horizon radius, or Hawking temperature, leading to new proposals for a generalized entropy.
INTERNATIONAL JOURNAL OF GEOMETRIC METHODS IN MODERN PHYSICS
(2023)
Article
Astronomy & Astrophysics
Valerio Faraoni, Robert Vanderwee
Summary: With examples and physical arguments, the Tolman-Ehrenfest criterion is extended to local thermal equilibrium in conformally static geometries, showing the temperature scaling of the rescaled fluid with the inverse of the conformal factor and its relation to cosmic evolution and black hole temperature.
Article
Physics, Particles & Fields
Valerio Faraoni, Sonia Jose
Summary: Formal analogies are established between the ordinary differential equations of geophysical flows and Friedmann cosmology. This leads to the development of Lagrangian and Hamiltonian formulations for these equations, and laboratory experiments investigating geophysical flows are identified as analogue gravity systems for cosmology.
EUROPEAN PHYSICAL JOURNAL C
(2023)
Article
Physics, Particles & Fields
Valerio Faraoni, Serena Giardino, Andrea Giusti, Robert Vanderwee
Summary: We revisit the analogy between a minimally coupled scalar field in general relativity and a perfect fluid, correcting previous identifications of effective temperature and chemical potential. This provides a useful complementary picture for the first-order thermodynamics of scalar-tensor gravity, paving the way for the Einstein frame formulation (which eluded previous attempts) and raises interesting questions to further develop the analogy.
EUROPEAN PHYSICAL JOURNAL C
(2023)
Article
Astronomy & Astrophysics
Valerio Faraoni, Andrea Giusti, Sonia Jose, Serena Giardino
Summary: In this paper, the possibility of zero-temperature equilibrium states other than Einstein gravity is discussed in the context of first-order thermodynamics of scalar-tensor gravity. Pathological theories such as Brans-Dicke theory, Palatini f(R) gravity, and cuscuton gravity, all with non-dynamical scalar fields, are considered. The formalism is also extended to Nordstrom gravity, which has only one scalar degree of freedom and exhibits negative temperature relative to general relativity.
Article
Astronomy & Astrophysics
Andrea Giusti, Stefan Zentarra, Lavinia Heisenberg, Valerio Faraoni
Summary: In this paper, the irreversible thermodynamics description is applied to the Horndeski realm, and a subclass of Horndeski theories is identified as an out-of-equilibrium state, while general relativity corresponds to an equilibrium state. By recasting the field equations, an effective heat current, "temperature of gravity," and shear viscosity are identified in the space of theories.
Article
Astronomy & Astrophysics
Marcello Miranda, Daniele Vernieri, Salvatore Capozziello, Valerio Faraoni
Summary: In this study, we investigate McVittie and generalized McVittie solutions for Horndeski gravity with a spatially homogeneous gravitational scalar field. The solutions show interesting behavior at different scales, providing insights into the properties of the gravitational field. By including matter and utilizing the extended cuscuton model, we obtain generalized solutions that are classified based on various parameters such as the time dependence of the gravitational coupling and the accretion rate onto the central object.
Article
Astronomy & Astrophysics
Valerio Faraoni, Genevieve Vachon, Robert Vanderwee, Sonia Jose
Summary: This study examines the Friedmann-Lema??tre-Robertson-Walker cosmology from the perspective of gravitoelectromagnetism, focusing on spacetime regions smaller than the Hubble radius. It discusses several nontrivial features and differences compared to the "standard" asymptotically flat gravitoelectromagnetism.
Article
Astronomy & Astrophysics
Valerio Faraoni, Theo B. Franconnet
Summary: This article investigates a puzzle in the recent thermodynamics of scalar-tensor gravity, where general relativity is a zero-temperature equilibrium state and scalar-tensor gravity is the nonequilibrium configuration of an effective dissipative fluid. A stealth solution of Brans-Dicke gravity with constant positive temperature is shown to be analogous to a metastable state for the effective fluid and to suffer from an instability. The stability analysis utilizes a modified gravity version of the Bardeen-Ellis-Bruni-Hwang gauge-invariant formalism for cosmological perturbations. The metastable state is destroyed by tensor perturbations.
Article
Physics, Particles & Fields
Behnaz Fazlpour, Ali Banijamali, Valerio Faraoni
Summary: This article discusses the physical features of two recent classes of analytical solutions of the Einstein equations sourced by an exotic perfect fluid. These geometries, characterized by up to four parameters, are static and spherically symmetric, describing compact spaces with naked central singularities.
EUROPEAN PHYSICAL JOURNAL C
(2022)
Article
Astronomy & Astrophysics
Valerio Faraoni, Sonia Jose, Alexandre Leblanc
Summary: In this paper, we revisit Wyman's scalar field solution of the Einstein equations and its Sultana generalization with a positive cosmological constant. This solution has a finite 3-space and corresponds to a special case of the stiff fluid solution proposed by Buchdahl and Land, as well as Ibanez and Sanz, for modeling relativistic stars. However, the existence of a hidden cosmological constant and the peculiar geometry prevent the use of this spacetime for modeling relativistic stars.
Article
Astronomy & Astrophysics
Shin'ichi Nojiri, Sergei D. Odintsov, Valerio Faraoni
Summary: We propose a new entropy construct that generalizes existing entropies and converges to known entropy in a specific limit. Applying this construct to Schwarzschild black hole and cosmology models, it is shown that it has the potential to describe inflation and/or holographic dark energy.
Article
Astronomy & Astrophysics
Nelson R. F. Braga, Octavio C. Junqueira
Summary: This study investigates the influence of rotation on the transition temperature of strongly interacting matter produced in non-central heavy ion collisions. By using a holographic description of an AdS black hole, the authors extend the analysis to the more realistic case where the matter spreads over a region around the rotational axis. The results show the coexistence of confined and deconfined phases and are consistent with the concept of local temperature in rotating frames developed by Tolman and Ehrenfest.
Article
Astronomy & Astrophysics
Bing Sun, Jiachen An, Zhoujian Cao
Summary: This paper investigates the effect of gravitational constant variation on the propagation of gravitational waves. By employing two analytical methods, the study finds that variations in the gravitational constant result in amplitude and phase corrections for gravitational waves, and the time variation of the gravitational constant can be constrained through the propagation of gravitational waves.
Article
Astronomy & Astrophysics
Abdellah Touati, Zaim Slimane
Summary: This letter presents the first study of Hawking radiation as a tunneling process within the framework of non-commutative gauge theory of gravity. The non-commutative Schwarzschild black hole is reconstructed using the Seiberg-Witten map and the star product. The emission spectrum of outgoing massless particles is computed using the quantum tunneling mechanism. The results reveal pure thermal radiation in the low-frequency scenario, but a deviation from pure thermal radiation in the high-frequency scenario due to energy conservation. It is also found that noncommutativity enhances the correlations between successively emitted particles.
Article
Astronomy & Astrophysics
Shahar Hod
Summary: The travel times of light signals between two antipodal points on a compact star's surface are calculated for two different trajectories. It is shown that, for highly dense stars, the longer trajectory along the surface may have a shorter travel time as measured by asymptotic observers. A critical value of the dimensionless density-area parameter is determined for constant density stars to distinguish cases where crossing through the star's center or following a semi-circular trajectory on the surface has a shorter travel time as measured by asymptotic observers.
