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Author |
Moretti, F.; Bombacigno, F.; Montani, G. |
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Title |
The Role of Longitudinal Polarizations in Horndeski and Macroscopic Gravity: Introducing Gravitational Plasmas |
Type |
Journal Article |
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Year |
2021 |
Publication |
Universe |
Abbreviated Journal |
Universe |
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Volume |
7 |
Issue |
12 |
Pages |
496 - 28pp |
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Keywords |
gravitational waves; gauge-invariant method; Landau damping; macroscopic gravity |
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Abstract |
We discuss some general and relevant features of longitudinal gravitational modes in Horndeski gravity and their interaction with matter media. Adopting a gauge-invariant formulation, we clarify how massive scalar and vector fields can induce additional transverse and longitudinal excitations, resulting in breathing, vector, and longitudinal polarizations. We review, then, the interaction of standard gravitational waves with a molecular medium, outlining the emergence of effective massive gravitons, induced by the net quadrupole moment due to molecule deformation. Finally, we investigate the interaction of the massive mode in Horndeski gravity with a noncollisional medium, showing that Landau damping phenomenon can occur in the gravitational sector as well. That allows us to introduce the concept of “gravitational plasma”, where inertial forces associated with the background field play the role of cold ions in electromagnetic plasma. |
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Address |
[Moretti, Fabio; Montani, Giovanni] Sapienza Univ Rome, Dept Phys, Ple Aldo Moro 5, I-00185 Rome, Italy, Email: fabio.moretti@uniroma1.it; |
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Mdpi |
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English |
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WOS:000741918900001 |
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no |
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Is ISI |
yes |
International Collaboration |
yes |
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Call Number |
IFIC @ pastor @ |
Serial |
5076 |
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Author |
Benisty, D.; Olmo, G.J.; Rubiera-Garcia, D. |
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Title |
Singularity-Free and Cosmologically Viable Born-Infeld Gravity with Scalar Matter |
Type |
Journal Article |
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Year |
2021 |
Publication |
Symmetry-Basel |
Abbreviated Journal |
Symmetry-Basel |
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Volume |
13 |
Issue |
11 |
Pages |
2108 - 24pp |
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Keywords |
metric-affine gravity; non-singular cosmologies; born-infeld gravity; observational constraints; scalar fields |
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Abstract |
The early cosmology, driven by a single scalar field, both massless and massive, in the context of Eddington-inspired Born-Infeld gravity, is explored. We show the existence of nonsingular solutions of bouncing and loitering type (depending on the sign of the gravitational theory's parameter, epsilon) replacing the Big Bang singularity, and discuss their properties. In addition, in the massive case, we find some new features of the cosmological evolution depending on the value of the mass parameter, including asymmetries in the expansion/contraction phases, or a continuous transition between a contracting phase to an expanding one via an intermediate loitering phase. We also provide a combined analysis of cosmic chronometers, standard candles, BAO, and CMB data to constrain the model, finding that for roughly |epsilon|& LSIM;5 & BULL;10-8m2 the model is compatible with the latest observations while successfully removing the Big Bang singularity. This bound is several orders of magnitude stronger than the most stringent constraints currently available in the literature. |
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[Benisty, David] Univ Cambridge, Ctr Math Sci, DAMTP, Wilberforce Rd, Cambridge CB3 0WA, England, Email: benidav@post.bgu.ac.il; |
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Mdpi |
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WOS:000726717400001 |
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no |
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Is ISI |
yes |
International Collaboration |
yes |
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Call Number |
IFIC @ pastor @ |
Serial |
5040 |
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Author |
Borja, E.F.; Garay, I.; Vidotto, F. |
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Title |
Learning about Quantum Gravity with a Couple of Nodes |
Type |
Journal Article |
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Year |
2012 |
Publication |
Symmetry Integrability and Geometry-Methods and Applications |
Abbreviated Journal |
Symmetry Integr. Geom. |
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Volume |
8 |
Issue |
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Pages |
015 - 44pp |
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Keywords |
discrete gravity; canonical quantization; spinors; spinfoam; quantum cosmology |
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Abstract |
Loop Quantum Gravity provides a natural truncation of the infinite degrees of freedom of gravity, obtained by studying the theory on a given finite graph. We review this procedure and we present the construction of the canonical theory on a simple graph, formed by only two nodes. We review the U(N) framework, which provides a powerful tool for the canonical study of this model, and a formulation of the system based on spinors. We consider also the covariant theory, which permits to derive the model from a more complex formulation, paying special attention to the cosmological interpretation of the theory. |
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[Borja, Enrique F.; Garay, Inaki] Univ Erlangen Nurnberg, Inst Theoret Phys 3, D-91058 Erlangen, Germany, Email: efborja@theorie3.physik.uni-erlangen.de; |
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Natl Acad Sci Ukraine, Inst Math |
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English |
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1815-0659 |
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Notes |
WOS:000303831400001 |
Approved |
no |
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Is ISI |
yes |
International Collaboration |
yes |
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Call Number |
IFIC @ pastor @ |
Serial |
1018 |
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Permanent link to this record |
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Author |
Beltran Jimenez, J.; Heisenberg, L.; Olmo, G.J.; Rubiera-Garcia, D. |
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Title |
Born-Infeld inspired modifications of gravity |
Type |
Journal Article |
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Year |
2018 |
Publication |
Physics Reports |
Abbreviated Journal |
Phys. Rep. |
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Volume |
727 |
Issue |
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Pages |
1-129 |
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Keywords |
Born-Infeld gravity; Astrophysics; Black holes; Cosmology; Early universe; Compact objects; Singularities |
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Abstract |
General Relativity has shown an outstanding observational success in the scales where it has been directly tested. However, modifications have been intensively explored in the regimes where it seems either incomplete or signals its own limit of validity. In particular, the breakdown of unitarity near the Planck scale strongly suggests that General Relativity needs to be modified at high energies and quantum gravity effects are expected to be important. This is related to the existence of spacetime singularities when the solutions of General Relativity are extrapolated to regimes where curvatures are large. In this sense, Born-Infeld inspired modifications of gravity have shown an extraordinary ability to regularise the gravitational dynamics, leading to non-singular cosmologies and regular black hole spacetimes in a very robust manner and without resorting to quantum gravity effects. This has boosted the interest in these theories in applications to stellar structure, compact objects, inflationary scenarios, cosmological singularities, and black hole and wormhole physics, among others. We review the motivations, various formulations, and main results achieved within these theories, including their observational viability, and provide an overview of current open problems and future research opportunities. |
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[Beltran Jimenez, Jose] Univ Autonoma Madrid, CSIC, Inst Fis Teor, E-28049 Madrid, Spain, Email: jose.beltran@uam.es; |
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Publisher |
Elsevier Science Bv |
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English |
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Series Volume |
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Series Issue |
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Edition |
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ISSN |
0370-1573 |
ISBN |
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Area |
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Expedition |
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Conference |
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Notes |
WOS:000425482900001 |
Approved |
no |
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Is ISI |
yes |
International Collaboration |
yes |
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Call Number |
IFIC @ pastor @ |
Serial |
3497 |
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Permanent link to this record |
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Author |
Olmo, G.J.; Rubiera-Garcia, D.; Wojnar, A. |
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Title |
Stellar structure models in modified theories of gravity: Lessons and challenges |
Type |
Journal Article |
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Year |
2020 |
Publication |
Physics Reports |
Abbreviated Journal |
Phys. Rep. |
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Volume |
876 |
Issue |
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Pages |
1-75 |
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Keywords |
Stellar structure; Modified gravity; Palatini formalism; Neutron stars; Brown dwarfs; Relativistic stars; Weak field; f(R) theories; Born-Infeld theory; Horndeski theory |
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Abstract |
The understanding of stellar structure represents the crossroads of our theories of the nuclear force and the gravitational interaction under the most extreme conditions observably accessible. It provides a powerful probe of the strong field regime of General Relativity, and opens fruitful avenues for the exploration of new gravitational physics. The latter can be captured via modified theories of gravity, which modify the Einstein-Hilbert action of General Relativity and/or some of its principles. These theories typically change the Tolman-Oppenheimer-Volkoff equations of stellar's hydrostatic equilibrium, thus having a large impact on the astrophysical properties of the corresponding stars and opening a new window to constrain these theories with present and future observations of different types of stars. For relativistic stars, such as neutron stars, the uncertainty on the equation of state of matter at supranuclear densities intertwines with the new parameters coming from the modified gravity side, providing a whole new phenomenology for the typical predictions of stellar structure models, such as mass-radius relations, maximum masses, or moment of inertia. For non-relativistic stars, such as white, brown and red dwarfs, the weakening/strengthening of the gravitational force inside astrophysical bodies via the modified Newtonian (Poisson) equation may induce changes on the star's mass, radius, central density or luminosity, having an impact, for instance, in the Chandrasekhar's limit for white dwarfs, or in the minimum mass for stable hydrogen burning in high-mass brown dwarfs. This work aims to provide a broad overview of the main such results achieved in the recent literature for many such modified theories of gravity, by combining the results and constraints obtained from the analysis of relativistic and non-relativistic stars in different scenarios. Moreover, we will build a bridge between the efforts of the community working on different theories, formulations, types of stars, theoretical modelings, and observational aspects, highlighting some of the most promising opportunities in the field. |
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Address |
[Olmo, Gonzalo J.] Univ Valencia, Dept Fis Teor, Ctr Mixto, CSIC, Valencia 46100, Spain, Email: gonzalo.olmo@uv.es; |
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Publisher |
Elsevier |
Place of Publication |
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Editor |
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Language |
English |
Summary Language |
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Original Title |
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Series Editor |
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Series Title |
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Abbreviated Series Title |
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Series Volume |
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Series Issue |
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Edition |
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ISSN |
0370-1573 |
ISBN |
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Area |
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Expedition |
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Conference |
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Notes |
WOS:000570298900001 |
Approved |
no |
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Is ISI |
yes |
International Collaboration |
yes |
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Call Number |
IFIC @ pastor @ |
Serial |
4531 |
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Permanent link to this record |