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Journal articles on the topic 'Extended theories of gravity'

Author: Grafiati
Published: 10 March 2023
Last updated: 14 March 2023

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1

Capozziello, Salvatore, and Mariafelicia De Laurentis. "Extended Theories of Gravity." Physics Reports 509, no. 4-5 (December 2011): 167–321. http://dx.doi.org/10.1016/j.physrep.2011.09.003.

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2

Fatibene, Lorenzo, and Simon Garruto. "Extended gravity." International Journal of Geometric Methods in Modern Physics 11, no. 07 (August 2014): 1460018. http://dx.doi.org/10.1142/s0219887814600184.

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We shall show equivalence between Palatini-f(ℛ) theories and Brans–Dicke (BD) theories at the level of action principles in generic dimension with generic matter coupling. We do that by introducing the Helmholtz Lagrangian associated to Palatini-f(ℛ) theory and then performing frame transformations in order to recover Einstein frame and BD frame. This clarifies the relation among different formulations and the transformations among different frames. Additionally, it defines a formulation a lá Palatini for the BD theory which is dynamically equivalent to metric BD (unlike the standard Palatini-formulation of metric BD theory which are not dynamically equivalent). In conclusion, we discuss interpretation of extended theories of gravitation and perspectives.
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3

Komada, Meguru. "Causality of 3D extended gravity theories." Modern Physics Letters A 34, no. 16 (May 29, 2019): 1950122. http://dx.doi.org/10.1142/s0217732319501220.

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Causality is one of the most important properties to understand gravity theories. It gives us not only a method to confirm that the gravity theories are really consistent, but also gives implications about the properties which unknown fundamental physics should obey. We investigate the causality of three-dimensional (3D) gravity theories, which are considered to be important, by using the Shapiro time delay effect in the Shock wave geometry. One of such gravity theories is the Zwei-Dreibein Gravity (ZDG) theory, which is a consistent 3D gravity theory. In ZDG theory, the serious problems can be removed that have appeared in another important gravity theory called New Massive Gravity (NMG). We study whether the ZDG theory could preserve the causality without losing the above good properties and how the causality structure is related to the structure of the NMG theory.
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4

Crisostomi, Marco, Kazuya Koyama, and Gianmassimo Tasinato. "Extended scalar-tensor theories of gravity." Journal of Cosmology and Astroparticle Physics 2016, no. 04 (April 21, 2016): 044. http://dx.doi.org/10.1088/1475-7516/2016/04/044.

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5

Sherf, Yotam. "Hyperbolicity constraints in extended gravity theories." Physica Scripta 94, no. 8 (June 4, 2019): 085005. http://dx.doi.org/10.1088/1402-4896/ab1352.

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6

de Mello, R. O. "Extended 2D generalized dilaton gravity theories." Classical and Quantum Gravity 25, no. 17 (August 13, 2008): 175003. http://dx.doi.org/10.1088/0264-9381/25/17/175003.

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7

Aschieri, Paolo, and Leonardo Castellani. "Extended gravity theories from dynamical noncommutativity." General Relativity and Gravitation 45, no. 2 (November 9, 2012): 411–26. http://dx.doi.org/10.1007/s10714-012-1479-4.

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8

Capozziello, Salvatore, Rocco D’Agostino, and Orlando Luongo. "Extended gravity cosmography." International Journal of Modern Physics D 28, no. 10 (July 2019): 1930016. http://dx.doi.org/10.1142/s0218271819300167.

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Cosmography can be considered as a sort of a model-independent approach to tackle the dark energy/modified gravity problem. In this review, the success and the shortcomings of the [Formula: see text]CDM model, based on General Relativity (GR) and standard model of particles, are discussed in view of the most recent observational constraints. The motivations for considering extensions and modifications of GR are taken into account, with particular attention to [Formula: see text] and [Formula: see text] theories of gravity where dynamics is represented by curvature or torsion field, respectively. The features of [Formula: see text] models are explored in metric and Palatini formalisms. We discuss the connection between [Formula: see text] gravity and scalar–tensor theories highlighting the role of conformal transformations in the Einstein and Jordan frames. Cosmological dynamics of [Formula: see text] models is investigated through the corresponding viability criteria. Afterwards, the equivalent formulation of GR (Teleparallel Equivalent General Relativity (TEGR)) in terms of torsion and its extension to [Formula: see text] gravity is considered. Finally, the cosmographic method is adopted to break the degeneracy among dark energy models. A novel approach, built upon rational Padé and Chebyshev polynomials, is proposed to overcome limits of standard cosmography based on Taylor expansion. The approach provides accurate model-independent approximations of the Hubble flow. Numerical analyses, based on Monte Carlo Markov Chain integration of cosmic data, are presented to bound coefficients of the cosmographic series. These techniques are thus applied to reconstruct [Formula: see text] and [Formula: see text] functions and to frame the late-time expansion history of the universe with no a priori assumptions on its equation-of-state. A comparison between the [Formula: see text]CDM cosmological model with [Formula: see text] and [Formula: see text] models is reported.
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9

Vinckers, U. K. Beckering, Á. de la Cruz-Dombriz, and F. J. Maldonado Torralba. "Focusing conditions for extended teleparallel gravity theories." Journal of Cosmology and Astroparticle Physics 2020, no. 12 (December 9, 2020): 020. http://dx.doi.org/10.1088/1475-7516/2020/12/020.

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10

Adami, Hamed, Mohammad Reza Setare, Tahsin Çağrı Şişman, and Bayram Tekin. "Conserved charges in extended theories of gravity." Physics Reports 834-835 (November 2019): 1–85. http://dx.doi.org/10.1016/j.physrep.2019.08.003.

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11

Capozziello, Salvatore, Carlo Alberto Mantica, and Luca Guido Molinari. "Geometric perfect fluids from Extended Gravity." Europhysics Letters 137, no. 1 (January 1, 2022): 19001. http://dx.doi.org/10.1209/0295-5075/ac525d.

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Abstract A main issue in cosmology and astrophysics is whether the dark sector phenomenology originates from particle physics, then requiring the detection of new fundamental components, or it can be addressed by modifying General Relativity. Extended Theories of Gravity are possible candidates aimed at framing dark energy and dark matter in a comprehensive geometric view. Considering the concept of perfect scalars, we show that the field equations of such theories naturally contain perfect fluid terms. Specific examples are developed for the Friedman-Lemaître-Roberson-Walker metric.
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12

Roshan, Mahmood, Indranil Banik, Neda Ghafourian, Ingo Thies, Benoit Famaey, Elena Asencio, and Pavel Kroupa. "Barred spiral galaxies in modified gravity theories." Monthly Notices of the Royal Astronomical Society 503, no. 2 (March 8, 2021): 2833–60. http://dx.doi.org/10.1093/mnras/stab651.

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ABSTRACT When bars form within galaxy formation simulations in the standard cosmological context, dynamical friction with dark matter (DM) causes them to rotate rather slowly. However, almost all observed galactic bars are fast in terms of the ratio between corotation radius and bar length. Here, we explicitly display an 8σ tension between the observed distribution of this ratio and that in the EAGLE simulation at redshift 0. We also compare the evolution of Newtonian galactic discs embedded in DM haloes to their evolution in three extended gravity theories: Milgromian Dynamics (MOND), a model of non-local gravity, and a scalar–tensor–vector gravity theory (MOG). Although our models start with the same initial baryonic distribution and rotation curve, the long-term evolution is different. The bar instability happens more violently in MOND compared to the other models. There are some common features between the extended gravity models, in particular the negligible role played by dynamical friction − which plays a key role in the DM model. Partly for this reason, all extended gravity models predict weaker bars and faster bar pattern speeds compared to the DM case. Although the absence of strong bars in our idealized, isolated extended gravity simulations is in tension with observations, they reproduce the strong observational preference for ‘fast’ bar pattern speeds, which we could not do with DM. We confirm previous findings that apparently ‘ultrafast’ bars can be due to bar-spiral arm alignment leading to an overestimated bar length, especially in extended gravity scenarios where the bar is already fast.
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13

Choudhury, S., M. Sen, and S. Sadhukhan. "From Extended Theories of Gravity to Dark Matter." Acta Physica Polonica B Proceedings Supplement 9, no. 4 (2016): 789. http://dx.doi.org/10.5506/aphyspolbsupp.9.789.

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14

Izadi, A., and A. Shojai. "The speed of light in extended gravity theories." Classical and Quantum Gravity 26, no. 19 (September 22, 2009): 195006. http://dx.doi.org/10.1088/0264-9381/26/19/195006.

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15

García-Bellido, J., and M. Quirós. "Extended inflation in scalar-tensor theories of gravity." Physics Letters B 243, no. 1-2 (June 1990): 45–51. http://dx.doi.org/10.1016/0370-2693(90)90954-5.

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16

DE LAURENTIS, M., S. CAPOZZIELLO, and G. BASINI. "GRAVITATIONAL CHERENKOV RADIATION FROM EXTENDED THEORIES OF GRAVITY." Modern Physics Letters A 27, no. 23 (July 24, 2012): 1250136. http://dx.doi.org/10.1142/s0217732312501362.

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We linearize the field equations for higher order theories of gravity that contain scalar invariants other than the Ricci scalar. We find that besides a massless spin-2 field (the standard graviton), the theory contains also spin-0 and spin-2 massive modes with the latter being, in general, ghost modes. The rate at which such particles would emit gravitational Cherenkov radiation is calculated for some interesting physical cases.
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17

Mimoso, J. P., F. S. N. Lobo, and S. Capozziello. "Extended Theories of Gravity with Generalized Energy Conditions." Journal of Physics: Conference Series 600 (April 28, 2015): 012047. http://dx.doi.org/10.1088/1742-6596/600/1/012047.

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18

Wands, David. "Extended gravity theories and the Einstein--Hilbert action." Classical and Quantum Gravity 11, no. 1 (January 1, 1994): 269–79. http://dx.doi.org/10.1088/0264-9381/11/1/025.

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19

Astashenok, Artyom V., Salvatore Capozziello, and Sergei D. Odintsov. "Extreme neutron stars from Extended Theories of Gravity." Journal of Cosmology and Astroparticle Physics 2015, no. 01 (January 7, 2015): 001. http://dx.doi.org/10.1088/1475-7516/2015/01/001.

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20

Bhattacharjee, Snehasish. "Gravitational baryogenesis in extended teleparallel theories of gravity." Physics of the Dark Universe 30 (December 2020): 100612. http://dx.doi.org/10.1016/j.dark.2020.100612.

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21

Capozziello, S., R. de Ritis, and A. A. Marino. "Recovering the Effective Cosmological Constant in Extended Gravity Theories." General Relativity and Gravitation 30, no. 8 (August 1998): 1247–72. http://dx.doi.org/10.1023/a:1026651129626.

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22

Englert, Francois, Marc Henneaux, and Laurent Houart. "From very-extended to overextended gravity and M-theories." Journal of High Energy Physics 2005, no. 02 (March 9, 2005): 070. http://dx.doi.org/10.1088/1126-6708/2005/02/070.

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23

Allemandi, Gianluca, and Matteo Luca Ruggiero. "Constraining extended theories of gravity using Solar System tests." General Relativity and Gravitation 39, no. 9 (May 22, 2007): 1381–88. http://dx.doi.org/10.1007/s10714-007-0441-3.

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24

Giacchini, Breno L., and Ilya L. Shapiro. "Light bending in F[g(□)R] extended gravity theories." Physics Letters B 780 (May 2018): 54–60. http://dx.doi.org/10.1016/j.physletb.2018.02.055.

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25

Tretyakov, Petr V. "Dynamical stability of extended teleparallel gravity." Modern Physics Letters A 31, no. 14 (May 5, 2016): 1650085. http://dx.doi.org/10.1142/s0217732316500851.

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We discuss modified teleparallel gravity with function [Formula: see text] in the action, where the function depends on two arguments: torsion scalar [Formula: see text] and analogue of Gauss–Bonnet invariant [Formula: see text]. In contradistinction to usual teleparallel gravity [Formula: see text], this theory contains higher derivative terms, which may produce different instabilities. We discuss Minkowski stability problem in such kind of theories and explicitly demonstrate that for stability it must be [Formula: see text], [Formula: see text]. We apply these restrictions for the few types of functions discussed by the early authors.
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26

Borka, Duško, Vesna Borka Jovanović, Salvatore Capozziello, Alexander F. Zakharov, and Predrag Jovanović. "Estimating the Parameters of Extended Gravity Theories with the Schwarzschild Precession of S2 Star." Universe 7, no. 11 (October 28, 2021): 407. http://dx.doi.org/10.3390/universe7110407.

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After giving a short overview of previous results on constraining of Extended Gravity by stellar orbits, we discuss the Schwarzschild orbital precession of S2 star assuming the congruence with predictions of General Relativity (GR). At the moment, the S2 star trajectory is remarkably fitted with the first post-Newtonian approximation of GR. In particular, both Keck and VLT (GRAVITY) teams declared that the gravitational redshift near its pericenter passage for the S2 star orbit corresponds to theoretical estimates found with the first post-Newtonian (pN) approximation. In 2020, the GRAVITY Collaboration detected the orbital precession of the S2 star around the supermassive black hole (SMBH) at the Galactic Center and showed that it is close to the GR prediction. Based on this observational fact, we evaluated parameters of the Extended Gravity theories with the Schwarzschild precession of the S2 star. Using the mentioned method, we estimate the orbital precession angles for some Extended Gravity models including power-law f(R), general Yukawa-like corrections, scalar–tensor gravity, and non-local gravity theories formulated in both metric and Palatini formalism. In this consideration, we assume that a gravitational field is spherically symmetric, therefore, alternative theories of gravity could be described only with a few parameters. Specifically, considering the orbital precession, we estimate the range of parameters of these Extended Gravity models for which the orbital precession is like in GR. Then we compare these results with our previous results, which were obtained by fitting the simulated orbits of S2 star to its observed astrometric positions. In case of power-law f(R), generic Yukawa-like correction, scalar–tensor gravity and non-local gravity theories, we were able to obtain a prograde orbital precession, like in GR. According to these results, the method is a useful tool to evaluate parameters of the gravitational potential at the Galactic Center.
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27

Scipioni, R. "Isomorphism between non-Riemannian gravity and Einstein-Proca-Weyl theories extended to a class of scalar gravity theories." Classical and Quantum Gravity 16, no. 7 (January 1, 1999): 2471–78. http://dx.doi.org/10.1088/0264-9381/16/7/320.

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28

Kibaroğlu, Salih, Oktay Cebecioğlu, and Ahmet Saban. "Gauging the Maxwell Extended GLn,R and SLn+1,R Algebras." Symmetry 15, no. 2 (February 9, 2023): 464. http://dx.doi.org/10.3390/sym15020464.

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We consider the extension of the general-linear and special-linear algebras by employing the Maxwell symmetry in D space-time dimensions. We show how various Maxwell extensions of the ordinary space-time algebras can be obtained by a suitable contraction of generalized algebras. The extended Lie algebras could be useful in the construction of generalized gravity theories and the objects that couple to them. We also consider the gravitational dynamics of these algebras in the framework of the gauge theories of gravity. By adopting the symmetry-breaking mechanism of the Stelle–West model, we present some modified gravity models that contain the generalized cosmological constant term in four dimensions.
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29

Suvorov, A. G. "Signatures of Extended Theories of Gravity in Black Hole Oscillations." Acta Physica Polonica B Proceedings Supplement 13, no. 2 (2020): 283. http://dx.doi.org/10.5506/aphyspolbsupp.13.283.

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30

Akbar, M. "Generalized Second Law of Thermodynamics in Extended Theories of Gravity." International Journal of Theoretical Physics 48, no. 9 (June 9, 2009): 2672–78. http://dx.doi.org/10.1007/s10773-009-0056-4.

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31

Capozziello, Salvatore, and Mauro Francaviglia. "Extended theories of gravity and their cosmological and astrophysical applications." General Relativity and Gravitation 40, no. 2-3 (December 19, 2007): 357–420. http://dx.doi.org/10.1007/s10714-007-0551-y.

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32

Perivolaropoulos, Leandros. "Accelerating universe: Recent observations and implications for extended gravity theories." Journal of Physics: Conference Series 189 (October 1, 2009): 012031. http://dx.doi.org/10.1088/1742-6596/189/1/012031.

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33

Capozziello, Salvatore, and Mariafelicia De Laurentis. "Noether symmetries in extended gravity quantum cosmology." International Journal of Geometric Methods in Modern Physics 11, no. 02 (February 2014): 1460004. http://dx.doi.org/10.1142/s0219887814600044.

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We summarize the use of Noether symmetries in Minisuperspace Quantum Cosmology. In particular, we consider minisuperspace models, showing that the existence of conserved quantities gives selection rules that allow to recover classical behaviors in cosmic evolution according to the so-called Hartle criterion. Such a criterion selects correlated regions in the configuration space of dynamical variables whose meaning is related to the emergence of classical observable universes. Some minisuperspace models are worked out starting from Extended Gravity, in particular coming from scalar-tensor, f(R) and f(T) theories. Exact cosmological solutions are derived.
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34

Kadoyoshi, Tomoko, and Shin'ichi Nojiri. "N=3 and N=4 Two-Form Supergravities." Modern Physics Letters A 12, no. 16 (May 30, 1997): 1165–74. http://dx.doi.org/10.1142/s0217732397001199.

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We construct the Lagrangians of N=3 and N=4 two-form supergravities. The two-form gravity theories are classically equivalent to the Einstein gravity theories and can be formulated as gauge theories. The gauge algebras used here can be identified with the subalgebra of N=3 superconformal algebra and SU (2)× SU (2)× U(1) -extended N=4 superconformal algebra.
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35

Capozziello, Salvatore, and Francesco Bajardi. "Gravitational waves in modified gravity." International Journal of Modern Physics D 28, no. 05 (April 2019): 1942002. http://dx.doi.org/10.1142/s0218271819420021.

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Gravitational waves (GWs) in Modified Gravity are discussed and compared with General Relativity in view of possible detection of new modes. After a summary of some classes of modified theories, we recall GWs properties in General Relativity as the basic standard by which to compare any modified theory. Then, we discuss GWs in Extended Theories like [Formula: see text] gravity, scalar–tensor gravity, Gauss–Bonnet gravity and other gauge theories, as well as Teleparallel Equivalent General Relativity (TEGR), taking into account also torsion instead of curvature, to describe the gravitational field. As a general remark, all these theories lead to new polarizations in addition to the two standard modes of General Relativity. It is possible to show that, in 4 dimensions, the polarizations are at most 6. Theoretical and experimental implications of new modes are briefly discussed.
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36

ALDAYA, V., and E. SÁNCHEZ-SASTRE. "GAUGE THEORIES OF GRAVITY AND MASS GENERATION." International Journal of Geometric Methods in Modern Physics 05, no. 02 (March 2008): 197–232. http://dx.doi.org/10.1142/s0219887808002710.

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The compensating Utiyama's method including space-time symmetries is revisited as well as the gauge gravitational theories associated with translation, Poincaré and Weyl groups. Then we propose an extension of the gauge symmetry, allowing for the incorporation of the gauge group parameters into the theory as dynamical fields by considering the jet-gauge group as fundamental symmetry. As a consequence, a natural mass-generating mechanism for the gauge potentials arises without damaging gauge invariance. We also present, as a simple example, some sort of generalized Stueckelberg model for the Weyl group, thus accounting for massive dilatonic gauge field. Finally, the standard diffeomorphism symmetry of gravitation is extended by resorting to the jet-diffeomorphism group, formalism which helps to fix the Hilbert-Einstein Lagrangian in the teleparallelism version.
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37

Järv, Laur, Manuel Hohmann, Martin Krššák, and Christian Pfeifer. "Flat Connection for Rotating Vacuum Spacetimes in Extended Teleparallel Gravity Theories." Universe 5, no. 6 (June 10, 2019): 142. http://dx.doi.org/10.3390/universe5060142.

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Teleparallel geometry utilizes Weitzenböck connection which has nontrivial torsion but no curvature and does not directly follow from the metric like Levi–Civita connection. In extended teleparallel theories, for instance in f ( T ) or scalar-torsion gravity, the connection must obey its antisymmetric field equations. Thus far, only a few analytic solutions were known. In this note, we solve the f ( T , ϕ ) gravity antisymmetric vacuum field equations for a generic rotating tetrad ansatz in Weyl canonical coordinates, and find the corresponding spin connection coefficients. By a coordinate transformation, we present the solution also in Boyer–Lindquist coordinates, often used to study rotating solutions in general relativity. The result hints for the existence of another branch of rotating solutions besides the Kerr family in extended teleparallel gravities.
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38

Allemandi, Gianluca, Monica Capone, Salvatore Capozziello, and Mauro Francaviglia. "Conformal aspects of the Palatini approach in Extended Theories of Gravity." General Relativity and Gravitation 38, no. 1 (January 2006): 33–60. http://dx.doi.org/10.1007/s10714-005-0208-7.

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39

Prokopov, Vjacheslav, and Stanislav Alexeyev. "Shadow from a rotating black hole in an extended gravity." International Journal of Modern Physics A 35, no. 02n03 (January 30, 2020): 2040060. http://dx.doi.org/10.1142/s0217751x20400606.

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We focus on the consequences of that the Event Horizon Telescope obtained images of the black hole shadow in the center of the M87 galaxy. We show that to test extended theories of gravity the improving of the resolution by 3 orders is necessary. In addition it is demonstrated that the rotation distorts the shape of the shadow and corrections from the extended gravity may affect on this distortion.
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40

PERRET, R. E. C. "THREE-DIMENSIONAL FIELD THEORIES FROM INFINITE-DIMENSIONAL LIE ALGEBRAS." International Journal of Modern Physics A 09, no. 23 (September 20, 1994): 4063–76. http://dx.doi.org/10.1142/s0217751x94001643.

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A procedure for constructing topological actions from centrally extended Lie algebras is introduced. For a Kac–Moody algebra, this produces the three-dimensional Chern–Simons theory, while for the Virasoro algebra, the result is a new three-dimensional topological field theory whose physical states satisfy the Virasoro Ward identity. This topological field theory is shown to be a first order formulation of two-dimensional induced gravity in the chiral gauge. The extension to W3 gravity is discussed.
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41

CAPOZZIELLO, S., M. DE LAURENTIS, L. FATIBENE, and M. FRANCAVIGLIA. "THE PHYSICAL FOUNDATIONS FOR THE GEOMETRIC STRUCTURE OF RELATIVISTIC THEORIES OF GRAVITATION: FROM GENERAL RELATIVITY TO EXTENDED THEORIES OF GRAVITY THROUGH EHLERS–PIRANI–SCHILD APPROACH." International Journal of Geometric Methods in Modern Physics 09, no. 08 (October 29, 2012): 1250072. http://dx.doi.org/10.1142/s0219887812500727.

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We discuss in a critical way the physical foundations of geometric structure of relativistic theories of gravity by the so-called Ehlers–Pirani–Schild formalism. This approach provides a natural interpretation of the observables showing how relate them to General Relativity and to a large class of Extended Theories of Gravity. In particular we show that, in such a formalism, geodesic and causal structures of space-time can be safely disentangled allowing a correct analysis in view of observations and experiment. As specific case, we take into account the case of f(R)-gravity.
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42

Shu, Fu-Wen, and Tao Zhang. "Extended Hořava Gravity with Physical Ground-State Wavefunction." Symmetry 13, no. 1 (January 8, 2021): 100. http://dx.doi.org/10.3390/sym13010100.

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We propose a new extended theory of Hořava gravity based on the following three conditions: (i) power-counting renormalizable, (ii) healthy IR behavior and (iii) a stable vacuum state in a quantized version of the theory. Compared with other extended theories, we stress that any realistic theory of gravity must have physical ground states when quantization is performed. To fulfill the three conditions, we softly break the detailed balance but keep its basic structure unchanged. It turns out that the new model constructed in this way can avoid the strong coupling problem and remains power-counting renormalizable, moreover, it has a stable vacuum state by an appropriate choice of parameters.
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43

Shu, Fu-Wen, and Tao Zhang. "Extended Hořava Gravity with Physical Ground-State Wavefunction." Symmetry 13, no. 1 (January 8, 2021): 100. http://dx.doi.org/10.3390/sym13010100.

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We propose a new extended theory of Hořava gravity based on the following three conditions: (i) power-counting renormalizable, (ii) healthy IR behavior and (iii) a stable vacuum state in a quantized version of the theory. Compared with other extended theories, we stress that any realistic theory of gravity must have physical ground states when quantization is performed. To fulfill the three conditions, we softly break the detailed balance but keep its basic structure unchanged. It turns out that the new model constructed in this way can avoid the strong coupling problem and remains power-counting renormalizable, moreover, it has a stable vacuum state by an appropriate choice of parameters.
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44

Momeni, D., R. Myrzakulov, and E. Güdekli. "Cosmological viable mimetic f(R) and f(R, T) theories via Noether symmetry." International Journal of Geometric Methods in Modern Physics 12, no. 10 (October 25, 2015): 1550101. http://dx.doi.org/10.1142/s0219887815501017.

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Extended f(R) theories of gravity have been investigated from the symmetry point of view. We briefly has been investigated Noether symmetry of two types of extended f(R) theories: f(R, T) theory, in which curvature is coupled non-minimally to the trace of energy–momentum tensor Tμν and mimetic f(R) gravity, a theory with a scalar field degree of freedom, but ghost-free and with internal conformal symmetry. In both cases we write point-like Lagrangian for flat Friedmann–Lemaitre–Robertson–Walker (FLRW) cosmological background in the presence of ordinary matter. We have been shown that some classes of models existed with Noether symmetry in these viable extensions of f(R) gravity. As a motivated idea, we have been investigating the stability of the solutions and the bouncing and ΛCDM models using the Noether symmetries. We have been shown that in mimetic f(R) gravity bouncing and ΛCDM solutions are possible. Also a class of solutions with future singularities has been investigated.
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45

Papini, Giorgio. "Covariance and gauge invariance in relativistic theories of gravity." Modern Physics Letters A 29, no. 14 (May 8, 2014): 1450075. http://dx.doi.org/10.1142/s0217732314500758.

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Any metric theory of gravity whose interaction with quantum particles is described by a covariant wave equation is equivalent to a vector theory that satisfies Maxwell-type equations identically. This result does not depend on any particular set of field equations for the metric tensor, but only on covariance. It is derived in the linear case, but can be extended to any order of approximation in the metric deviation. In this formulation of the interaction of gravity with matter, angular momentum and momentum are conserved locally.
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46

HOSONO, SHINOBU. "ALGEBRAIC DEFINITION OF TOPOLOGICAL W GRAVITY." International Journal of Modern Physics A 07, no. 21 (August 20, 1992): 5193–211. http://dx.doi.org/10.1142/s0217751x92002374.

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We propose a definition of the topological W gravity using some properties of the principal three-dimensional subalgebra of a simple Lie algebra due to Kostant. In our definition, structures of the two-dimensional topological gravity are naturally embedded in the extended theories. In accordance with the definition, we will present some explicit calculations for the W3 gravity.
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47

Miranda, Marcello, Daniele Vernieri, Salvatore Capozziello, and Francisco S. N. Lobo. "Bouncing Cosmology in Fourth-Order Gravity." Universe 8, no. 3 (March 4, 2022): 161. http://dx.doi.org/10.3390/universe8030161.

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The Big Bang initial singularity problem can be solved by means of bouncing solutions. In the context of extended theories of gravity, we will look for covariant effective actions whose field equations contain up to fourth-order derivatives of the metric tensor. In finding such bouncing solutions, we will make use of an order reduction technique based on a perturbative approach. Reducing the order of the field equations to second-order, we are able to find solutions which are perturbatively close to General Relativity. We will build the covariant effective actions of the resulting order reduced theories.
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48

Capozziello, Salvatore, D. J. Cirilo-Lombardo, and Mariafelicia De Laurentis. "The affine structure of gravitational theories: Symplectic groups and geometry." International Journal of Geometric Methods in Modern Physics 11, no. 10 (November 2014): 1450081. http://dx.doi.org/10.1142/s0219887814500819.

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We give a geometrical description of gravitational theories from the viewpoint of symmetries and affine structure. We show how gravity, considered as a gauge theory, can be consistently achieved by the nonlinear realization of the conformal-affine group in an indirect manner: due to the partial isomorphism between CA(3, 1) and the centrally extended Sp( 8), we perform a nonlinear realization of the centrally extended ( CE )Sp( 8) in its semi-simple version. In particular, starting from the bundle structure of gravity, we derive the conformal-affine Lie algebra and then, by the nonlinear realization, we define the coset field transformations, the Cartan forms and the inverse Higgs constraints. Finally, we discuss the geometrical Lagrangians where all the information on matter fields and their interactions can be contained.
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49

JEAN-MARC, LINA, and PRASANTA K. PANIGRAHI. "LAX-PAIR FORMULATION OF THE W-GRAVITY THEORIES IN TWO DIMENSIONS." International Journal of Modern Physics A 07, no. 27 (October 30, 1992): 6907–32. http://dx.doi.org/10.1142/s0217751x92003185.

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The Lax-pair formulation of the two-dimensional induced gravity in the light-cone gauge is extended to the more general wN theories. After presenting the w2 and w3 gravities, we give a general prescription for an arbitrary wN case. This is further illustrated with the w4 gravity to point out some peculiarities. The constraints and the possible presence of the cosmological constants are systematically exhibited in the zero-curvature condition, which also yields the relevant Ward identities. The restrictions on the gauge parameters in the presence of the constraints are pointed out too, and are contrasted with those of the ordinary 2D gravity.
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50

VACARU, SERGIU I. "PRINCIPLES OF EINSTEIN–FINSLER GRAVITY AND PERSPECTIVES IN MODERN COSMOLOGY." International Journal of Modern Physics D 21, no. 09 (September 2012): 1250072. http://dx.doi.org/10.1142/s0218271812500721.

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We study the geometric and physical foundations of Finsler gravity theories with metric compatible connections defined on tangent bundles, or (pseudo) Riemannian manifolds, endowed with nonholonomic frame structure. Several generalizations and alternatives to Einstein gravity are considered, including modifications with broken local Lorentz invariance. It is also shown how such theories (and general relativity) can be equivalently re-formulated in Finsler like variables. We focus on prospects in modern cosmology and Finsler acceleration of Universe. Einstein–Finsler gravity theories are elaborated following almost the same principles as in the general relativity theory but extended to Finsler metrics and connections. Finally, some examples of generic off-diagonal metrics and generalized connections, defining anisotropic cosmological Einstein–Finsler spaces are analyzed; certain criteria for the Finsler accelerating evolution are formulated.
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