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Author Bazeia, D.; Marques, M.A.; Olmo, G.J. url  doi
openurl 
  Title (up) Small and hollow magnetic monopoles Type Journal Article
  Year 2018 Publication Physical Review D Abbreviated Journal Phys. Rev. D  
  Volume 98 Issue 2 Pages 025017 - 8pp  
  Keywords  
  Abstract We deal with the presence of magnetic monopoles in a non-Abelian model that generalizes the standard 't Hooft-Polyakov model in three spatial dimensions. We investigate the energy density of the static and spherically symmetric solutions to find first order differential equations that solve the equations of motion. The system is further studied and two distinct classes of solutions are obtained, one that can also be described by analytical solutions and is called a small monopole, since it is significantly smaller than the standard 't Hooft-Polyakov monopole. The other type of structure is the hollow monopole, since the energy density is endowed with a hole at its core. The hollow monopole can be smaller or larger than the standard monopole, depending on the value of the parameter that controls the magnetic permeability of the model.  
  Address [Bazeia, D.; Marques, M. A.; Olmo, Gonzalo J.] Univ Fed Paraiba, Dept Fis, BR-58051970 Joao Pessoa, Paraiba, Brazil  
  Corporate Author Thesis  
  Publisher Amer Physical Soc Place of Publication Editor  
  Language English Summary Language Original Title  
  Series Editor Series Title Abbreviated Series Title  
  Series Volume Series Issue Edition  
  ISSN 2470-0010 ISBN Medium  
  Area Expedition Conference  
  Notes WOS:000439545000007 Approved no  
  Is ISI yes International Collaboration yes  
  Call Number IFIC @ pastor @ Serial 3672  
Permanent link to this record
 

 
Author Olmo, G.J.; Rubiera-Garcia, D. url  doi
openurl 
  Title (up) Some recent results on Ricci-based gravity theories Type Journal Article
  Year 2022 Publication International Journal of Modern Physics D Abbreviated Journal Int. J. Mod. Phys. D  
  Volume 31 Issue Pages 2240012 - 15pp  
  Keywords Metric-affine gravity; scalar fields; stellar models; junction conditions; compact objects  
  Abstract In this paper, metric-afline theories in which the gravity Lagrangian is built using (projectively invariant) contractions of the Ricci tensor with itself and with the metric (Ricci-based gravity theories, or RBGs for short) are reviewed. The goal is to provide a contextualized and coherent presentation of some recent results. In particular, we focus on the correspondence that exists between the field equations of these theories and those of general relativity, and comment on how this can be used to build new solutions of physical interest. We also discuss the formalism of junction conditions in the f (R) case, and provide a brief summary on current experimental and observational bounds on model parameters.  
  Address [Olmo, Gonzalo J.] Univ Valencia, CSIC, Ctr Mixto, Dept Fis Teor, Valencia 46100, Spain, Email: gonzalo.olmo@uv.es;  
  Corporate Author Thesis  
  Publisher World Scientific Publ Co Pte Ltd Place of Publication Editor  
  Language English Summary Language Original Title  
  Series Editor Series Title Abbreviated Series Title  
  Series Volume Series Issue Edition  
  ISSN 0218-2718 ISBN Medium  
  Area Expedition Conference  
  Notes WOS:000848888900001 Approved no  
  Is ISI yes International Collaboration yes  
  Call Number IFIC @ pastor @ Serial 5350  
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Author Delhom, A.; Mariz, T.; Nascimento, J.R.; Olmo, G.J.; Petrov, A.Y.; Porfirio, P.J. url  doi
openurl 
  Title (up) Spontaneous Lorentz symmetry breaking and one-loop effective action in the metric-affine bumblebee gravity Type Journal Article
  Year 2022 Publication Journal of Cosmology and Astroparticle Physics Abbreviated Journal J. Cosmol. Astropart. Phys.  
  Volume 07 Issue 7 Pages 018 - 27pp  
  Keywords gravity; modified gravity; quantum field theory on curved space; Quantum fields in curved spacetimes  
  Abstract The metric-affine bumblebee model in the presence of fermionic matter minimally coupled to the connection is studied. We show that the model admits an Einstein frame representation in which the matter sector is described by a non-minimal Dirac action without any analogy in the literature. Such non-minimal terms involve unconventional couplings between the bumblebee and the fermion field. We then rewrite the quadratic fermion action in the Einstein frame in the basis of 16 Dirac matrices in order to identify the coefficients for Lorentz/CPT violation in all orders of the non-minimal coupling xi. The exact result for the fermionic determinant in the Einstein frame, including all orders in xi, is also provided. We demonstrate that the axial contributions are at least of second order in the perturbative expansion of xi. Furthermore, we compute the one-loop effective potential within the weak field approximation.  
  Address [Delhom, Adria] Univ Tartu, Inst Phys, Lab Theoret Phys, W Ostwaldi 1, EE-50411 Tartu, Estonia, Email: adria.delhom@gmail.com;  
  Corporate Author Thesis  
  Publisher IOP Publishing Ltd Place of Publication Editor  
  Language English Summary Language Original Title  
  Series Editor Series Title Abbreviated Series Title  
  Series Volume Series Issue Edition  
  ISSN 1475-7516 ISBN Medium  
  Area Expedition Conference  
  Notes WOS:000834157900011 Approved no  
  Is ISI yes International Collaboration yes  
  Call Number IFIC @ pastor @ Serial 5314  
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Author Olmo, G.J.; Sanchis-Alepuz, H.; Tripathi, S. url  doi
openurl 
  Title (up) Stellar structure equations in extended Palatini gravity Type Journal Article
  Year 2012 Publication Physical Review D Abbreviated Journal Phys. Rev. D  
  Volume 86 Issue 10 Pages 104039 - 8pp  
  Keywords  
  Abstract We consider static spherically symmetric stellar configurations in Palatini theories of gravity in which the Lagrangian is an unspecified function of the form f(R, R μnu R μnu). We obtain the Tolman-Oppenheimer-Volkov equations corresponding to this class of theories and show that they recover those of f(R) theories and general relativity in the appropriate limits. We show that the exterior vacuum solutions are of Schwarzschild-de Sitter type and comment on the possible expected modifications, as compared to general relativity, of the interior solutions.  
  Address [Olmo, Gonzalo J.; Sanchis-Alepuz, Helios] Univ Valencia, CSIC, Fac Fis, Dept Fis Teor, E-46100 Valencia, Spain, Email: gonzalo.olmo@csic.es;  
  Corporate Author Thesis  
  Publisher Amer Physical Soc Place of Publication Editor  
  Language English Summary Language Original Title  
  Series Editor Series Title Abbreviated Series Title  
  Series Volume Series Issue Edition  
  ISSN 1550-7998 ISBN Medium  
  Area Expedition Conference  
  Notes WOS:000311143500004 Approved no  
  Is ISI yes International Collaboration yes  
  Call Number IFIC @ pastor @ Serial 1222  
Permanent link to this record
 

 
Author Olmo, G.J.; Rubiera-Garcia, D.; Wojnar, A. url  doi
openurl 
  Title (up) Stellar structure models in modified theories of gravity: Lessons and challenges Type Journal Article
  Year 2020 Publication Physics Reports Abbreviated Journal Phys. Rep.  
  Volume 876 Issue Pages 1-75  
  Keywords Stellar structure; Modified gravity; Palatini formalism; Neutron stars; Brown dwarfs; Relativistic stars; Weak field; f(R) theories; Born-Infeld theory; Horndeski theory  
  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.  
  Address [Olmo, Gonzalo J.] Univ Valencia, Dept Fis Teor, Ctr Mixto, CSIC, Valencia 46100, Spain, Email: gonzalo.olmo@uv.es;  
  Corporate Author Thesis  
  Publisher Elsevier Place of Publication Editor  
  Language English Summary Language Original Title  
  Series Editor Series Title Abbreviated Series Title  
  Series Volume Series Issue Edition  
  ISSN 0370-1573 ISBN Medium  
  Area Expedition Conference  
  Notes WOS:000570298900001 Approved no  
  Is ISI yes International Collaboration yes  
  Call Number IFIC @ pastor @ Serial 4531  
Permanent link to this record
 

 
Author Lobo, F.S.N.; Olmo, G.J.; Orazi, E.; Rubiera-Garcia, D.; Rustam, A. url  doi
openurl 
  Title (up) Structure and stability of traversable thin-shell wormholes in Palatini f(R) gravity Type Journal Article
  Year 2020 Publication Physical Review D Abbreviated Journal Phys. Rev. D  
  Volume 102 Issue 10 Pages 104012 - 11pp  
  Keywords  
  Abstract We study the structure and stability of traversable wormholes built as (spherically symmetric) thin shells in the context of Palatini f(R) gravity. Using a suitable junction formalism for these theories we find that the effective number of degrees of freedom on the shell is reduced to a single one, which fixes the equation of state to be that of massless stress-energy fields, contrary to the general relativistic and metric f(R) cases. Another major difference is that the surface energy density threading the thin shell, needed in order to sustain the wormhole, can take any sign and may even vanish, depending on the desired features of the corresponding solutions. We illustrate our results by constructing thin-shell wormholes by surgically grafting Schwarzschild space-times and show that these configurations are always linearly unstable. However, surgically joined Reissner-Nordstrom space-times allow for linearly stable, traversable thin-shell wormholes supported by a positive energy density provided that the (squared) mass-to-charge ratio, given by y = Q(2)/M-2, satisfies the constraint 1 < y < 9/8 (corresponding to overcharged Reissner-Nordstrom configurations having a photon sphere) and lies in a region bounded by specific curves defined in terms of the (dimensionless) radius of the shell x(0) = R/M.  
  Address [Lobo, Francisco S. N.] Univ Lisbon, Fac Ciencias, Inst Astrofis & Ciencias Espaco, Edificio C8,Campo Grande, P-1749016 Lisbon, Portugal, Email: fslobo@fc.ul.pt;  
  Corporate Author Thesis  
  Publisher Amer Physical Soc Place of Publication Editor  
  Language English Summary Language Original Title  
  Series Editor Series Title Abbreviated Series Title  
  Series Volume Series Issue Edition  
  ISSN 2470-0010 ISBN Medium  
  Area Expedition Conference  
  Notes WOS:000587286200006 Approved no  
  Is ISI yes International Collaboration yes  
  Call Number IFIC @ pastor @ Serial 4596  
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Author Beltran Jimenez, J.; Heisenberg, L.; Olmo, G.J. url  doi
openurl 
  Title (up) Tensor perturbations in a general class of Palatini theories Type Journal Article
  Year 2015 Publication Journal of Cosmology and Astroparticle Physics Abbreviated Journal J. Cosmol. Astropart. Phys.  
  Volume 06 Issue 6 Pages 026 - 16pp  
  Keywords modified gravity; inflation; gravity; dark energy theory  
  Abstract We study a general class of gravitational theories formulated in the Palatini approach and derive the equations governing the evolution of tensor perturbations. In the absence of torsion, the connection can be solved as the Christoffel symbols of an auxiliary metric which is non-trivially related to the space-time metric. We then consider background solutions corresponding to a perfect fluid and show that the tensor perturbations equations (including anisotropic stresses) for the auxiliary metric around such a background take an Einstein-like form. This facilitates the study in a homogeneous and isotropic cosmological scenario where we explicitly establish the relation between the auxiliary metric and the spacetime metric tensor perturbations. As a general result, we show that both tensor perturbations coincide in the absence of anisotropic stresses.  
  Address [Jimenez, Jose Beltran] Univ Louvain, Inst Math & Phys, Ctr Cosmol Particle Phys & Phenomenol, B-1318 Louvain, Belgium, Email: jose.beltran@cpt.univ.mrs.fr;  
  Corporate Author Thesis  
  Publisher Iop Publishing Ltd Place of Publication Editor  
  Language English Summary Language Original Title  
  Series Editor Series Title Abbreviated Series Title  
  Series Volume Series Issue Edition  
  ISSN 1475-7516 ISBN Medium  
  Area Expedition Conference  
  Notes WOS:000359215400027 Approved no  
  Is ISI yes International Collaboration yes  
  Call Number IFIC @ pastor @ Serial 2368  
Permanent link to this record
 

 
Author Capozziello, S.; Harko, T.; Lobo, F.S.N.; Olmo, G.J.; Vignolo, S. url  doi
openurl 
  Title (up) The Cauchy problem in hybrid metric-Palatini f(X)-gravity Type Journal Article
  Year 2014 Publication International Journal of Geometric Methods in Modern Physics Abbreviated Journal Int. J. Geom. Methods Mod. Phys.  
  Volume 11 Issue 5 Pages 1450042 - 12pp  
  Keywords Cauchy problem; modified gravity; hybrid metric-Palatini gravity  
  Abstract The well-formulation and the well-posedness of the Cauchy problem are discussed for hybrid metric-Palatini gravity, a recently proposed modified gravitational theory consisting of adding to the Einstein-Hilbert Lagrangian an f(R)-term constructed a la Palatini. The theory can be recast as a scalar-tensor one predicting the existence of a light long-range scalar field that evades the local Solar System tests and is able to modify galactic and cosmological dynamics, leading to the late-time cosmic acceleration. In this work, adopting generalized harmonic coordinates, we show that the initial value problem can always be well-formulated and, furthermore, can be well-posed depending on the adopted matter sources.  
  Address [Capozziello, Salvatore] Univ Naples Federico II, Dipartimento Sci Fis, Naples, Italy, Email: capozzie@na.infn.it;  
  Corporate Author Thesis  
  Publisher World Scientific Publ Co Pte Ltd Place of Publication Editor  
  Language English Summary Language Original Title  
  Series Editor Series Title Abbreviated Series Title  
  Series Volume Series Issue Edition  
  ISSN 0219-8878 ISBN Medium  
  Area Expedition Conference  
  Notes WOS:000336527100003 Approved no  
  Is ISI yes International Collaboration yes  
  Call Number IFIC @ pastor @ Serial 1813  
Permanent link to this record
 

 
Author Olmo, G.J.; Rubiera-Garcia, D. url  doi
openurl 
  Title (up) The quantum, the geon and the crystal Type Journal Article
  Year 2015 Publication International Journal of Modern Physics D Abbreviated Journal Int. J. Mod. Phys. D  
  Volume 24 Issue 9 Pages 1542013 - 15pp  
  Keywords Effective geometries; crystalline structures; modified gravity; metric-affine approach; geons  
  Abstract Effective geometries arising from a hypothetical discrete structure of spacetime can play an important role in the understanding of the gravitational physics beyond General Relativity (GR). To discuss this question, we make use of lessons from crystalline systems within solid state physics, where the presence of defects in the discrete microstructure of the crystal determine the kind of effective geometry needed to properly describe the system in the macroscopic continuum limit. In this work, we study metric-affine theories with nonmetricity and torsion, which are the gravitational analog of crystalline structures with point defects and dislocations. We consider a crystal-motivated gravitational action and show the presence of topologically nontrivial structures (wormholes) supported by an electromagnetic field. Their existence has important implications for the quantum foam picture and the effective gravitational geometries. We discuss how the dialogue between solid state physics systems and modified gravitational theories can provide useful insights on both sides.  
  Address [Olmo, Gonzalo J.] Univ Valencia, CSIC, Fac Fis, Dept Fis Teor, E-46100 Valencia, Spain, Email: drubiera@fudan.edu.cn  
  Corporate Author Thesis  
  Publisher World Scientific Publ Co Pte Ltd Place of Publication Editor  
  Language English Summary Language Original Title  
  Series Editor Series Title Abbreviated Series Title  
  Series Volume Series Issue Edition  
  ISSN 0218-2718 ISBN Medium  
  Area Expedition Conference  
  Notes WOS:000358793200014 Approved no  
  Is ISI yes International Collaboration yes  
  Call Number IFIC @ pastor @ Serial 2322  
Permanent link to this record
 

 
Author Alfonso, V.I.; Bejarano, C.; Beltran Jimenez, J.; Olmo, G.J.; Orazi, E. url  doi
openurl 
  Title (up) The trivial role of torsion in projective invariant theories of gravity with non-minimally coupled matter fields Type Journal Article
  Year 2017 Publication Classical and Quantum Gravity Abbreviated Journal Class. Quantum Gravity  
  Volume 34 Issue 23 Pages 235003 - 20pp  
  Keywords modified gravity; metric-affine theories; torsion; non-minimal couplings  
  Abstract We study a large family of metric-affine theories with a projective symmetry, including non-minimally coupled matter fields which respect this invariance. The symmetry is straightforwardly realised by imposing that the connection only enters through the symmetric part of the Ricci tensor, even in the matter sector. We leave the connection completely free (including torsion), and obtain its general solution as the Levi-Civita connection of an auxiliary metric, showing that the torsion only appears as a projective mode. This result justifies the widely used condition of setting vanishing torsion in these theories as a simple gauge choice. We apply our results to some particular cases considered in the literature, including the so-called Eddington-inspired-Born-Infeld theories among others. We finally discuss the possibility of imposing a gauge fixing where the connection is metric compatible, and comment on the genuine character of the non-metricity in theories where the two metrics are not conformally related.  
  Address [Alfonso, Victor I.] Univ Fed Campina Grande, Unidade Acad Fis, BR-58109970 Campina Grande, PB, Brazil, Email: viafonso@df.ufcg.edu.br;  
  Corporate Author Thesis  
  Publisher Iop Publishing Ltd Place of Publication Editor  
  Language English Summary Language Original Title  
  Series Editor Series Title Abbreviated Series Title  
  Series Volume Series Issue Edition  
  ISSN 0264-9381 ISBN Medium  
  Area Expedition Conference  
  Notes WOS:000414726500001 Approved no  
  Is ISI yes International Collaboration yes  
  Call Number IFIC @ pastor @ Serial 3353  
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