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Olmo, G. J., Rubiera-Garcia, D., & Wojnar, A. (2019). Minimum main sequence mass in quadratic Palatini f(R) gravity. Phys. Rev. D, 100(4), 044020–9pp.
Abstract: General relativity yields an analytical prediction of a minimum required mass of roughly similar to 0.08-0.09 M-circle dot for a star to stably burn sufficient hydrogen to fully compensate photospheric losses and, therefore, to belong to the main sequence. Those objects below this threshold ( brown dwarfs) eventually cool down without any chance to stabilize their internal temperature. In this work we consider quadratic Palatini f(R) gravity and show that the corresponding Newtonian hydrostatic equilibrium equation contains a new term whose effect is to introduce a weakening/strengthening of the gravitational interaction inside astrophysical bodies. This fact modifies the general relativity prediction for this minimum main sequence mass. Through a crude analytical modeling we use this result in order to constraint a combination of the quadratic f(R) gravity parameter and the central density according to astrophysical observations.
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Olmo, G. J., & Sanchis-Alepuz, H. (2011). Hamiltonian formulation of Palatini f(R) theories a la Brans-Dicke theory. Phys. Rev. D, 83(10), 104036–11pp.
Abstract: We study the Hamiltonian formulation of f(R) theories of gravity both in metric and in Palatini formalism using their classical equivalence with Brans-Dicke theories with a nontrivial potential. The Palatini case, which corresponds to the omega = -3/2 Brans-Dicke theory, requires special attention because of new constraints associated with the scalar field, which is nondynamical. We derive, compare, and discuss the constraints and evolution equations for the omega = -3/2 and omega not equal -3/2 cases. Based on the properties of the constraint and evolution equations, we find that, contrary to certain claims in the literature, the Cauchy problem for the omega = -3/2 case is well formulated and there is no reason to believe that it is not well posed in general.
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Olmo, G. J., Sanchis-Alepuz, H., & Tripathi, S. (2012). Stellar structure equations in extended Palatini gravity. Phys. Rev. D, 86(10), 104039–8pp.
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.
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Rosa, J. L., Lobo, F. S. N., & Olmo, G. J. (2021). Weak-field regime of the generalized hybrid metric-Palatini gravity. Phys. Rev. D, 104(12), 124030–11pp.
Abstract: In this work we explore the dynamics of the generalized hybrid metric-Palatini theory of gravity in the weak-field, slow-motion regime. We start by introducing the equivalent scalar-tensor representation of the theory, which contains two scalar degrees of freedom, and perform a conformal transformation to the Einstein frame. Linear perturbations of the metric in a Minkowskian background are then studied for the metric and both scalar fields. The effective Newton constant and the PPN parameter. of the theory are extracted after transforming back to the (original) Jordan frame. Two particular cases where the general method ceases to be applicable are approached separately. A comparison of these results with observational constraints is then used to impose bounds on the masses and coupling constants of the scalar fields.
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Sepehri, A., Pincak, R., & Olmo, G. J. (2017). M-theory, graphene-branes and superconducting wormholes. Int. J. Geom. Methods Mod. Phys., 14(11), 1750167–32pp.
Abstract: Exploiting an M-brane system whose structure and symmetries are inspired by those of graphene (what we call a graphene-brane), we propose here a similitude between two layers of graphene joined by a nanotube and wormholes scenarios in the brane world. By using the symmetries and mathematical properties of the M-brane system, we show here how to possibly increase its conductivity, to the point of making it as a superconductor. The questions of whether and under which condition this might point to the corresponding real graphene structures becoming superconducting are briefly outlined.
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Silva, J. E. G., Maluf, R. V., Olmo, G. J., & Almeida, C. A. S. (2022). Braneworlds in f(Q) gravity. Phys. Rev. D, 106(2), 024033–15pp.
Abstract: We propose a braneworld scenario in a modified symmetric teleparallel gravitational theory, where the dynamics for the gravitational field is encoded in the nonmetricity tensor rather than in the curvature. Assuming a single real scalar field with a sine-Gordon self-interaction, the generalized quadratic nonmetricity invariant Q controls the brane width while keeping the shape of the energy density. By considering power corrections of the invariant Q in the gravitational Lagrangian, the sine-Gordon potential is modified exhibiting new barriers and false vacuum. As a result, the domain wall brane obtains an inner structure, and it undergoes a splitting process. In addition, we also propose a nonminimal coupling between a bulk fermion field and the nonmetricity invariant Q. Such geometric coupling leads to a massless chiral fermion bound to the 3-brane and a stable tower of nonlocalized massive states.
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