Stars and quark stars in bumblebee gravity

The European Physical Journal C, 2020

We study non-rotating and isotropic strange quark stars in Lorentz-violating theories of gravity, and in particular in Hořava gravity and Einstein-æther theory. For quark matter we adopt both linear and non-linear equations of state, corresponding to the MIT bag model and color flavor locked state, respectively. The new structure equations describing hydrostatic equilibrium generalize the usual Tolman–Oppenheimer–Volkoff (TOV) equations of Einstein’s general relativity. A dimensionless parameter $$\nu $$ν measures the deviation from the standard TOV equations, which are recovered in the limit $$\nu \rightarrow 0$$ν→0. We compute the mass, the radius as well as the compactness of the stars, and we show graphically the impact of the parameter $$\nu $$ν on the mass-to-radius profiles for different equations of state describing quark matter. The energy conditions and stability criteria are also considered, and they are all found to be fulfilled.

2020

We study non-rotating and isotropic strange quark stars in Lorentz-violating theories of gravity, and in particular in Hořava gravity and Einstein-aether theory. For quark matter we adopt both linear and non-linear equations of state, corresponding to the MIT bag model and color flavor locked state, respectively. The new structure equations describing hydrostatic equilibrium generalize the usual Tolman-Oppenheimer-Volkoff (TOV) equations of Einstein's general relativity. A dimension-less parameter ν measures the deviation from the standard TOV equations, which are recovered in the limit ν → 0. We compute the mass, the radius as well as the compactness of the stars, and we show graphically the impact of the parameter ν on the mass-to-radius profiles for different equations of state describing quark matter. The energy conditions and stability criteria are also considered, and they are all found to be fulfilled.

Quark star models with realistic equation of state in nonperturbative f (R) gravity are considered. The mass-radius relation for f (R) = R + αR 2 model is obtained. Considering scalar curvature R as an independent function, one can find out, for each value of central density, the unique value of central curvature for which one has solutions with the required asymptotic R → 0 for r → ∞. In another words, one needs a fine-tuning for R to achieve quark stars in f (R) gravity. We consider also the analogue description in corresponding scalar-tensor gravity. The fine-tuning on R is equivalent to the fine-tuning on the scalar field φ in this description. For distant observers, the gravitational mass of the star increases with increasing α (α > 0) but the interpretation of this fact depends on frame where we work. Considering directly f (R) gravity, one can say that increasing of mass occurs by the "gravitational sphere" outside the star with some "effective mass". On the other hand, in conformal scalar-tensor theory, we also have a dilaton sphere (or "disphere") outside the star but its contribution to gravitational mass for distant observer is negligible. We show that it is possible to discriminate modified theories of gravity from General Relativity due to the gravitational redshift of the thermal spectrum emerging from the surface of the star.

Physical Review D, 2021

In this work, we present black hole solutions with a cosmological constant in bumblebee gravity, which provides a mechanism for the Lorentz symmetry violation by assuming a nonzero vacuum expectation value for the bumblebee field. From the gravitational point of view, such solutions are spherically symmetric black holes with an effective cosmological constant and are supported by an anisotropic energy-momentum tensor, conceived of as the manifestation of the bumblebee field in the spacetime geometry. Then we calculate the shadow angular radius for the proposed black hole solution with a positive effective cosmological constant. In particular, our results are the very first relation between the bumblebee field and the shadow angular size.

2017

We derive a working model for the Tolman-Oppenheimer-Volkoff equation for quark star systems within the modified f(T, T)-gravity class of models. We consider f(T, T)-gravity for a static spherically symmetric space-time. In this instance the metric is built from a more fundamental tetrad vierbein from which the metric tensor can be derived. We impose a linear f(T) parameter parameter, namely taking f=α T(r) + βT(r) + φ and investigate the behavior of a linear energy-momentum tensor trace, T. We also outline the restrictions which modified f(T, T)-gravity imposes upon the coupling parameters. Finally we incorporate the MIT bag model in order to derive the mass-radius and mass-central density relations of the quark star within f(T, T)-gravity.

The European Physical Journal C

We derive a working model for the Tolman-Oppenheimer-Volkoff equation for quark star systems within the modified f (T, T)-gravity class of models. We consider f (T, T)-gravity for a static spherically symmetric spacetime. In this instance the metric is built from a more fundamental tetrad vierbein from which the metric tensor can be derived. We impose a linear f (T) parameter, namely taking f = αT (r) + βT (r) + ϕ and investigate the behaviour of a linear energy-momentum tensor trace, T. We also outline the restrictions which modified f (T, T)-gravity imposes upon the coupling parameters. Finally we incorporate the MIT bag model in order to derive the mass-radius and mass-central density relations of the quark star within f (T, T)-gravity.

SSRN Electronic Journal

We study the structure of quark stars (QSs) adopting homogeneously confined matter inside the star with a 3-flavor neutral charge and a fixed strange quark mass m s. We explore the internal structure, and the physical properties of specific classes of QSs in the recently proposed energy-momentum squared gravity (EMSG). Also, we obtain the mass-radius (M − R) and mass-central energy density (M − ρ c) relations for QS using the QCD motivated EoS. The maximum mass for QSs in EMSG is investigated depending on the presence and absence of the free parameter α. Furthermore, the stability of stars is determined by the condition dM dρ c > 0. We observe that consideration of the EMSG has specific contributions to the structure of QSs.

Annals of Physics, 2023

Kasner cosmology is a vacuum and anisotropically expanding spacetime in the general relativity context. In this work, such a cosmological model is studied in another context, the bumblebee model, where the Lorentz symmetry is spontaneously broken. By using the bumblebee context it is possible to justify the anisotropic feature of the Kasner cosmology. Thus, the origin of the anisotropy in this cosmological model could be in the Lorentz symmetry breaking. Lastly, an application in the pre-inflationary cosmology is suggested.

2020

This paper deals with the existence of a compact stellar object, precisely strange (quark) star, in the framework of Einstein's General Theory of Relativity with Tolman $V$ metric potential, which is one of the simplest forms of potential among his proposals. The potential is given by $e^{\nu}=Kr^{2n}$, where $K$ is the constant and $n$ is a parameter [R.C. Tolman, Phys. Rev. {\bf55}, 364 (1939)]. Considering charged, static, spherically symmetric, isotropic fluid sphere we have studied different physical features of some strange star candidates namely $EXO\ 1785-248$, $LMC\ X-4$, $SMC\ X-1$, $SAX\ J1808.4-3658$, $4U\ 1538-52$ and $Her\ X-1$. To represent the strange quark matter (SQM) distribution we have employed the simplest form of MIT bag equation of state (EOS), which provides a linear relationship between pressure and density of the matter through Bag constant $B$. We have done several tests for the stability criteria and the physical acceptability of the proposed model. ...

The European Physical Journal C

We have considered the bumblebee gravity model where lorentz-violating (LV) scenario gets involved through a bumblebee field vector field $$B_\mu $$ B μ . A spontaneous symmetry breaking allows the field to acquires a vacuum expectation value that generates LV into the system. A Kerr–Sen-like solution has been found out starting from the generalized form of a radiating stationery axially symmetric black hole metric. We compute the effective potential offered by the null geodesics in the bumblebee rotating black hole spacetime. The shadow has been sketched for different variations of the parameters involved in the system. A careful investigation has been carried out to study how the shadow gets affected when Lorentz violation enters into the picture. The emission rate of radiation has also been studied and how it varies with the LV parameter $$\ell $$ ℓ is studied scrupulously.