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Thermodynamics of 4D dS/AdS Gauss–Bonnet black holes according to consistent gravity theory in the presence of a cloud of strings

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Communications in Theoretical Physics 2022年第4期74卷 72-84页

作者： hossein ghaffarnejad Faculty of Physics Semnan UniversityP.C.35131-19111SemnanIran

By looking at the Lovelock theorem one can infer that the gravity model given by[1]cannot be applicable for all types of 4D Einstein–Gauss–Bonnet(EGB)curved *** reason for this is that in 4D space-time,the Gauss–Bonnet invariant is a total derivative and hence it does not contribute to gravitational ***,the authors of[2]presented an alternative consistent EGB gravity model instead of[1]by applying a break-of-diffeomorphism *** this work,we use the alternative model to produce a de Sitter(dS)/Anti-de Sitter(AdS)black hole metric and then investigate its thermodynamic behavior in the presence of a cloud of Nambu–Goto *** derivations show that the resulting diagrams of pressure vs specific volume at a constant temperature are similar to that for a van der Waals gas/fluid in an ordinary thermodynamic system in the d S sector but not in the Ad S *** this,we infer that the black hole participates in the small-to-large black hole phase transition in the dS background,while it exhibits a Hawking–Page phase transition in the AdS *** the latter case,an evaporating black hole eventually reaches an AdS vacuum space because of its instability.

关键词： Lovelock gravity string Nambu-Goto Gauss-Bonnet black hole thermodynamic cosmological constant phase transition

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Weak gravitational lensing of quantum perturbed lukewarm black holes and cosmological constant effect

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Research in Astronomy and Astrophysics 2017年第6期17卷 17-36页

作者： hossein ghaffarnejad Mojtaba Amir Mojahedi Faculty of Physics Semnan University Semnan 35131-19111 Iran

The aim of the paper is to study weak gravitational lensing of quantum （perturbed） and classical lukewarm black holes （QLBHs and CLBHs respectively） in the presence of cosmological parameter A. We apply a numerical method to evaluate the deflection angle of bending light rays, image locations θ of sample sourceβ = π- 4, and corresponding magnifications μ. There are no obtained real values for Einstein ring locations θE（β = 0） for CLBHs but we calculate them for QLBHs. As an experimental test of our calculations, we choose mass M of 60 types of the most massive observed galactic black holes acting as a gravitational lens and study quantum matter field effects on the angle of bending light rays in the presence of cosmological constant effects. We calculate locations of non-relativistic images and corresponding magnifications. Numerical diagrams show that the quantum matter effects cause absolute values of the quantum deflection angle to be reduced with respect to the classical ones. The sign of the quantum deflection angle is changed with respect to the classical values in the presence of the cosmological constant. This means dominance of the anti-gravity counterpart of the cosmological horizon on the angle of bending light rays with respect to absorbing effects of 60 local types of the most massive observed black holes. Variations of the image positions and magnifications are negligible when increasing dimensionless cosmological constant ∈ = 16AM2 /2The deflection angle takes positive （negative） values for CLBHs （QLBHs） and they decrease very fast （slowly） by increasing the closest distance x0 of bending light ray and/or dimensionless cosmological parameter for sample giant black holes with 0.001 〈 ∈ 〈 0.01.

关键词： gravitational lensing weak - strong - micro - stars black holes

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