Clouds of strings in $4D$ Einstein-Gauss-Bonnet black holes

Abstract

Recently it has been shown that the Einstein-Gauss-Bonnet (EGB) gravity, by rescaling the coupling constant as $\alpha /(D-4)$ and taking the limit $D\to 4$ at the level of the equations of motion, becomes nontrivially ghost-free in $4D$ - namely the novel $4D$ EGB gravity. We present an exact charged black hole solution to the theory surrounded by clouds of string (CS) and also analyze their thermodynamic properties to calculate exact expressions for the black hole mass, temperature, and entropy. Owing to the corrected black hole due to the background CS, the thermodynamic quantities have also been corrected except for the entropy, which remains unaffected by a CS background. However, as a result of the novel $4D$ EGB theory, the Bekenstein-Hawking area law turns out to be corrected by a logarithmic area term. The heat capacity $C_{+}$ diverges at a critical radius $r=r_C$, where incidentally the temperature has a maximum, and the Hawking-Page transitions even in absence of the cosmological term and $C_{+}>0$ for $r_{+}<r_C$ allowing the black hole to become thermodynamically stable. In addition, the smaller black holes are globally preferred with negative free energy $F_{+}<0$. Our solution can also be identified as a $4D$ monopole-charged EGB black hole. We regain results of spherically symmetric black hole solutions of general relativity and that of novel $4D$ EGB, respectively, in the limits $\alpha \to 0$ and $a=0$.

12 pages, 4 figures, 2 tables, Phys.Dark Univ. 30 (2020) 100730

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