Regular Black Holes in Quasi-Topological Gravity

• Regular black holes in quasi-topological gravity are nonsingular solutions achieved by incorporating higher-order curvature invariants into the gravitational action.
• The theory transforms complex field equations into tractable algebraic constraints, yielding rich horizon structures and modified thermodynamic properties.
• These models play a pivotal role in holography, linking gravitational couplings with dual CFT parameters and enabling controlled studies of finite-coupling effects.

Regular black holes in quasi-topological gravity are solutions to gravitational field equations whose singularities are resolved or smoothed by the inclusion of higher-order curvature invariants in the action. Quasi-topological gravity generalizes Lovelock models, enabling nontrivial higher-curvature corrections in arbitrary dimensions, retaining analytic control over spherically symmetric black holes, and supporting applications in holography and thermodynamics. The resulting black holes are characterized by branches of nonsingular, asymptotically anti-de Sitter (AdS) or flat solutions with tunable causal structure, thermodynamic behavior, and stability properties.

1. Quasi-Topological Gravitational Actions and Field Equations

Quasi-topological gravity augments the Einstein–Hilbert action with higher-order curvature invariants—typically up to cubic (and beyond) in the Riemann tensor—where the additional terms are engineered so that the spherically symmetric sector remains analytically treatable. In its canonical five-dimensional form (Myers et al., 2010), the action is

$$I = \frac{1}{16\pi G_5} \int d^5x \sqrt{-g} \Bigg[ \frac{12}{L^2} + R + \frac{\lambda L^2}{2} X_4 + \frac{7\mu L^4}{4} Z_5 \Bigg],$$

where $X_4$ is the Gauss–Bonnet combination and %%%%1%%%% is a curvature-cubed quasi-topological term.

For static, planar or spherically symmetric ansätze,

$$ds^2 = \frac{r^2}{L^2}\left[-N(r)^2 f(r) dt^2 + \sum_{i=1}^3 dx_i^2\right] + \frac{L^2}{r^2 f(r)} dr^2,$$

the gravitational equations reduce to a single algebraic constraint for $f(r)$, e.g.

$$1 - f(r) + \lambda f(r)^2 + \mu f(r)^3 = \frac{\omega^4}{r^4},$$

where $\omega$ is an integration constant proportional to the ADM mass. Regular solutions are those where $f(r)$ is monotonic, real, and positive-definite outside the event horizon, with real, non-degenerate roots corresponding to the horizon locations. For larger curvature orders (quartic, quintic, and beyond) (Dehghani et al., 2011, Cisterna et al., 2017), the algebraic equation for the metric function typically becomes a polynomial of degree $K$, where $K$ is the highest order of the curvature term.

More generally, for $K$-th order quasitopological gravity in $(n+1)$ spacetime dimensions (with $n\neq 2K-1$), the reduced one-dimensional action and constraint are

$$I_G = \int dt\,dr\,N(r)\left[ r^n \sum_{k=0}^K \hat{\mu}_k \psi^k \right]',$$

$$\sum_{k=0}^K \hat{\mu}_k \psi^k = \frac{m}{r^n}, \qquad \psi = \frac{\ell^2}{r^2} (k - f(r)),$$

ensuring the existence of regular branches provided real, acceptable algebraic roots exist over the relevant domain.

2. Horizon Structure, Phase Space, and Regularity Conditions

The regularity of black holes in quasi-topological gravity is intimately tied to the structure of the master equation’s roots. For each coupling (e.g., $\lambda, \mu$ in cubics; $\hat{\mu}_k$ in higher orders), the set of real solutions to the master polynomial dictates whether there are zero, one, or multiple black hole horizons (Myers et al., 2010, Dehghani et al., 2011). Typically, a regular black hole is identified with the branch that:

• Admits an outer event horizon (the smallest positive real root $r_+$ where $f(r_+) = 0$);
• Is free of naked singularities (the metric function $f(r)$ is smoothly defined down to the horizon);
• Exhibits AdS asymptotics with $f(r)\to f_\infty > 0$ at infinity, where $f_\infty$ satisfies a modified “vacuum” equation (e.g., $$1 - f_\infty + \lambda f_\infty^2 + \mu f_\infty^3 = 0$$);
• Corresponds, in the coupling parameter space, to ghost-free branches (the sign of the kinetic term for the graviton fluctuation is correct, i.e., $h'(f_\infty)<0$ or the analog for higher order).

The horizon structure, such as the number of horizons or the possible occurrence of extremal configurations, can be tuned by adjusting both gravitational couplings and charges (if present) (Brenna et al., 2012, Ghanaatian et al., 2013). Notably, higher-curvature terms (especially negative-coupling cubics, as in the parameter $\mu$) can produce double-horizon structures even for uncharged black holes, making the phase diagram richer than in Einstein or Lovelock gravity.

3. Thermodynamic Quantities and First Law

Black holes in quasi-topological gravity exhibit modified thermodynamic properties compared to their Einstein or Lovelock counterparts. The presence of higher curvature interactions modifies the Bekenstein–Hawking formula and reshapes the temperature, entropy, and free energy. For cubic and quartic actions in $n+1$ dimensions, the entropy per unit volume becomes (Dehghani et al., 2011, Ghanaatian et al., 2013): $$s = \frac{r_+^{n-1}}{4}\left[ 1 + 2\hat{\mu}_2(n-1)\frac{\ell^2 k}{(n-3) r_+^2} + 3\hat{\mu}_3(n-1)\frac{\ell^4 k^2}{(n-5) r_+^4} + 4\hat{\mu}_4 (n-1)\frac{\ell^6 k^3}{(n-7) r_+^6} \right],$$ where each term represents successive corrections due to higher curvature invariants.

The Hawking temperature is computed from regularity at the horizon: $T = \frac{1}{4\pi} f'(r_+),$ where the explicit dependence on curvature couplings arises by differentiating the master polynomial at the horizon. For charged solutions with non-linear electromagnetic sources (Ghanaatian et al., 2013), the mass, charge, electric potential, and chemical potential can also be computed and they satisfy the first law: $dM = T dS + \Phi dQ.$ These thermodynamic relations have been consistently shown to hold and allow for the analysis of local and global stability—extending to heat capacities and free energy landscapes.

4. Stability Analysis and Ghost-Free Conditions

Ensuring physical viability requires that the theory propagate only healthy (non-ghost) degrees of freedom on relevant backgrounds. Quasi-topological gravity typically yields fourth (or higher)-order field equations, but crucially, the graviton’s linearized equations about AdS (or Minkowski) backgrounds reduce to second order (modulo an overall prefactor) (Myers et al., 2010, Dehghani et al., 2013). The absence of ghosts is then imposed by demanding positivity of this prefactor, e.g.,

$1 - 2\lambda f_\infty - 3\mu f_\infty^2 > 0,$

where all higher-derivative contributions cancel in the maximally symmetric vacuum. The branch chosen for the black hole solution must therefore correspond to a region of the coupling space where this “kinetic coefficient” remains positive.

Thermodynamic stability is analyzed via heat capacities and the sign of $\log S$ vs. $\log T$ plots. In neutral and charged quartic/cubic quasitopological models, all three horizon topologies—spherical ($k=1$), planar ($k=0$), hyperbolic ($k=-1$)—can be shown to support locally stable black holes in large sectors of coupling space (Ghanaatian et al., 2013). For Yang–Mills charged solutions (Naeimipour et al., 2021), stability is guaranteed in the canonical ensemble, while the grand canonical ensemble generally leads to instability.

5. Holographic Implications and Dual CFT Structure

Quasi-topological gravity models play a crucial role in the AdS/CFT correspondence. The three independent gravitational couplings map onto the three parameters of the dual CFT stress tensor three-point function (Myers et al., 2010), and the addition of higher-order corrections (e.g., cubic and quartic quasi-topological terms) allows precise holographic modeling of classes of CFTs with adjustable central charges (Dehghani et al., 2013). The linearized graviton equations ensure standard (Einstein-like) behavior of metric fluctuations near the AdS boundary, preserving the well-posedness of the holographic dictionary and the structure of the boundary stress tensor (Chernicoff et al., 2016).

Central charges $a$ and $c$ of the dual CFT are directly related to the gravitational couplings and appear both in the Weyl anomaly

$$\langle T^a_a \rangle = \frac{c}{16\pi^2} I_4 - \frac{a}{16\pi^2} E_4,$$

and in the coefficient of the graviton kinetic term. The theory also satisfies a holographic c-theorem if all couplings are taken in appropriate ranges.

Higher-order quasi-topological corrections induce nontrivial causality constraints (especially when both quartic and cubic couplings are present), but unlike in Lovelock theory, they do not necessarily provide a lower positive bound on the $\eta/s$ ratio in dual hydrodynamics (Dehghani et al., 2013). Hence, such models offer a flexible laboratory for exploring holographic dualities and finite-coupling corrections.

6. Extensions: Gauge Fields, Critical Phenomena, and Universal Structures

Nonabelian gauge fields can be incorporated, yielding analytic black hole solutions with SO(n) and SO(n–1,1) gauge symmetries (Naeimipour et al., 2021). The inclusion of both non-linear electromagnetic fields and higher curvature terms supports a zoo of horizon structures, from single to double horizon topologies, and the appearance of extremal or near-extremal regular black holes. The richness of the phase space allows for Van der Waals–like phase transitions, critical exponent analyses, swallowtail Gibbs free energy diagrams, and criticality analogous to the liquid–gas system (Naeimipour et al., 2021, Mir et al., 2019).

A recurrent feature is the universality in the structure of the field equations across dimensions and curvature order: for each Kth order quasi-topological theory, the metric function is algebraically determined by a degree K polynomial, and the branching and regularity properties can be systematically classified in the ($\lambda, \mu, \cdots$) coupling space (Dehghani et al., 2011, Cisterna et al., 2017).

7. Summary Table: Key Features of Quasi-Topological Regular Black Holes

Aspect	Features in Quasi-Topological Gravity	Standard Lovelock/Einstein
Field equations	Reduce to degree-K algebraic polynomial for static, symmetric metrics	Algebraic only up to (dimension/2)th order
Regularity (r→0)	No curvature singularity in suitable coupling regions; de Sitter-like core possible	Singular at r = 0 (unless coupled to nonlinear E&M)
Horizon structure	Multiple real roots allow for single/double/extremal horizons; phase transitions possible	Fewer possibilities; double horizons only with charge
Ghost-freeness	Imposed by root of kinetic coefficient h’(f_∞); quadratic/cubic/ quartic coupling space constraint	Guaranteed in Einstein; nontrivial in Lovelock
Thermodynamics	Modified entropy, temperature via Wald’s method; area law with corrections in couplings; first law holds exactly	Standard area/4 law; simple Hawking temperature
Holography/CFT data	Three independent couplings → CFT 3-point stress tensor params; central charges a, c adjustable; holographic c-theorem possible	Limited adjustability; less control of dual CFT

8. Concluding Perspective

Regular black holes in quasi-topological gravity demonstrate that the inclusion of higher-curvature invariants, constructed so as to retain second-order field equations for relevant ansätze, yields a tractable, richly-structured spectrum of physically viable, non-singular black hole solutions. They enable analytic explorations across arbitrary dimension and topological type, possess well-posed thermodynamics and phase structures, and underpin a generically ghost-free duality to a broad class of conformal field theories in the AdS/CFT context (Myers et al., 2010, Dehghani et al., 2011, Dehghani et al., 2013, Naeimipour et al., 2021). The resulting framework provides a controlled setting for modeling finite-coupling corrections, studying critical phenomena, and investigating the links between horizon regularity, causality, and holographic data.