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Sepideh Bakhoda

Publications and source records attributed to Sepideh Bakhoda.

8 recordsLinked to original sources

Thermodynamics of polymerized vacuum regular black holes in anti-de Sitter spacetime

We derive a class of vacuum regular black holes inspired by effective loop quantum gravity dynamics and extend the construction to asymptotically anti-de Sitter spacetimes. The derivation is based on a deparameterized Lema\^itre--Tolman--Bondi formulation, where an auxiliary dust field is introduced only to define an internal time and does not act as a matter source. In spherical symmetry, the dynamics reduces to a set of independent radial shells, giving rise to a factorized shell Hamiltonian and to a Birkhoff-type property: for a fixed reconstruction function and cosmological constant, the static geometry is uniquely determined by the mass. Within this framework, we construct several regular black hole models with de Sitter cores and corresponding models with anti-de Sitter cores. We then study their thermodynamics in the extended phase space, with particular emphasis on the Hawking--Page transition. For the class of models considered, the dominant transition is of Hawking--Page type, determined by the crossing of the black hole free energy with the corresponding thermal-AdS background. The regularization affects the quantitative transition temperature by deforming the physical outer-horizon branch, including its endpoint structure. In the large anti-de Sitter radius regime, the de Sitter core solutions exhibit a higher Hawking--Page temperature than their anti-de Sitter-core counterparts, while the ordering can be modified close to the lower admissible range of the AdS scale. Thus, the thermodynamic differences between the two classes are not a consequence of regularity alone, but arise from how the core deformation modifies the horizon branch relative to the thermal-AdS reference background.

gr-qc

Asymptotic Symmetries of the Holst Action at Spatial Infinity: Including Supertranslations

We investigate the asymptotic symmetries of General Relativity at spatial infinity within the first-order formalism described by the Holst action. Employing the covariant phase space method, we propose a set of relaxed boundary conditions for the co-tetrad and Lorentz connection that admit the full Bondi-Metzner-Sachs (BMS) group, including non-trivial supertranslations, which are typically eliminated in standard treatments. We demonstrate that the logarithmic divergences appearing in the symplectic structure can be removed by imposing specific, symmetry-preserving parity conditions on the asymptotic fields without suppressing the supertranslation sector. A detailed analysis of the conserved charges reveals that the Holst term contributes non-trivially to the charge variations due to the linear growth of Lorentz generators. We show that the naive surface integrals for the Holst charges exhibit linear divergences arising from the rotation of the background tetrad. These divergences are successfully regularized by supplementing the asymptotic symmetry generator with a compensating internal Lorentz gauge transformation defined to preserve the background structure. The resulting charges are manifestly finite and integrable. Crucially, we prove that while the Holst modification shifts the charges associated with Lorentz boosts and rotations, it leaves the supertranslation charges identically invariant. This framework provides a consistent derivation of the full BMS algebra at spatial infinity in terms of Ashtekar-Barbero variables, offering new insights into the role of the Immirzi parameter in classical and quantum gravity.

gr-qc

Geometrical Quantum Time in the $U(1)^3$ Model of Euclidean Quantum Gravity

Loop Quantum Gravity faces challenges in constructing a well-defined Hamiltonian constraint and understanding the quantum notion of time. In this paper these issues are studied by quantizing the $U(1)^3$ model, a simplified system exhibiting features similar to general relativity. By isolating a holonomy component within the Hamiltonian constraint, a discrete relative time evolution equation for quantum states is obtained. Then a Shr\"{o}dinger-like equation is derived in continuous limit. Thus the physical states solving this Shr\"{o}dinger-like equation can be written out. The emergence of the time parameter and its corresponding quantum operator are analyzed. It indicates the notion of a geometrical quantum time for quantum gravity.

gr-qc

Boundary conditions for Ashtekar-Barbero variables in the context of asymptotically flat spacetimes which lead to supertranslations at spatial infinity

This paper delves into the exploration of suitable boundary conditions for the asymptotically flat scenario of general relativity presented in terms of Ashtekar-Barbero variables. While the standard parity conditions have been extensively studied in \cite{Thiemann, Campiglia}, it turns out that they fail to produce non-trivial supertranslations at spatial infinity. We propose new parity conditions for the Ashtekar-Barbero variables that do yield non-trivial supertranslation charges at spatial infinity. We compare our findings with those presented in \cite{Henneaux} and demonstrate that the new boundary conditions ensure the finiteness of the symplectic structure. Moreover, when embarking on the quest for appropriate parity conditions, it is essential to ensure that the selected parities remain invariant under hypersurface deformations. Given that working with Ashtekar-Barbero variables provides more asymptotic structure as compared to the ADM variables, it is shown that by fixing the Lagrange multiplier corresponding to the Gauss constraint, the invariance of certain parity conditions can be guaranteed.

gr-qc

Reduced Phase Space Approach to the $U(1)^3$ model for Euclidean Quantum Gravity

If one replaces the constraints of the Ashtekar-Barbero $SU(2)$ gauge theory formulation of Euclidean gravity by their $U(1)^3$ version, one arrives at a consistent model which captures significant structure of its $SU(2)$ version. In particular, it displays a non trivial realisation of the hypersurface deformation algebra which makes it an interesting testing ground for (Euclidean) quantum gravity as has been emphasised in a recent series of papers due to Varadarajan et al. In this paper we consider a reduced phase space approach to this model. This is especially attractive because, after a canonical transformation, the constraints are at most {\it linear} in the momenta. In suitable gauges, it is therefore possible to find a closed and explicit formula for the physical Hamiltonian which depends only on the physical observables. Not surprisingly, that physical Hamiltonian is generically neither polynomial nor spatially local. The corresponding reduced phase space quantisation can be confronted with the constraint quantisation due to Varadarajan et al to gain further insights into the quantum realisation of the hypersurface deformation algebra.

gr-qc

Covariant Origin of the $U(1)^3$ model for Euclidean Quantum Gravity

The utility of the U(1)$^3$ model as a test laboratory for quantum gravity has recently been emphasized in a recent series of papers due to Varadarajan et al. The simplification from SU(2) to U(1)$^3$ can be performed simply by hand within the Hamiltonian formulation by dropping all non-Abelian terms from the Gauss, spatial diffeomorphism and Hamiltonian constraints respectively. However, one may ask from which Lagrangian formulation this theory descends. For the SU(2) theory it is known that one can choose the Palatini action, Holst action or (anti-)selfdual action (Euclidian signature) as starting point all leading to equivalent Hamiltonian formulations. In this paper we systematically analyse this question directly for the U(1)$^3$ theory. Surprisingly, it turns out that the Abelian analog of the Palatini or Holst formulation is a consistent but topological theory without propagating degrees of freedom. On the other hand, a twisted Abelian analog of the (anti-)selfdual formulation does lead to the desired Hamiltonian formulation. A new aspect of our derivation is that we work with 1. half-density valued tetrads which simplifies the analysis, 2. without the simplicity constraint (which admits one undesired solution that is usually neglected by hand) and 3. without imposing the time gauge from the beginning. As a byproduct we show that also the non-Abelian theory admits a twisted (anti-)selfdual formulation. Finally we also derive a pure connection formulation of Euclidian GR including a cosmological constant by extending previous work due to Capovilla, Dell, Jacobson and Peldan which may be an interesting starting point for path integral investigations and displays (Euclidian) GR as a Yang-Mills theory with non-polynomial Lagrangian.

gr-qc

Asymptotically flat boundary conditions for the $U(1)^3$ model for Euclidean Quantum Gravity

A generally covariant $U(1)^3$ gauge theory describing the $G_N \to 0$ limit of Euclidean general relativity is an interesting test laboratory for general relativity, specially because the algebra of the Hamiltonian and diffeomorphism constraints of this limit is isomorphic to the algebra of the corresponding constraints in general relativity. In the present work, we study boundary conditions and asymptotic symmetries of the $U(1)^3$ model and show that while asymptotic spacetime translations admit well-defined generators, boosts and rotations do not. Comparing with Euclidean general relativity, one finds that exactly the non-Abelian part of the $SU(2)$ Gauss constraint which is absent in the $U(1)^3$ model plays a crucial role in obtaining boost and rotation generators.

gr-qc

Asymptotic conformal symmetry at spatial infinity

In this paper, the effects of adding spatial conformal symmetry to the asymptotic symmetry group of an asymptotically conformally flat spacetime are studied. It is shown that, in addition to the BMS group, only the dilations of the spatial conformal generators keep the corresponding boundary conditions conformally invariant under hypersurface deformations. We prove that in order to attain (i) a well-defined symplectic structure and (ii) a finite and (iii) integrable conserved charge, these conditions are satisfied simultaneously when admitting Regge-Teitelboim and twisted Henneaux-Troessaert parity conditions, where the latter also contain supertranslation invariance. The conserved dilation charge contains nonzero terms independent of the field variables, giving a nonvanishing effect on the boundary. The dilation symmetry also modifies the ADM mass, which is another physical effect of the conformal symmetry.

gr-qc