arXiv subjects

Shubhashis Mallik

Publications and source records attributed to Shubhashis Mallik.

7 recordsLinked to original sources

arXiv2026

Boundary conditions for axionic wormholes, imaginary distance bound and KSW allowability

We study four-dimensional axion wormholes in the Lorentzian mini-superspace path integral using dual scalar and three-form flux formulations. We analyze this duality for generic metric boundary conditions and real lapse integration contour, and show that it holds on any background, including complex ones. With the Dirichlet condition on the metric, the fixed-flux path integral is evaluated exactly. In the Euclidean regime, saddle geometries organize into same-side and cross-throat segments of the Giddings-Strominger (GS) wormhole, depending on whether the two boundaries lie on the same or opposite sides of the throat. A Picard-Lefschetz analysis in the covering plane of lapse reveals that the cross-throat saddle has vanishing intersection number, and therefore does not contribute. In the asymptotically flat limit, it corresponds to the imaginary wormhole that saturates the imaginary distance bound (IDB). The same conclusion follows independently on the scalar side of the duality, and is confirmed by the exact amplitude. Replacing the Dirichlet condition with a one-parameter Neumann condition, the cross-throat saddle that leads to the imaginary wormholes remains irrelevant for the physical lift of the contour. This purely imaginary parameter characterizing the Neumann condition interpolates continuously between the half- and complete-wormhole geometries. Convergence of the sum over fixed-charge sectors imposes a corresponding interpolating bound not only on the imaginary part of the boundary axion but also on the parameter characterizing the Neumann boundary condition. An independent analysis based on the KSW criterion reproduces exactly the same bound, emphasizing its compatibility with the IDB.

hep-th↗

arXiv2026

IR behaviour of one-loop complex $\mathbb{R}\times S^3$ saddles

Gravitational path-integral over $\mathbb{R}\times S^3$ complex metrics with fluctuations is studied in 4D for Einstein-Hilbert gravity in Lorentzian signature, with the aim to investigate the IR properties of complex saddles for various boundary choices. General covariance doesn't allow arbitrary boundary choices for the background and fluctuations. In the ADM-decomposition, while imposing ``no-boundary'' condition at the initial boundary, two scenarios are considered for the final boundary: Dirichlet and fixed extrinsic curvature. Universe undergoes transition from a Euclidean to Lorentzian phase in either scenario, where the dominant saddle in Euclidean phase correspond to a Euclidean metric (imaginary time), while the Lorentzian phase has two complex metrics as dominant saddles which superimpose. One-loop corrected lapse action is computed using Hurwitz-Zeta regularization. UV-divergences canceled by suitable counter terms lead to a renormalized lapse action. One-loop renormalized Hartle-Hawking wave-function is computed using the Picard-Lefschetz and WKB methods, where the contributions coming from the metric-fluctuations show secularly growing infrared divergences as the Universe expands. This is compared with the situation in pure Lorentzian dS, corresponding to a Universe transitioning from an initial state of vanishing conjugate momenta to final state of fixed extrinsic curvature, thereby giving real saddles. Picard-Lefschetz methods alone are not sufficient to overcome the technical hurdles in the one-loop computation, which needs to be supplemented by an $i\epsilon$-prescription, achieved via slight complexification of the cosmological constant $\Lambda$. The UV renormalized one-loop dS wavefunction has the same leading IR divergence as for the Hartle-Hawking no-boundary Universe. Interestingly for all boundary choices considered, the saddles remain KSW-allowed.

hep-th↗

arXiv2025

Complex Saddles of Charged-AdS Gravitational partition function

In this paper, we consider the Euclidean partition function of uncharged and charged $AdS_{d+1}$ black hole geometries in canonical and grand canonical ensemble for $d\geq3$. It is seen that the partition function can be reduced to a one-dimensional integral, which can be investigated using methods of Picard-Lefschetz. The saddles of the system correspond to either naked-singular geometry, thermal-AdS, small-, intermediate- or large-sized black hole for different ranges of parameter space. These are solutions of Einstein's equation, which are dominant saddles of the partition function in various regimes of parameter space. A naive analysis of the partition function involving these saddles would lead to conflicts with the standard understanding of black hole thermodynamics and also with AdS/CFT. However, when the partition function is analysed using Picard-Lefschetz, it is seen that naked-singular geometries turn out to be irrelevant and therefore do not contribute. This also aligns well with the Cosmic Censorship hypothesis. Depending on the ensemble, saddles corresponding to negative specific heat are either small- or intermediate-sized black holes. Although they are relevant in the partition function but are sub-dominant. They drop out under homology averaging. Only saddles corresponding to non-negative specific heat contribute to the Euclidean partition function. Finally, we analyze the allowability of these complex geometries using the KSW criterion.

hep-th↗

arXiv2025

Resolving Degeneracies in Complex $\mathbb{R}\times S^3$ and $\theta$-KSW

Lorentzian gravitational path integral for the Gauss-Bonnet gravity in $4D$ is studied in the mini-superspace ansatz for metric. The gauge-fixed path-integral for Robin boundary choice is computed exactly using {\it Airy}-functions, where the dominant contribution comes from No-boundary geometries. The lapse integral is further analysed using saddle-point methods to compare with exact results. Picard-Lefschetz methods are utilized to find the {\it relevant} complex saddles and deformed contour of integration, thereby using WKB methods to compute the integral along the deformed contour in the saddle-point approximation. However, their successful application is possible only when system is devoid of degeneracies, which in present case appear in two types: {\bf type-1} where the flow-lines starting from neighbouring saddles overlap leading to ambiguities in deciding the {\it relevance} of saddles, {\bf type-2} where saddles merge for specific choices of boundary parameters leading to failure of WKB. Overcoming degeneracies using artificial {\it defects} introduces ambiguities due to the choice of {\it defects} involved. Corrections from quantum fluctuations of scale-factor overcome degeneracies only partially (lifts {\bf type-2} completely with partial resolution of {\bf type-1}), with the residual lifted fluently by complex deformation of $(G\hbar)$. {\it Anti-linear} symmetry present in various forms in the lapse action is the reason behind all the {\bf type-1} degeneracies. Any form of {\it defect} or {\it deformation} breaking anti-linearity resolves {\bf type-1} degeneracies, indicating complex deformation of $(G\hbar)$ as an ideal choice. Compatibility with the KSW criterion is analyzed after symmetry breaking. Complex deformation of $(G\hbar)$ modifies the KSW criterion, imposing a strong constraint on the deformation if No-boundary geometries are required to be always KSW-allowed.

hep-th↗

arXiv2024

Boundary choices and one-loop complex gravitational path integral

The path integral of 4D Einstein-Hilbert gravity for the de Sitter-like Universe with fluctuations is investigated, and the transition amplitude from one boundary configuration to another is computed. The gravitational system is described by lapse, scale factor and metric-fluctuation field. Variational consistency demands augmenting the bulk theory with suitable boundary action. A given boundary choice on scale factor is seen to be achievable via an infinite family of covariant boundary actions, each restricting the boundary choices for the fluctuation field. General covariance intimately ties the two boundary choices, which no longer can be chosen independently. For vanishing metric fluctuations at the boundaries, the gauge-fixed gravitational path integral disintegrates into path integral over scale factor and metric-fluctuation field, connected via only lapse integration. While the former is exactly doable, the latter is computed up to one loop, leading to one-loop corrected lapse action. Ultraviolet (UV) divergences are systematically extracted and removed by the addition of suitable counterterms, leading to finite effective action for the lapse. The lapse effective action is then utilized for computing finite transition amplitude. Contributions from virtual gravitons are seen to be secularly growing with Universe size, leading to an infrared divergent transition amplitude. The presence of nonvanishing metric fluctuation at the boundaries implies that the ``no-boundary'' saddles of the theory without metric fluctuations are no longer the saddles of the one-loop corrected action. The corrected saddles have the Universe starting from a nonzero size.

gr-qc↗

arXiv2024

Lorentzian Robin Universe of Gauss-Bonnet Gravity

The gravitational path-integral of Gauss-Bonnet gravity is investigated and the transition from one spacelike boundary configuration to another is analyzed. Of particular interest is the case of Neumann and Robin boundary conditions which is known to lead to a stable Universe in Einstein-Hilbert gravity in four spacetime dimensions. After setting up the variational problem and computing the necessary boundary terms, the transition amplitude is computed \emph{exactly} in the mini-superspace approximation. The $\hbar\to0$ limit brings out the dominant pieces in the path-integral which is traced to an initial configuration corresponding to Hartle-Hawking no-boundary Universe. A deeper study involving Picard-Lefschetz methods not only allow us to find the integration contour along which the path integral becomes convergent but also aids in understanding the crossover from Euclidean to Lorentzian signature. Saddle analysis further highlights the boundary configurations giving dominant contribution to the path-integral which is seen to be those corresponding to Hartle-Hawking no-boundary proposal and agrees with the exact computation. To ensure completeness, a comparison with the results from Wheeler-DeWitt equation is done.

gr-qc↗

arXiv2023

Lorentzian Robin Universe

In this paper, we delve into the gravitational path integral of Gauss-Bonnet gravity in four spacetime dimensions, in the mini-superspace approximation. Our primary focus lies in investigating the transition amplitude between distinct boundary configurations. Of particular interest is the case of Robin boundary conditions, known to lead to a stable Universe in Einstein-Hilbert gravity, alongside Neumann boundary conditions. To ensure a consistent variational problem, we supplement the bulk action with suitable surface terms. This study leads us to compute the necessary surface terms required for Gauss-Bonnet gravity with the Robin boundary condition, which wasn't known earlier. Thereafter, we perform an exact computation of the transition amplitude. Through $\hbar\to0$ analysis, we discover that the Gauss-Bonnet gravity inherently favors the initial configuration, aligning with the Hartle-Hawking no-boundary proposal. Remarkably, as the Universe expands, it undergoes a transition from the Euclidean (imaginary time) to the Lorentzian signature (real time). To further reinforce our findings, we employ a saddle point analysis utilizing the Picard-Lefschetz methods. The saddle point analysis allows us to find the initial configurations which lead to Hartle-Hawking no-boundary Universe that agrees with the exact computations. Our study concludes that for positive Gauss-Bonnet coupling, initial configurations corresponding to the Hartle-Hawking no-boundary Universe gives dominant contribution in the gravitational path-integral.

gr-qc↗