arXiv subjects

Rikpratik Sengupta

Publications and source records attributed to Rikpratik Sengupta.

At least 19 recordsLinked to original sources

arXiv2026

Cycles Without End: An Ekpyrotic Braneworld Bounce with a Kinetically Coupled Entropic Mechanism

Cyclic cosmologies replace inflation's single creation event with an eternal sequence of smooth turnarounds, but three problems have historically stood in their way: singular bounces, the blue tilt generic to single-field ekpyrotic contraction, and Tolman's observation that entropy production should make successive cycles longer and larger without bound. We resolve all three within one explicit model. A Randall-Sundrum brane with a timelike extra dimension regularizes the bounce without violating the null energy condition, and a single scalar field's potential carries the universe from a thawing dark-energy plateau through a sign-changing turnaround into a BKL-safe ekpyrotic contraction and back. We show that no correction internal to the brane's Friedmann equation, and no straight two-field trajectory, can rescue the spectral index from the generic ekpyrotic result $n_s\approx3$. We then build the minimal completion that can: a kinetically coupled entropy field, following the general mechanism of Ijjas, Lehners, and Steinhardt, whose coupling steepness has a closed-form, exact-scale-invariance value for this model's ekpyrotic potential, with only a percent-level offset needed to match the measured tilt $n_s=0.9649$. This fixes the local non-Gaussianity ($f_{\rm NL}\approx-0.02$) and tensor-to-scalar ratio ($r\approx7.6\times10^{-26}$). Matching the observed amplitude fixes the conversion coupling and brane tension to unremarkable values ($\lambda\approx5.7\times10^{-3}$, $\rho_c^{1/4}=10^{13}\,$GeV). Finally, because the brane construction is spatially flat, Tolman's entropy argument does not bound the number of cycles here: we derive $N_{\rm DE}\sim70$-80 e-folds of accelerated expansion needed each cycle to dilute the entropy produced back to a negligible density before the next contraction.

gr-qc↗

arXiv2026

Accelerating expansion and isotropic sky-hemisphere consistency in Pantheon+ supernovae: a revised analysis in the dark energy debate

We perform four independent decompositions of the deceleration parameter $q_0$ using 1564 Type Ia supernovae (SNe Ia) from the Pantheon+ catalogue: by redshift bin, sky hemisphere, host galaxy mass, and supernova colour, correcting a coordinate error identified by Sah, Rameez & Sarkar (SRS26) in the sky-hemisphere direction used in our original analysis. Without any progenitor-age correction, the full sample yields $q_m=-0.490$, consistent with the $\Lambda$CDM expectation of $-0.55$. Applying the Son et al. (S25) progenitor-age correction shifts this to $q_m=-0.267$, remaining in the accelerating regime. The sky hemisphere test, using the corrected CMB dipole direction (RA$=167.80^\circ$, Dec$=-7.10^\circ$), shows consistent results between the CMB dipole ($q_m=-0.527$) and anti-dipole ($q_m=-0.464$) hemispheres, supporting isotropy but not the strong deceleration values reported previously. Our revised results are consistent with \lcdm{} and do not support either the original claim of near-zero baseline acceleration or the S25/SRS26 claim of a decelerating universe.

astro-ph.CO↗

arXiv2026

A Potential Black Hole Mimicker From Non-Minimal Coupling

We present a class of horizonless, regular ultra-compact objects arising in a theory of gravity which allows curvature-fluid coupling. The non-minimal interaction between fluid variables and the Ricci scalar generates a vacuum-like equation of state in the interior, while the exterior remains exactly Schwarzschild. The two spacetimes are glued through a shell at the junction. The interior metric is non-singular, the shell acquires a stiff-matter equation of state, and near-horizon compactness can potentially mimic black-hole phenomenology without event horizons. Unlike the Mazur-Mottola gravastar and its variants, the present model naturally selects a typical ultra-compact mass-radius window, with masses in the range $1.4$-$2.1 M_\odot$ and radii in the range 5-7 km. This framework predicts a unique geometric-thermodynamic shell temperature in the ultra-compact limit distinctly different from the Hawking expression and the other unique observational feature of the model is the prediction of mass independent luminosity.

gr-qc↗

arXiv2026

Bounce, Turnaround, and the Anisotropy Problem in Cyclic Cosmology on a Brane with a Timelike Extra Dimension

We study cosmological bounces, turnarounds, and cyclic evolution on an anisotropic Bianchi-I brane embedded in a five-dimensional bulk with a \emph{timelike} extra dimension, within the Shtanov--Sahni braneworld framework. Restricting to the flat, dark-radiation-free, effective-$\Lambda$-free branch of the general anisotropic brane Friedmann equation, we drive the dynamics with a single canonical scalar field obeying the uniform-rate condition $\dot\phi=-\lambda=\mathrm{const}$, with shear anisotropy encoded through a geometric term $\Omega_\sigma(a)\propto a^{-6}$. We derive a general turning-point classification valid for any fluid obeying the null energy condition: turnarounds at negative energy density occur unconditionally, while bounces at $\rho>\rho_c$ occur only when the negative high-energy brane correction dominates the decelerating shear term. Specializing to the uniform-rate scalar, we obtain closed-form bounce and turnaround conditions, the leading-order excess of the bounce density above critical, and a matching condition for a finite cyclic branch connecting a bounce at $N_B$ to a turnaround at $N_T$. We identify post-bounce superinflationary and post-shear-dilution ordinary-inflationary regimes, compute the single-field curvature power spectrum, and derive parameter relations fixing $H_*$, $\lambda$, $\rho_c$, and the shear amplitude $\Sigma_g^2$ in terms of the observed amplitude $A_s$ and tilt $n_{s*}$. An explicit CMB-normalized parameter point shows that sustaining a long, weak-shear cyclic phase compatible with observations requires the shear amplitude suppressed by $10^{2}$--$10^{3}$ orders of magnitude below $H_*^2$, depending sensitively on the pivot density fraction $x_*=\rho_{\phi*}/\rho_c$. We discuss the physical origin of this anisotropy problem, its parametric dependence, and the status of the periodicity condition required for a genuinely cyclic $V(\phi)$.

gr-qc↗

arXiv2026

Gravastars on the brane with a timelike extra dimension

We construct a gravastar configuration within the Shtanov-Sahni (SS) braneworld scenario, characterized by a timelike extra dimension and negative brane tension. Unlike classical black holes, which inevitably culminate in central curvature singularities, our model demonstrates that the SS braneworld dynamics naturally regularize the interior geometry and prevent singularity formation. By solving the modified Einstein field equations induced on the brane, we obtain explicit interior, shell, and exterior solutions without invoking the idealized thin-shell approximation. The gravastar core is modeled as a Bose--Einstein condensate, while the intermediate shell consists of ultra-dense stiff matter. Bulk Weyl corrections induce anisotropic effective pressures on the brane, a feature that emerges intrinsically in this scenario and supports stability. We analyze the active gravitational mass, energy, entropy, and proper thickness of the shell, and establish the junction conditions at the interfaces. Our analysis reveals that the SS gravastar exhibits suppressed or even negative effective mass, reflecting the repulsive nature of the interior condensate, and admits stable equilibrium solutions consistent with energy conditions. This highlights the SS braneworld gravastar as a physically viable compact object and a compelling alternative to black holes. A key novelty of our construction is that the stabilizing pressure anisotropy and suppressed effective gravitational mass arise dynamically from higher-dimensional Weyl corrections, rather than being imposed through ad hoc matter sources or thin-shell idealizations. This provides the first fully analytic realization of a finite-thickness, stable gravastar in the Shtanov-Sahni braneworld, highlighting a genuinely geometric mechanism for singularity avoidance in compact objects.

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arXiv2025

Gravitational collapse and singularity avoidance of a homogeneous dust fluid on a brane with timelike extra dimension

We investigate the gravitational collapse of a homogeneous dust cloud in the Shtanov Sahni braneworld model, which incorporates an extra timelike dimension. The interior of the collapsing configuration is modeled by a Friedmann Lemaitre spacetime, while the exterior is described by a Vaidya radiation envelope that eventually settles into a static Reissner Nordstrom (RN) geometry with a positive tidal charge. Although a smooth matching between the interior and the static exterior is precluded by the breakdown of Birkhoff's theorem in the braneworld scenario, we show that as long as braneworld effects remain significant, the brane tension stays finite. Consequently, the scalar curvature remains bounded, thereby preventing the formation of a singularity.

gr-qc↗

arXiv2025

Co-existence of alternative Generalized Chaplygin Gas and other Dark Energies in the Framework of Fractal Universe

We have explored the possibility of the co-existence of two forms of dark energy in the form of an alternative Generalized Chaplygin Gas (GCG) along with a scalar field of field theoretic or extra dimensional origin in the background of a fractal universe. The essential physical model parameters have been computed and their variations have been plotted. Fractal cosmology in an universe dominated by the alternative GCG (overcoming the drawbacks of the conventional form of GCG) has also been studied by computing the equation of state, deceleration and statefinder parameters. Their variations with the redshift have also been studied. We have tried to construct alternative cosmological models deviating from standard cosmology that can be put to observational testing.

gr-qc↗

arXiv2025

Observationally constrained emergent universe scenario with non-conventional late-time dynamics

In this paper, we attempt to explore the possibility of a obtaining a viable emergent universe scenario supported by a type of fluid known as the extended Chaplygin gas, which extends a modification to the equation of state of the well known modified Chaplygin gas by considering additional higher order barotropic fluid terms. We consider quadratic modification only. Such a fluid is capable of explaining the present cosmic acceleration and is a possible dark energy candidate. We construct a theoretical model of the emergent universe assuming it is dominated by such a fluid at late times. Our model results in non-conventional late-time behavior and deviates from the standard $Λ$-CDM model. Dark energy is found to cross the \textit{phantom} divide in the past and present besides exhibiting \textit{thawing} behaviour in the future, asymptotically leading to transition into a decelerating phase making dark energy a \textit{transient} phenomenon. The qualitative nature of variation of the cosmological parameters resulting from model parameters observationally constrained through Markov Chain Monte Carlo sampling of Pantheon+OHD data is interestingly found to resemble the DESI results. Also,the value of $H(z)$ at a redshift $z=2.34$ and present value of Hubble parameter fits much better than $Λ$-CDM with recent observations. This leads us to the realization that such a fluid is not only a probable candidate for dark energy, but also sources an emergent universe unlike modified Chaplygin gas and the initial singularity problem can be resolved in a flat universe within the standard relativistic context.

gr-qc↗

arXiv2023

A novel model of non-singular oscillating cosmology on flat Randall-Sundrum II braneworld

We obtain a \textit{novel} model of oscillating non-singular cosmology on the spatially flat Randall-Sundrum (RS) II brane. At early times, the universe is dominated by a scalar field with an inflationary emergent potential $V(ϕ)=A(e^{Bϕ}-1)^2$, $A$ and $B$ being constants. Interestingly, we find that such a scalar field can source a non-singular bounce, replacing the big bang on the brane. The turnaround again happens naturally on the brane dominated by a phantom dark energy (favoured by observations\cite{E1,E2,E3} at late times), thus avoiding the big rip singularity and leading upto the following non-singular bounce via a contraction phase. There is a smooth non-singular transition of the brane universe through both the bounce and turnaround, leading to alternate expanding and contracting phases. This is the \textit{first} model where a single braneworld of positive tension can be made to recycle as discussed in details in the concluding section.

gr-qc↗

arXiv2023

Gravastar in the framework of Loop Quantum Cosmology

In this paper we attempt to construct a regular gravastar model using the UV corrected framework of Loop Quantum Cosmology. We find that a stable gravastar model can be constructed with a number of unique features: (i) no thin shell approximation needs to be invoked to obtain solutions in the shell which can be considered to be of a finite thickness, (ii) the central singularity of a self gravitating object can be averted by a bounce mechanism, such that the interior density of the gravastar reaches a maximum critical density and cannot be raised further due to an operative repulsive force, (iii) the inherent isotropy of the effective fluid description does not prevent the formation of a stable gravastar and anisotropic pressures is not an essential requirement.

gr-qc↗

arXiv2023

Lorentzian wormhole in the framework of loop quantum cosmology

In this paper, we construct a traversable static Lorentzian wormhole in the effective scenario of Loop Quantum Cosmology (LQC), where the field equations are modified due to the ultraviolet (UV) corrections introduced at large space-time curvatures. A stable wormhole can be constructed in the effective scenario without the violation of Null energy condition (NEC) by physical matter at the throat. The NEC is effectively violated due to the corrections in the field equations from LQC, resolving the Weyl curvature singularity at the throat. However, the physical matter does violate the Strong energy condition (SEC), suggesting the interesting possibility that dark energy can be harnessed into a wormhole. A possible explanation for this is the presence of inherent pressure isotropy in the UV-corrected field equations (discussed and compared to braneworld wormholes in the discussion). No additional exotic ingredient (violating NEC) is required, avoiding quantum instabilities. The tidal forces at the throat do not diverge and also the throat is found to be stable. The wormhole features an attractive geometry. LQC can resolve both types of curvature singularities appearing at the black hole center and wormhole throat, without exotic matter.

gr-qc↗

arXiv2023

Traversable Lorentzian wormhole on the Shtanov-Sahni braneworld with matter obeying the energy conditions

In this paper we have explored the possibility of constructing a traversable wormhole on the Shtanov-Sahni braneworld with a timelike extra dimension. We find that the Weyl curvature singularity at the throat of the wormhole can be removed with physical matter satisfying the NEC $ρ+p \geq 0$, even in the absence of any effective $Λ$-term or any type of charge source on the brane. (The NEC is however violated by the effective matter description on the brane arising due to effects of higher dimensional gravity.) Besides satisfying NEC the matter constituting the wormhole also satisfies the Strong Energy Condition (SEC), $ρ+3p \geq 0$, leading to the interesting possibility that normal matter on the brane may be harnessed into a wormhole. Incidentally, these conditions also need to be satisfied to realize a non-singular bounce and cyclic cosmology on the brane\cite{Sahni4} where both past and future singularities can be averted. Thus, such a cyclic universe on the brane, constituted of normal matter can naturally contain wormholes. The wormhole shape function on the brane with a time-like extra dimension represents the tubular structure of the wormhole spreading out at large radial distances much better than in wormholes constructed in a braneworld with a spacelike extra dimension and have considerably lower mass resulting in minimization of the amount of matter required to construct a wormhole. Wormholes in the Shtanov-Sahni (SS) braneworld also have sufficiently low tidal forces, facilitating traversability. Additionally they are found to be stable and exhibit a repulsive geometry. We are left with the intriguing possibilty that both types of curvature singularity can be resolved with the SS model, which we discuss at the end of the concluding section.

gr-qc↗

arXiv2023

Lorentzian wormholes in an emergent universe

A non-singular Emergent Universe (EU) scenario within the realm of standard Relativistic physics requires a generalization of the Equation of State (EoS) connecting the pressure and energy density. This generalized EoS is capable of describing a composition of exotic matter, dark energy and cosmological dust matter. Since the EU scenario is known to violate the Null Energy Condition, we investigate the possibility of presence of static, spherically symmetric and traversable Lorentzian wormholes in an EU. The obtained shape function is found to satisfy the criteria for wormhole formation, besides the violation of the NEC at the wormhole throat and ensuring traversability such that tidal forces are within desirable limits. Also, the wormhole is found to be stable through linear stability analysis. Most ${importantly}$, the numerical value of the emergent universe parameter $B$ as estimated by our wormhole model is in agreement with and lies within the range of values as constrained by observational data in a cosmological context. Also, the negative sign of the second EU parameter $A$ as obtained from our wormhole model is in agreement with the one required for describing an EU, which further indicates on the existence of such wormholes in an emergent universe ${without}$ accounting for any additional exotic matter field or any modification to the gravitational sector.

gr-qc↗

arXiv2023

Non-singular flat universes in braneworld and Loop Quantum Cosmology

In this paper we take matter source with non-linear Equation of state (EoS) that has produced non-singular Emergent cosmology for spatially flat universe in General Relativity and minimally coupled scalar field with two different potentials that produce an inflationary emergent universe for positive spatial curvature in the relativistic context. We study all these three cases both in the context of Randall-Sundrum braneworld and effective Loop quantum cosmology (LQC) for zero spatial curvature that is observationally favoured and in the absence of any effective cosmological constant term. We solve the modified Friedmann equation in each case to obtain the time evolution of the scale factor and use it to check whether the initial singularity can be averted. In almost all the cases we find the initial singularity is absent. We study the nature of the slow roll inflation in the cases where we obtain inflationary emergent universes. The inflationary scenario is found to be improved than in a standard relatvistic context and we compare the improved scenario for both the braneworld and LQC models. Interestingly, we also obtain bouncing and cyclic universes from our analysis in some cases. We find that the initial singularity can be averted for a spatially flat universe with specific choice of matter EoS or scalar field potential, which do not violate the Null Energy condition in most cases, taking into account effective high energy (curvature) corrections with or without extra dimensions.

gr-qc↗

arXiv2022

Modified Power law Inflation: solution to the graceful exit problem and improvement of dark energy models

We study power law inflation (PLI) with a monomial potential and find a novel exact solution. It is well known that conventional PLI with exponential potential is inconsistent with the Planck data. Unlike the standard PLI, present model does not suffer from graceful exit problem and it agrees fairly well with recent observations. We have calculated the spectral index and the tensor-to-scalar ratio which are in very good agreement with recent observational data and also comparable with other modified inflationary models. A technique has been used which shows that the large cosmological constant reduces with expansion of the Universe in case of the power law inflation. The coupling of the inflaton with gravitation is the main point in this technique. The basic assumption here is that the two metric tensors in the gravitational and the inflaton parts correspond to different conformal frames which is in contradiction with the conventional power law inflation where the inflaton directly coupled with the background metric tensor. This fact has direct application to different dark energy models and assisted quintessence theory.

gr-qc↗

arXiv2022

Weyl transformation: a dynamical degree of freedom in the light of Dirac's Large Number Hypothesis

In Einstein's Field Equation(EFE) the geometry of the space-time is connected with the matter distribution. The geometry or the gravitational sector deals with classical macroscopic objects involving gravitational units while the matter sector can be better described by quantum theory involving atomic units. It has been argued by Bisabr that there exists an epoch-dependent conversion factor between these two unit systems present in two different conformal frames,i.e. the conformal factor is epoch dependent. We argue that the conformal transformation is a dynamical degree of freedom describing it's possible relevance in inflation in context to the graceful exit problem, dynamics of the cosmological constant Λ and justify the argument in the light of consequences of Dirac's Large Number hypothesis(LNH).

gr-qc↗

arXiv2022

Lorentzian wormholes supported by tachyon matter

Wormholes with Ellis geometry have been successfully constructed using tachyon matter \cite{Das}. However, for such a wormhole, it is obtained that the redshift function is necessarily a constant, and also the wormhole is plagued with an imaginary tachyon potential and a constant field if the solutions are obtained in the absence of a cosmological constant term. So, a physically plausible wormhole solution is possible only in the presence of a $Λ$ term. In this paper, we try to construct a wormhole from tachyon matter with three $other$ geometries \textit{different} from the Ellis geometry and see whether it is possible to construct them successfully, besides checking whether the restrictions of the Ellis wormhole can be overcome with these geometries. Among others, we obtain one very interesting result that for all three of these geometries \textit{different} from the Ellis, the $Λ$ term is no longer an essential ingredient in constructing physically plausible traversable wormholes and the tachyon matter, capable of providing explanations for the "\textbf{dark sector}" of the universe,is itself sufficient for this purpose.

gr-qc↗

arXiv2022

Wormhole on the Brane with Ordinary Matter: The Broader View

In this paper we attempt to examine the possibility of construction of a traversable wormhole on the Randall-Sundrum braneworld with ordinary matter employing the Kuchowicz potential as one of the metric potentials. In this scenario, the wormhole shape function is obtained and studied, along with validity of Null Energy Condition (NEC) and the junction conditions at the surface of the wormhole are used to obtain a few of the model parameters. The investigation, besides giving an estimate for the bulk equation of state parameter, draws important constraints on the brane tension which is a novel attempt in this aspect and very interestingly the constraints imposed by a physically plausible traversable wormhole is in high confirmity with those drawn from more general space-times or space-time independent situations involved in fundamental physics. Also, we go on to claim that the possible existence of a wormhole may very well indicate that we live on a three-brane universe.

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