arXiv subjects

Sukanta Panda

Publications and source records attributed to Sukanta Panda.

At least 19 recordsLinked to original sources

arXiv2026

Modifications of CMB Temperature and Polarization Quadrupole Signals in Thurston Spacetimes

Recent cosmological tests have discovered a fresh new set of anomalies in the large-scale isotropy of the universe. Motivated thus by the numerous pieces of evidence for large-scale cosmic isotropy violation with the advent of the 'precision cosmology' era, we are led to explore the viability of anisotropic Thurston geometries, described in William Thurston's geometrization conjecture. In this work, we examine the coherent temperature and polarization signals generated in the CMB sky by such geometries. We begin with introducing Thurston spacetimes as our background model and the formalism we use to obtain the patterns. We then construct a set of transfer equations relative to a given background and solve them for each spacetime geometry. We finally discuss the role of spatial curvature in these FLRW limiting models along with their underlying geometry, and attempt to establish some general results on the symmetries of the patterns produced by their time evolution in terms of the Stokes parameters P, Q, U and V. We show the evolution of temperature and polarization amplitudes in terms of such Stokes parameters at different timestamps and attempt to isolate individual Thurston geometries.

gr-qc↗

arXiv2026

Oscillon Formation in Palatini Modified Gravity Theories

We investigate the formation of spatially localized, oscillatory in time and non topological solitonic, quasi-stable energy configurations, Oscillons, which are formed at the end of Inflationary epoch, during the preheating phase and decay over long periods of time. Oscillons have been previously studied in literature in the regime of General Relativity using Metric Formalism. In this paper we look for formation of these energy lumps by modifying the gravity part of the Einstein Hilbert Action, considering a non minimal coupling of the scalar field with Ricci Scalar,$R$, and working in an alternative formulation of General Relativity known as Palatini Formalism. The potential we consider is of polynomial form. We demonstrate numerically, using CosmoLattice, that the equation governing the dynamics of the Inflaton scalar field give oscillatory and decaying solutions as it is expected in the case of Oscillons, with power spectrum governing the growth of perturbations of $k$ modes. The equation of state reveals an extended period of Oscillon domination in the early universe. Along with this, the Primordial Gravitational Wave spectrum due to asymmetric distribution of these energy configurations have also been studied. We observe that these generate Ultra-High Frequency regime Gravitational Waves, which lie in the range of the planned future detectors and experiments.

gr-qc↗

arXiv2026

Thurston geometries and parameter constraints from SNIa data

Following the numerous evidence for large-scale cosmic isotropy violation with the advent of the `precision cosmology' era, we explore the possible advantages of extending the flat $\Lambda$CDM model to more general models in order to constrain anisotropies in the universe, otherwise absent in the standard model based on FLRW spacetime. Such extensions are offered by the topologically unique Thurston geometries, which are homogeneous but anisotropic spacetime models. In this work, we attempt to distinguish Thurston geometries from one another by introducing anisotropies via different scale factors in different directions, thereby introducing additional model parameters such as shear, eccentricity, curvature, and a preferred axis. We used the latest compilation of Pantheon+ \& SH0ES Type Ia supernova data for deriving model constraints, and found mild evidence of large-scale isotropy violation.

gr-qc↗

arXiv2025

Phantom Menace in general Palatini $f(R,\phi)$ theories

We study general $f(R,\phi)$ theories in Palatini formalism and attempt to constrain the behavior of ones that could support both inflationary and late-time expansion era in a unified model. In particular, we find conditions for which the theories remain consistent in weak gravity regimes as well as cosmic expansion eras in both early and late universe. Assuming that the curvature part of the $f(R,\phi)$ behaves as Starobinsky gravity, we assess post-inflation dynamical stability of the theory in Einstein frame and proceed to isolate two distinct fixed points that provide a stable late-time accelerating universe. Comparison with DESI, Cosmic Chronometers, and SNeIa datasets adds more stringent constraints to the behavior of the theory near the present epoch, giving us one stable fixed point where expansion is driven by a phantom scalar field. However, time scales of the two fixed points suggest that this fixed point may be transient and may eventually evolve toward a stable expansion stage driven potential domination in the distant future of the universe.

gr-qc↗

arXiv2025

The PAU Survey: Uncovering the connection between intrinsic and observed galaxy properties using symbolic regression

Estimating stellar masses for billions of galaxies in upcoming surveys requires methods that are both accurate and computationally efficient. We present a new approach using symbolic regression trained on a simulation to derive simple, explicit mathematical expressions that estimate galaxy stellar masses from basic observables: photometry and redshift. Using a mock catalogue from the GALFORM semi-analytical model that reproduces the Physics of the Accelerating Universe Survey (PAUS), we show that a linear combination of just four observables -- minimally processed $u$- and $i$- band magnitudes, observed $(g-r)$ colour, and redshift -- can recover stellar masses with accuracy comparable to traditional spectral energy distribution (SED) fitting, but with negligible computational cost. Our expressions can be evaluated instantaneously for millions of galaxies, making them ideal for next-generation surveys like LSST and Euclid. When observational errors are included, symbolic regression achieves a similar accuracy to deep neural networks while maintaining transparency. Validation against CIGALE SED fitting on PAUS data shows agreement within 0.13 dex for galaxies with $M_{*} > 10^8 M_{\odot}$. We demonstrate that the stellar mass function can be recovered at $z < 0.5$, though with distortions at the extremes: the high-mass end is overestimated by a factor of $\sim 3$ at $10^{11.5} h^{-1} M_{\odot}$ due to scatter. Our approach offers a fast, transparent alternative to traditional methods without sacrificing accuracy for the bulk of the galaxy population.

astro-ph.GA↗

arXiv2025

Revisiting Lagrangian Formulation of Stochastic inflation

We revisit the Lagrangian formulation of stochastic inflation, where the path-integral approach is employed to derive the Langevin equation governing the dynamics of long-wavelength fields, in contrast to the standard method where the Langevin equation is derived directly from the equation of motion of the full quantum field. Focusing on a massless, minimally coupled scalar field with quartic self-interaction in a de Sitter background, we re-derive the formal expression for the influence functional that encapsulates the effects of short-wavelength fields up to second order in the coupling constant, and compare our results with those obtained in earlier works. In doing so, we highlight certain subtleties that have been previously overlooked, including the non-orthogonality between long- and short-wavelength modes, which we analyze in detail, as well as the absence of a consistent prescription for handling general interaction terms in the imaginary part of the influence functional. The latter issue points to a broader challenge: the lack of a universally accepted framework for treating the imaginary component of effective actions.

hep-th↗

arXiv2025

Cosmological viability of anisotropic inflation in Thurston spacetimes

Recent observations of large-scale statistical isotropy violations have prompted the adoption of anisotropic cosmological models that account for inherent directional curvature. Studies of these anisotropic spacetimes have shown how they can explain the evolutionary dynamics and light propagation in the universe. Here, we consider one such interesting set of spacetimes that preserve homogeneity but place no constraint on isotropy during the inflationary epoch, to examine whether we can address the possibility of anisotropic inflation in the universe. Researchers have proposed inflationary models in which a vector field coupled to the inflaton is found to violate the cosmic no-hair theorem for the anisotropic Bianchi type I spacetime, due to the existence of a stable anisotropically inflationary fixed point. Lately, this study has been extended to axisymmetric spacetimes of Bianchi type II, III, and the Kantowski-Sachs metric, and it has been inferred that the entire family of spacetimes is attracted to the anisotropic Bianchi I fixed point. By constructing inflationary models where the spatial slices are anisotropic Thurston 3-geometries, we demonstrate that the intrinsic eccentricity of the background geometry induces an isotropy-violating vector field. This field, through its coupling to the inflaton, triggers a secondary phase of anisotropic inflation. We perform dynamical stability and phase-space analyses to assess the feasibility of anisotropic inflation. The results for the considered set of Thurston geometries showed the presence of a unique, stable inflationary fixed point that converges, similar to those in Bianchi spacetimes, thereby indicating the cosmological viability of inflation with anisotropic hair.

gr-qc↗

arXiv2025

Exploring Coupled Quintessence in light of CMB and DESI DR2 measurements

We perform a detailed analysis of a theoretically motivated dark energy quintessence model which interacts with the dark matter sector of the universe. Utilising the current observational datasets from the Cosmic Microwave Background, Baryon Acoustic Oscillations and Type Ia Supernovae, we constrain the parameters that characterise the strength of the time dependent interaction. We also look at the effect of a warm dark matter component in the context of coupled quintessence. Analysis using Deviance Information Criterion indicates strong preference for the quintessence model coupled with warm dark matter. However, Bayesian evidence analysis shows favor in the direction of $\Lambda$CDM model.

gr-qc↗

arXiv2025

Scalar and vector modes in inflation with antisymmetric tensor field

We investigate the scalar and vector modes arising from cosmological perturbations within the framework of an inflationary scenario driven by an antisymmetric tensor field, minimally coupled to gravity. After eliminating gauge artifacts, there remain four scalar and six vector modes of interest which can be studied separately. We analyze the stability of these modes, while looking for generic instabilities like ghost and gradient instabilities that could potentially plague the theory. Further, we investigate the evolution of these modes across different regimes, particularly subhorizon and superhorizon scales.

gr-qc↗

arXiv2025

Eff-ACT-ive Starobinsky pre-inflation

We consider quantum corrections to a recently obtained perturbative form of Starobinsky model to extract information about the initial conditions of the universe leading to cosmological inflation. Integrating out graviton modes, we find higher-derivative instabilities that are shown to decay into scalarons, causing an effective kinetic-domination stage which is shown to lead naturally to inflation without the need for fine-tuning of initial inflaton amplitude. We find a perturbative upper bound on scalaron magnitude that matches Planck constraints on inflaton energy density near the pivot scale. This modified history also affects observables and resolves other anomalies such as the low-$\ell$ power deficit in the TT spectrum as well as the disfavorment of Starobinsky model based on updated predictions of scalar spectral index accounting for data from Atacama Cosmology Telescope (ACT).

gr-qc↗

arXiv2025

Testing the Starobinsky model of inflation with resonant cavities

We show that the Starobinsky inflation model based on $R^2$ gravity has a special feature that it provides a unique scalaron-two-graviton vertex with a coupling proportional to $1/M_P$. In this model stochastic gravitational waves are produced when the scalaron - which is the massive scalar mode of the metric - decays into gravitons during reheating. This decay is accompanied by decay of scalaron into matter as well through a similar coupling, providing an efficient reheating. The stochastic gravitational waves thus produced have characteristic strain $h_c\sim 10^{-35}-10^{-34}$ in the frequency range $10^{6}-10^{12}\, {\rm Hz}$ which makes them accessible to resonant cavity searches for graviton to photon conversions. The detection of these high frequency gravitational waves would be a significant step in experimentally testing the Starobinsky inflation model.

gr-qc↗

arXiv2025

Observable Primordial Gravitational Waves from Non-minimally Coupled $R^2$ Palatini Modified Gravity

We probe the spectrum of primordial gravitational waves (GWs) produced during the eras of hyperkination, kination, and reheating in a non-minimally coupled, $\mathcal{L} \propto (1+ \xi \chi /M_{\text{Pl}})^t (R+\alpha R^2)$, modified gravity using the Palatini formulation. We consider a runaway potential, which gives an era of kinetic domination after the end of inflation. The coupling order $t$ is varied to examine a large class of theories up to $\chi^2 R^2$. For models with $t>0$, reheating is not achieved naturally; hence, we supplement such theories with a reheating mechanism based on the interaction of inflaton and radiation produced at the end of inflation due to cosmological expansion. We demonstrate that the energy density of the GWs is enhanced as a function of the coupling during kination for all considered theories, and a short-lived phase of hyperkination truncates the boost and avoids the over-production of GWs. Hyperkination, and thus the $R^2$ term, should be deemed necessary in all theories with a runaway potential as it prevents the GW enhancement during kination from destabilizing the Big Bang Nucleosynthesis. The spectrum remains flat for the period of hyperkination and reheating. We examine the available parameter space for which the theories remain valid and place bounds on the Hubble parameter ($H$) and radiation energy density ($\Omega_r^{\text{end}}$) at the end of inflation. We find that as we decrease the order of the coupling, the spectra shift towards a more observable regime of future GW experiments. The observation of the plateau during reheating will constrain the $H$ and $\Omega_r^{\text{end}}$ values, while the spectral shape of the boost obtained during kination will confirm the nature of the theory. The bounds from hyperkination lie in the kHz-GHz frequency range.

astro-ph.CO↗

arXiv2024

Quantizable Ghost-ridden theories using Kinetic Positivity Constraints

We present a novel way to constrain the ghost field with respect to other physical fields present in a given theory such that the theory becomes quantizable. This is achieved by imposing positivity of the total kinetic energy of the system and performing Lorentz transformations in the field space manifold to arrive at an effective Lagrangian containing only physical degrees of freedom. Since models containing ghost fields such as quintom models are relevant in the cosmological context, this method can help ensure that such theories don't violate unitarity and can be treated as realistic candidates without the need to completely eliminate ghost(s).

hep-th↗

arXiv2024

One-particle irreducibility of Vilkovisky-DeWitt effective action

The effective action formalism introduced by Vilkovisky and later modified by DeWitt is completely covariant suitable for obtaining an effective action that is independent of the parametrization of the quantum fields. Among a few basic properties of an effective action, one property is that it can be written as a sum of one-particle irreducible diagrams. In this work, we verify this property up to three-loop order for Vilkovisky-Dewitt effective action (VDEA) by solving the equations satisfied by VDEA written down in the original parametrization for non-gauge theories.

hep-th↗

arXiv2024

Role of Spatial Curvature in a Dark Energy Interacting Model

This paper investigates the effects of spatial curvature in a model where dark matter and dark energy interact. The analysis employs a range of datasets, including CMB, BAO, Type Ia Supernova, $H(z)$ from cosmic chronometers, $H_0$ measurements from Megamasers and SH0ES, growth rate data and strong lensing time delay measurements, to assess the model's fit and explore the late-time dynamics of the interacting dark sector in a non-flat cosmological framework. The study indicates that introducing curvature does affect the Hubble constant ($H_0$) and the structure growth parameter ($S_8$), and also helps in alleviating the tensions between early and late universe observations to some extent. The observational data shows an indication for an open universe. This implies that the presence of curvature and its influence on Universe's evolution cannot be neglected entirely.

astro-ph.CO↗

arXiv2024

Open EFT treatment of Inflation with Thermal Initial Conditions

Investigating the thermal inflationary model, we introduce stochastic effects, incorporating a cutoff parameter $\sigma$ which distinguishes between quantum and classical modes. Testing the model against Planck 2018 data, we observe a preference for a non-zero $\sigma$ at least at 68\% C.L., suggesting the classicalization of most modes and providing a theoretical foundation for the quantum to classical transition. As a result of introducing the stochastic effects, we find that the solution to the large-scale power deficit requires a lower comoving temperature of inflaton.

gr-qc↗

arXiv2023

Constraints on Bianchi-I type universe with SH0ES anchored Pantheon+ SNIa data

We study the Bianchi-I cosmological model motivated by signals of statistical isotropy violation seen in cosmic microwave background (CMB) observations and others. To that end, we consider various kinds of anisotropic matter that source anisotropy in our model, specifically Cosmic strings, Magnetic fields, Domain walls and Lorentz violation generated magnetic fields. These anisotropic matter sources, taking one at a time, are studied for their co-evolution with standard model (isotropic) sources viz., dust-like (dark/normal) matter, and dark energy modelled as cosmological constant. We constrain the Hubble parameter, density fractions of anisotropic matter, cold dark matter (CDM), and dark energy ($\Lambda$) in a Bianchi-I universe with planar symmetry i.e., which has a global ellipsoidal geometry, and try to find signatures of a cosmic preferred axis if any. The latest compilation of Type Ia Supernova (SNIa) data from Pantheon+SH0ES collaboration is used in our analysis to obtain constraints on cosmological parameters and any preferred axis for our universe. In our analysis, we found mild evidence for a cosmic preferred axis. It is interesting to note that this preferred axis lies broadly in the vicinity of other prominent cosmic anisotropy axes reported in the literature from diverse data sets. Also we find some evidence for non-zero (negative) cosmic shear and eccentricity that characterize different expansion rates in different directions and deviation from an isotropic scale factor respectively. The energy density fractions of two of the sources considered are found to be non-zero at a $2\sigma$ confidence level. To be more conclusive, we require more SNIa host galaxy data for tighter constraints on distance and absolute magnitude calibration which are expected to be available from the future JWST observations and others.

astro-ph.CO↗

arXiv2023

Coupled Quintessence scalar field model in light of observational datasets

We do a detailed analysis of a well-theoretically motivated interacting dark energy scalar field model with a time-varying interaction term. Using current cosmological datasets from CMB, BAO, Type Ia Supernova, $H(z)$ measurements from cosmic chronometers, angular diameter measurements from Megamasers, growth measurements, and local SH0ES measurements, we found that dark energy component may act differently than a cosmological constant at early times. The observational data also does not disfavor a small interaction between dark energy and dark matter at late times. When using all these datasets in combination, our value of $H_0$ agrees well with SH0ES results but in 2.5$\sigma$ tension with Planck results. We also did AIC and BIC analysis, and we found that the cosmological data prefer coupled quintessence model over $\Lambda$CDM, although the chi-square per number of degrees of freedom test prefers the latter.

astro-ph.CO↗