arXiv subjects

Anjan Ananda Sen

Publications and source records attributed to Anjan Ananda Sen.

10 recordsLinked to original sources

arXiv2025

Anchoring the Universe with Characteristic Redshifts using Raychaudhuri Equation Informed Reconstruction Algorithm (REIRA)

We study the robustness and physical implications of a set of characteristic redshifts that capture key features of the late-time Universe. Using both model-independent reconstructions as well as different dark energy (DE) parameterizations, we show that these redshifts remain stable across cosmological models and reconstruction algorithm, making them reliable geometric anchors of the expansion history. Moreover, the Alcock-Paczy\'nski corrections at these redshift anchors are found to be unity with high statistical significance, making them natural isotropy points in the comoving distance-redshift relation. We also find that certain redshifts anchors $(z < 1)$ coincide with epochs where strong deviations from the Planck $\Lambda$CDM baseline are apparent irrespective of DE parametrisation like CPL or reconstruction algorithm, indicating their potential as probes of new physics in cosmological evolution. Finally, we demonstrate, for the first time, that a Raychaudhuri Equation Informed Reconstruction Algorithm, substantially enhances the precision of the inferred distance measures and the Hubble expansion rate as well as results tighter constraints in the DE parameter space. These results demonstrate that combining geometric reconstruction with physics-informed kinematic information offers a powerful and consistent algorithm to probe new physics in the late-time dynamics of our Universe.

astro-ph.CO↗

arXiv2025

Can an Anti-de Sitter Vacuum in the Dark Energy Sector Explain JWST High-Redshift Galaxy and Reionization Observations?

The unexpectedly large abundance of UV-bright galaxies at $z>10$ discovered by the James Webb Space Telescope poses a significant challenge to the standard $\Lambda$CDM cosmology. This work tests whether modifying the cosmological background, and thereby the growth of structures, can resolve this tension without invoking significant evolution in the astrophysical properties of early galaxies. We investigate an alternative framework featuring an anti-de Sitter vacuum in the dark energy sector, which naturally arises in quantum gravity models like string theory and can enhance early structure formation. Using a self-consistent semi-analytical model that couples galaxy evolution with reionization, we confront this scenario with a wide range of observations. We show that while a model tailored to fit the high-$z$ UV luminosity functions (UVLFs) appears promising, it is in strong tension with cosmological constraints from the CMB and other low-redshift probes. Conversely, models within this framework that satisfy these constraints provide only a modest boost to structure formation and fail to reproduce the observed galaxy abundances at $z>10$. Although these models remain consistent with the cosmic reionization history, we find that this class of cosmological modifications is insufficient on its own to explain the galaxy excess. Our study underscores the importance of holistic testing for any beyond-$\Lambda$CDM proposal; apparent success in one observational regime does not guarantee overall viability. By demonstrating the limitations of a purely cosmological solution, our results strengthen the case that evolving astrophysical properties are a necessary ingredient for solving the challenge of early galaxy formation.

astro-ph.CO↗

arXiv2025

The CosmoVerse White Paper: Addressing observational tensions in cosmology with systematics and fundamental physics

The standard model of cosmology has provided a good phenomenological description of a wide range of observations both at astrophysical and cosmological scales for several decades. This concordance model is constructed by a universal cosmological constant and supported by a matter sector described by the standard model of particle physics and a cold dark matter contribution, as well as very early-time inflationary physics, and underpinned by gravitation through general relativity. There have always been open questions about the soundness of the foundations of the standard model. However, recent years have shown that there may also be questions from the observational sector with the emergence of differences between certain cosmological probes. In this White Paper, we identify the key objectives that need to be addressed over the coming decade together with the core science projects that aim to meet these challenges. These discordances primarily rest on the divergence in the measurement of core cosmological parameters with varying levels of statistical confidence. These possible statistical tensions may be partially accounted for by systematics in various measurements or cosmological probes but there is also a growing indication of potential new physics beyond the standard model. After reviewing the principal probes used in the measurement of cosmological parameters, as well as potential systematics, we discuss the most promising array of potential new physics that may be observable in upcoming surveys. We also discuss the growing set of novel data analysis approaches that go beyond traditional methods to test physical models. [Abridged]

astro-ph.CO↗

arXiv2025

Towards a Composite Framework for Simultaneous Exploration of New Physics in Background and Perturbed Universe

We investigate deviations from $\Lambda$CDM by independently parameterizing modifications in the background evolution and the growth of structures. The background is characterized by two parameters, $A$ and $B$, which reduce to $A=\Omega_{m0}$ and $B=2/3$ in the $\Lambda$CDM limit, while deviations in the growth of structures are captured through a fitting function for $f\sigma_8$ involving the growth index $\gamma$. Using recent observational datasets involving background expansion and growth of structures (related to observations involving redshift space distortions), we find significant evidence for departures from $\Lambda$CDM in the background expansion whereas there is no finite evidence for deviations from $\Lambda$CDM behaviour in the growth of structures. This suggests that with the current precision in observational data involving background and perturbed Universe, a deviation from $\Lambda$CDM behaviour is confirmed (as shown in the recent DESI-DR2 results). But whether this deviation is due to an evolving Dark Energy or due to the modification of gravity at cosmological scales is still an open question, largely due to less precise data from perturbed Universe. We further demonstrate how the future high-precision growth data (from Euclid, for example) can answer such question using a forecast study.

astro-ph.CO↗

arXiv2024

Thirty Meter Telescope International Observatory Detailed Science Case 2024

The Thirty Meter Telescope (TMT) International Observatory (TIO) will be a revolutionary leap forward in astronomical observing capabilities, enabling us to address some of the most profound questions about the universe. From unraveling the mysteries of dark matter and dark energy to exploring the origins of stars and planets, TMT will transform our understanding of the cosmos. The TIO Detailed Science Case (DSC) presents science goals that inform the top-level requirements for the observatory's design and operations, including the telescope, enclosure, instruments, and adaptive optics system.

astro-ph.IM↗

arXiv2024

The Excess of JWST Bright Galaxies: a Possible Origin in the Ground State of Dynamical Dark Energy in the light of DESI 2024 Data

Recent observations by JWST yield a large abundance of luminous galaxies at $z\gtrsim 10$ compared to that expected in the CDM scenario based on extrapolations of the star formation efficiency measured at lower redshifts. While several astrophysical processes can be responsible for such observations, here we explore to what extent such an effect can be rooted in the assumed Dark Energy (DE) sector of the current cosmological model. This is motivated by recent results from different cosmological probes combined with the last data release of the Dark Energy Spectroscopic Instrument (DESI), which indicate a tension in the DE sector of the concordance $Λ$ CDM model. We have considered the effect of assuming a DE characterized by a negative Λ as the ground state of a quintessence field on the galaxy luminosity function (LF) at high redshifts. We find that such models naturally affect the galaxy UV luminosities in the redshift range $10 \lesssim z\lesssim 15$ needed to match the JWST observations, and with the value of $Ω_Λ$=[-0.6,-0.3] remarkably consistent with that required by independent cosmological probes. A sharp prediction of such models is the steep decline of the abundance of bright galaxies in the redshift range $15 \lesssim z\lesssim 16$.

astro-ph.CO↗

arXiv2024

Model-independent cosmological inference post DESI DR1 BAO measurements

In this work, we implement Gaussian process regression to reconstruct the expansion history of the universe in a model-agnostic manner, using the Pantheon-Plus SN-Ia compilation in combination with two different BAO measurements (SDSS-IV and DESI DR1). In both the reconstructions, the $\Lambda$CDM model is always included in the 95\% confidence intervals. We find evidence that the DESI LRG data at $z_{\text{eff}} = 0.51$ is not an outlier within our model-independent framework. We study the $\mathcal{O}m$-diagnostics and the evolution of the total equation of state (EoS) of our universe, which hint towards the possibility of a quintessence-like dark energy scenario with a very slowly varying EoS, and a phantom-crossing in higher $z$. The entire exercise is later complemented by considering two more SN-Ia compilations - DES-5YR and Union3 - in combination with DESI BAO. Reconstruction with the DESI BAO + DES-5YR SN data sets predicts that the $\Lambda$CDM model lies outside the 3$\sigma$ confidence levels, whereas with DESI BAO + Union3 data, the $\Lambda$CDM model is always included within 1$\sigma$. We also report constraints on $H_0 r_d$ from our model-agnostic analysis, independent of the pre-recombination physics. Our results point towards an $\approx$ 2$\sigma$ discrepancy between the DESI + Pantheon-Plus and DESI + DES-5YR data sets, which calls for further investigation.

astro-ph.CO↗

arXiv2024

Gravitational atoms in the braneworld scenario

In the Randall-Sundrum (RS) II braneworld scenario, general relativity (GR) is modified by adding an extra dimension such that it is indistinguishable from GR in the weak gravity limit. However, such modifications may leave a mark in the strong field regime. We therefore analyze massive scalar perturbations around rotating black holes in the RS II model. Unlike black holes in GR, these braneworld black holes carry a tidal charge that contains information about the extra spatial dimension, and the rotation parameter for such black holes can exceed unity. Through the method of continued fractions, we investigate the quasinormal mode spectra, and the superradiant instabilities associated with the existence of quasibound states, that is, gravitational atoms. In comparison to the four-dimensional Kerr black hole, we report distinctive signatures of the tidal charge and the rotation parameter, which manifest as signals of the extra dimension, on both the fundamental quasinormal mode and the formation of gravitational atoms. These findings offer insights into testing modifications to GR and detecting ultralight bosonic particles around black holes.

gr-qc↗

arXiv2012

Standard Cosmology Delayed

The introduction of a delay in the Friedmann equation of cosmological evolution is shown to result in the very early universe undergoing the necessary accelerated expansion in the usual radiation (or matter) dominated phase. Occurring even without a violation of the strong energy condition, this expansion slows down naturally to go over to the decelerated phase, namely the standard Hubble expansion. This may obviate the need for a scalar field driven inflationary epoch.

gr-qc↗

arXiv2002

Brane Dynamics in the Randall-Sundrum model, Inflation and Graceful Exit

We study the averaged action of the Randall-Sundrum model with a time dependent metric ansatz. It can be reformulated in terms of a Brans-Dicke action with time dependent Newton's constant. We show that the physics of early universe, particularly inflation, is governed by the Brans-Dicke theory. The Brans-Dicke scalar, however, quickly settles to its equilibrium value and decouples from the post-inflationary cosmology. The deceleration parameter is negative to start with but changes sign before the Brans-Dicke scalar settles to its equilibrium value. Consequently, the brane metric smoothly exits inflation. We have also studied the slow-roll inflation in our model and investigated the spectra of the density perturbation generated by the radion field and find them consistent with the current observations.

hep-th↗