arXiv subjects

Sandeep Haridasu

Publications and source records attributed to Sandeep Haridasu.

11 recordsLinked to original sources

arXiv2026

Extended Universal Rotational Curve of Spiral Galaxies

In this work, we aim to advance the Universal Rotation Curve (URC) paradigm by leveraging new data and extending its observational domain. Building on previous studies that established the URC using optical rotation curves reaching the galaxy optical radius $R_{opt}$, we exploit the SPARC sample's extended HI rotation curves to construct the URC out to $2R_{opt}$. This crucial extension enables us to investigate the mass distribution of spiral galaxies in a region dominated by dark matter an important step to better constrain galaxy mass models and to explore the nature of the dark matter particle. We find that the URC constructed from the SPARC sample's extended HI rotation curves maintains its universal character out to $2R_{opt}$, with the double-normalized rotation curves collapsing onto a single profile. This extended URC provides new insights into the interplay between baryonic matter and dark matter in shaping galaxy rotation curves, particularly in the outer regions where dark matter dominates. Our results not only reinforce the URC paradigm but also refine our understanding of the mass distribution in spiral galaxies, offering new constraints on galaxy mass models and implications for the nature of dark matter.

astro-ph.GA↗

arXiv2025

Dynamical Dark Energy or Modified Gravity? Signatures in Gravitational Wave Propagation

We discuss the implications of the most recent DESI Baryon Acoustic Oscillations (BAO) DR2 and DESyr5 compilation of supernovae (SNe) datasets for modified gravity focusing on non-metricity based $f(Q)$ theory, by employing a `model-independent' approach. We reconstruct of dark energy density which is then extended to estimate the perturbation-level quantities, sourced by the modified gravity, namely the effective gravitational coupling, $\mu$, and the amplitude damping parameter of gravitational wave propagation, $\nu$. In light of the remarkable hints for a dynamical dark energy emerging from the analysis of background cosmological data, we discuss the possibility to distinguish between dark energy and modified gravity scenarios from their scalar and tensor perturbation-level signatures. We contrast our findings between the older Pantheon+ and the newer DESyr5 compilations of SNe datasets, which predict a significant distinct signature in the damping parameter of the gravitational wave propagation amplitude. On the other hand, the prediction for the effective gravitational coupling remains insensitive to the choice of the SNe datasets. Our results provide a strong case for the study of gravitational wave propagation in modified gravity theories, in light of the new generation of gravitational wave detectors such as LISA, Einstein Telescope and Cosmic Explorer.

gr-qc↗

arXiv2025

Constraints on Logarithmic Model Extensions of Symmetric Teleparallel Gravity

We address various cosmological phenomenologies in the symmetric teleparallel framework both in background and perturbation such as cosmic expansion, gravitational coupling constant, gravitational waves propagation. Focusing on logarithmic extensions of $f(Q)$ models, we performed Bayesian analysis using the most-recent cosmological data, DESI DR2, Pantheon+. We also utilized a compilation of redshift space distortions ($f \sigma_8$) dataset to constrain the growth of structures in each of the models modulated by the effective gravitational coupling. We find that our extended Logarithmic $f(Q)$ models are well-constrained by the current cosmological data and are able to describe the late-time cosmic acceleration. The inverse Logarithmic model we introduce is also able to accommodate a phantom-like dark energy equation of state at late times, which is consistent with the recent DESI DR2 observations. We report explicitly predictions for the effective gravitational coupling ($\mu$), and the amplitude damping parameter of gravitational wave ($\nu$) solely based on the background data, which can be tested against future observations. While the two Log-based extensions we have introduced here perform equivalently on the background level, they provide contrasting predictions for the evolution of effective Gravitational constant and propagation of gravitational waves, which should be constrained against the future perturbation data.

astro-ph.CO↗

arXiv2025

Outer regions of galaxy clusters as a new probe to test modifications to gravity

We apply the caustic technique to samples of galaxy clusters stacked in redshift space to estimate the gravitational potential in the cluster's outer region and test modifications to the standard theory of gravity. We separate 122 galaxy clusters from the HeCS-SZ, HeCS-redMapper, and HeCS samples into four samples with increasing mass; we estimate four robust, highly constraining caustic profiles for these samples. The caustic masses of the four stacked clusters agree within $ 10\%$ with the corresponding median values of each cluster sample. By adopting the NFW density profile to model the gravitational potential, we recover the caustic profile $\mathcal{A}(r)$ up to radius $r_{\rm p} \sim 4.0\, {\rm Mpc}$. This comparison is a first-order validation of the mass-concentration relation for galaxy clusters expected in the $\Lambda$CDM model. We thus impose this correlation as a prior in our analysis. Based on our stacked clusters, we estimate the value of the filling factor, which enters the caustic technique, $\mathcal{F}_{\beta} = 0.59\pm 0.05$; we derive this value using real data alone and find it consistent with the value usually adopted in the literature. We then use the caustic profiles $\mathcal{A}(r)$ of the stacked clusters to constrain the chameleon gravity model. We find that the caustic profiles provide a stringent upper limit of $|f_{\rm R0}| \lesssim 4 \times 10^{-6}$ at $95\%$ C.L. limits in the $f(\mathcal{R})$ scenario. The formalism developed here shall be further refined to test modifications to gravity in the extended outer weak gravitational regions of galaxy clusters.

astro-ph.CO↗

arXiv2025

Constraints on Generalized Gravity-Thermodynamic Cosmology from DESI DR2

We explore the cosmological implications of generalized entropic models within the framework of Gravity-Thermodynamics (GT) approaches. These models, characterized by three or four additional free parameters, are designed to capture deviations from the standard Bekenstein-Hawking entropy and can reproduce well-known entropic formulations, including Tsallis, R\'enyi, Sharma-Mittal, Barrow, Kaniadakis, and Loop Quantum Gravity entropies in various analytical limits. We implement the corresponding cosmological models using a fully numerical GT approach to constrain the model parameters and to study the evolution of the dark energy equation of state as a function of the scale factor. Our Bayesian analysis, which incorporates the Pantheon+ and DESy5 supernovae data alongside the recently released DESI-DR2/DR1 Baryon Acoustic Oscillation (BAO) measurements, shows that the data favor the standard Bekenstein-Hawking entropy, leading to a $\Lambda$CDM-like late-time behavior. In this context, the three-parameter ($\mathcal{S}_3$) entropic model appears to be sufficient to capture the observed dark energy phenomenology. Furthermore, a direct comparison of the Bayesian evidence indicates that the three-parameter model is preferred over the four-parameter ($\mathcal{S}_4$) variant by a factor of $\Delta\log\mathcal{B} \sim -6$, while the GT approach as a whole is significantly disfavored relative to the $\Lambda$CDM model with at least $\Delta\log\mathcal{B} \sim -8$ ($\mathcal{S}_3$) to $\Delta\log\mathcal{B} \sim -13$ ($\mathcal{S}_4$), when using the DESy5 and DESI-DR2 datasets.

astro-ph.CO↗

arXiv2025

Unveiling the Coma Cluster Structure: From the Core to the Hubble Flow

The Coma cluster, embedded in a cosmic filament, is a complex and dynamically active structure in the local Universe. Applying a density-based member selection dbscan to data from the Sloan Digital Sky Survey (SDSS), we identify cluster member galaxies from its virialised core out to the zero-velocity boundary in the least model-dependent way. From dbscan, we infer a projected virial radius of $r_{\rm vir} = \left(1.95 \pm 0.12\right)\,h^{-1}~\text{Mpc}$ and projected zero-velocity radius of $r_{\rm ta} \geq 4.87~{h}^{-1}~\mbox{Mpc}$. Assuming that the barycentre of Coma has zero peculiar velocity, its distance from us is $r_\mathrm{c}=(69.959 \pm 0.012_\mathrm{stat}) \, h^{-1}~\text{Mpc}$ determined from the redshifts of 1092 member galaxies. Cross-correlating with the Cosmicflows-4 (CF4) catalogue enables a velocity-distance analysis. This reveals, for the first time, the Hubble flow surrounding Coma, a first step to investigate the entanglement between Coma's dark matter halo and the dark energy driving the expansion of the surroundings. If $v_\mathrm{c}$ is moving with the cosmic expansion, the CF4 distances yield a Hubble constant $H_0 = (73 \pm {1_\mathrm{stat} \pm 7_\mathrm{sys}})~\mbox{km}/\mbox{s}/\mbox{Mpc}$ with a dominating systematic error from different calibrations for the distance moduli. Mass estimates via caustics, the virial theorem, and the Hubble-flow method yield $M = [0.77, 2.0] \times 10^{15}\,h^{-1}\,M_{\odot}$ consistent with prior mass estimates. Our mass estimates are based on fewer model assumptions in the member selection and require $\sim20\%$ members to attain the same precision. Our approach maps the structure of Coma into its Hubble flow and shows degeneracies between the Hubble constant, the virial radius, and the total mass only using data and models from the single line-of-sight towards Coma.

astro-ph.CO↗

arXiv2024

Modified gravity in galaxy clusters: Joint analysis of Hydrostatics and Caustics

We present a comprehensive joint analysis of two distinct methodologies for measuring the mass of galaxy clusters: hydrostatic measurements and caustic techniques. We show that by including cluster-specific assumptions obtained from hydrostatic measurements in the caustic method, the potential mass bias between these approaches can be significantly reduced. Applying this approach to two well-observed massive galaxy clusters A2029 and A2142. We find no discernible mass bias, affirming the method's validity. We then extend the analysis to modified gravity models and draw a similar conclusion when applying our approach. Specifically, our implementation allows us to investigate Chameleon and Vainshtein screening mechanisms, tightening the posteriors and enhancing our understanding of these modified gravity scenarios.

astro-ph.CO↗

arXiv2024

The shape of the Chameleon fifth-force on the mass components of galaxy clusters

In the context of Chameleon gravity, we present a semi-analytical solution of the chameleon field profile in accurately modelled galaxy cluster's mass components, namely: the stellar mass of the Brightest Cluster Galaxy (BCG), the baryonic mass in galaxies (other than the BCG), the mass of the Intra-Cluster Medium (ICM) and the diffuse cold dark matter (CDM). The obtained semi-analytic profile is validated against the numerical solution of the chameleon field equation and implemented in the \textsc{MG-MAMPOSSt} code for kinematic analyses of galaxy clusters in modified gravity scenarios. By means of mock halos, simulated both in GR and in modified gravity, we show that the combination of velocities and positions of cluster member galaxies, along with data of the stellar velocity dispersion profile of the BCG, can impose constraints on the parameter space of the Chameleon model; for a cluster generated in GR, these constraints are at the same level as a joint lensing+kinematics analysis of a cluster modelled with a single mass profile, without the BCG data.

astro-ph.CO↗

arXiv2024

Inferring dark energy properties from the scale factor parametrisation

We propose and implement a novel test to assess deviations from well-established concordance $\Lambda$CDM cosmology while inferring dark energy properties. In contrast to the commonly implemented parametric forms of the dark energy equation-of-state (EoS), we test the validity of the cosmological constant on the more fundamental scale factor [$a(t)$] which determines the expansion rate of the Universe. We constrain our extended `general model' using the late-time observables. The posterior of the dark energy EoS is mainly constrained to be quintessence-like naturally excluding physically unviable regions such as phantom crossings or exponential growth.

astro-ph.CO↗

arXiv2024

Mass Modeling and Kinematics of Galaxy Clusters in Modified Gravity

The chameleon screening mechanism has been constrained many a time using dynamic and kinematic galaxy cluster observables. Current constraints are, however, insensitive to different mass components within galaxy clusters and have been mainly focused on a single mass density profile, the Navarro-Frenk-While mass density model. In this work, we extend the study of the Chameleon screening mechanism in galaxy clusters by considering a series of mass density models, namely: generalized-Navarro-Frenk-While, b-Navarro-Frenk-While, Burket, Isothermal and Einasto. The coupling strength ($\beta$) and asymptotic value of the chameleon field ($\phi_\infty$) are constrained by using kinematics analyses of simulated galaxy clusters, generated both assuming General Relativity and a strong chameleon scenario. By implementing a Bayesian analysis we comprehensively show that the biases introduced due to an incorrect assumption of the mass model are minimal. Similarly, we also demonstrate that a spurious detection of evidence for modifications to gravity is highly unlikely when utilizing the kinematics of galaxy clusters.

astro-ph.CO↗

arXiv2024

Holographic and Gravity-Thermodynamic Approaches in Entropic Cosmology: Bayesian Assessment using late-time Data

We investigate the cosmological implications of entropy-based approaches in the context of Holographic Dark Energy (HDE) and Gravity-Thermodynamics (GT) formalisms. We utilise the extended Barrow entropy form, with the index parameter $\Delta$, representing the fractal dimension of the horizon. We also test implementing different parameter ranges for $\Delta$, which can be extended to Tsallis' interpretation within the same formal cosmology. We perform a Bayesian analysis to constrain the cosmological parameters using the Pantheon+, more recent DESy5, DESI, and, as a supplement, Quasar datasets. We find that the HDE model within almost all data combinations performs extremely well in comparison to the GT approach, which is usually strongly disfavored. Using the combination of DESy5+DESI alone, we find that the GT approaches are disfavored at $|\log \mathcal{B}| \sim 5.8$ and $|\log \mathcal{B}| \sim 6.2$ for the Barrow and Tsallis limits on $\Delta$, respectively, wrt $\Lambda$CDM model. While the HDE approach is statistically equivalent to $\Lambda$CDM when comparing the Bayesian evidence. We also investigate the evolution of the dark energy equation of state and place limits on the same, consistent with quintessence-like behaviour in the HDE approaches.

astro-ph.CO↗