arXiv subjects

Abhijith Ajith

Publications and source records attributed to Abhijith Ajith.

8 recordsLinked to original sources

arXiv2026

Cosmological consequences of a dynamical dark matter in the light of DESI DR2 measurements

Recent DESI results exhibit preference for a Null Energy Condition violating dynamical dark energy, with early phantom behaviour. We explore an alternative interpretation in which this preference arises from unconventional dark matter dynamics rather than from dynamical dark energy. We propose a dynamical dark matter (DDM) model, with a non-zero equation of state (EoS) that smoothly interpolates between early time and late time asymptotes across a transition scale factor $a_t$, and study its consequences against cosmological datasets including CMB, DESI DR2 BAO, SNeIa (PantheonPlus, Union3, and DESY5) and growth rate data. We find the early time EoS to be consistent with zero, while the present day value is negative at a significance ranging from $0.42\sigma$ to $3.02\sigma$ depending on the dataset combination. The strongest preference occurs from the combination of CMB, DESI, and DESY5 giving the present day EoS to be $-0.060^{+0.013}_{-0.028}$ and $a_t = 0.41^{+0.088}_{-0.13}$ at 68\% CL. This preference for a non-zero, late time DM EoS persists when growth rate data are included and across all three SNeIa compilations considered, while the matter density $\Omega_m$ mildly shifts to higher values relative to $\Lambda$CDM. The model also predicts a lower $\sigma_8$ and $S_8$ than $\Lambda$CDM, in better agreement with weak-lensing data, while $H_0$ remains unchanged and in tension with local distance-ladder measurements. The DDM model is preferred over $\Lambda$CDM ($\Delta \chi^2_{\rm MAP} = -14.093$, $\Delta {\rm DIC} = -7.838$ for Planck+DESI+DESY5) but disfavored relative to the CPL parameterization of DE ($\Delta \chi^2_{\rm MAP} = 6.755$, $\Delta {\rm DIC} = 7.966$). This preference is consistent among other combination of datasets as well.

astro-ph.CO↗

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

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

Evidence for non-cold dark matter from DESI DR2 measurements

We investigate potential deviations from cold dark matter (CDM) using the latest Baryon Acoustic Oscillations (BAO) measurements from the Dark Energy Spectroscopic Instrument (DESI). Analyzing DESI data alone constrains the dark matter equation of state parameter $w_{\mathrm{dm}} = -0.042^{+0.047}_{-0.024}$, revealing a mild preference for non-cold dark matter. This preference strengthens significantly in combined analyses, but reveals a striking tension in the inferred $w_{\mathrm{dm}}$ values from observations of different nature. The DESI+DESY5 combination yields $w_{\mathrm{dm}} = -0.084 \pm 0.035$, excluding CDM ($w_{\mathrm{dm}}=0$) at 2.4$\sigma$ significance. In contrast, Planck+DESI gives $w_{\mathrm{dm}} = 0.00077\pm0.00038$, differing from concordance model at 2$\sigma$ significance. The non-vanishing $w_{\mathrm{dm}}$ preference is particularly driven by low-redshift BAO measurements ($z<1.1$), while higher redshift data remain consistent with $\Lambda$CDM. The evidence for non-cold dark matter is more pronounced in DESI compared to the previous BAO surveys. All dataset combinations show significant improvement over the $\Lambda$CDM paradigm, providing compelling evidence for non-cold dark matter scenario.

astro-ph.CO↗

arXiv2022

Inflation using a triplet of Antisymmetric tensor fields

We study an inflation model driven by a triplet of antisymmetric tensor fields, with minimal and nonminimal couplings to gravity. First, we show that the presence of a triplet of antisymmetric tensor fields can provide inherent background isotropy in the stress-energy tensor contrary to the past studies using an antisymmetric tensor field. Inflation is supported in the presence of non-minimal couplings with gravity. We perform the slow roll analysis and also analyse perturbations to the antisymmetric tensor field as well as the tensor modes of perturbed metric. The speed of gravitational waves manifested from the tensor perturbations is tuned to $c$. We also study the evolution of the gravitational waves, calculate their power spectrum and tensor spectral index.

gr-qc↗

arXiv2022

Inflation with antisymmetric tensor field: new candidates

We study classes of inflation models driven by antisymmetric tensor field, with minimal and nonminimal couplings to gravity, that address known issues of such models considered in the past. First we show that with a different choice of the background structure of antisymmetric tensor field, inflation is supported even for the minimal model with quadratic potential contrary to past results. We also include the nonminimal coupling to gravity and analyse perturbations to the antisymmetric tensor as well as the tensor modes of perturbed metric. The two models differ in terms of the behaviour of tensor modes, where the speed of gravitational wave can be tuned to $c$ in the latter model. The power spectrum and spectral index receive slight scale dependence. Finally, we consider a quartic potential motivated by the graceful exit to reheating phase, which requires a nonminimal coupling to support inflation. The two tensor modes of perturbed metric are found to evolve differently in this model, and give rise to a highly scale-dependent power spectrum.

gr-qc↗