Devaraja Mallesha Naik

Recent results from the second data release (DR2) from the Dark Energy Spectroscopic Instrument (DESI) hint at a possible dynamical form of dark energy. This motivates a fresh look at physically motivated alternatives to the standard Lambda cold dark matter model. In this work, we test two such scenarios – holographic dark energy (HDE) and Ricci dark energy (RDE) – against a wide range of observations. The data sets include cosmic chronometers, gamma-ray bursts, three major Type Ia supernova compilations (Pantheon+SH0ES, DES-Dovekie, and Union3), the new DESI DR2 baryon acoustic oscillation (BAO) measurements, and cosmic microwave background (CMB) distance priors from Planck 2018 + ACT-DR6. Our analysis shows that the RDE model is observationally non-viable. While late-time probes suggest a good fit by favouring a low matter density ($\Omega _{m0} \approx 0.19$–0.22), the inclusion of CMB priors forces $\Omega _{m0}$ to standard values, causing a catastrophic internal tension ($\,\gt\,5\sigma$) and decisive statistical rejection. For the HDE model, we identify a clear tension among data sets. Supernova-dominated samples tend to favour a quintessence-like state ($c \,\gt\, 1.2$), while the combination of DESI BAO and CMB data strongly shifts constraints towards a phantom behaviour ($c \approx 0.69$). We confirm that neither model resolves the Hubble tension. A formal model selection analysis shows that while late-time data yield a spurious preference for dynamical models ($\Delta \text{AIC} \,\lt\, 0$), this vanishes completely when early-Universe constraints are applied. We conclude, in agreement with other analyses, that the apparent hints for new physics are likely statistical artefacts of underlying data set tensions, such as the reported $\Omega _m$ tension. The key result is the CMB-driven indication of a phantom regime for HDE, which points towards interacting holographic dark energy as a promising next step to avoid a Big Rip singularity.

This article presents a novel parametrization of the deceleration parameter (DP) to investigate the cosmological scenario. The newly proposed parametric form of the DP is both physically plausible and model-independent. Constrained by a combined dataset of 31 cosmic chronometers (CC) data points, 26 non-correlated baryonic acoustic oscillations (BAO) points, and 1701 Pantheon $$+$$+ data points from supernovae type Ia (SNeIa), we determine the model parameters using a Markov Chain Monte Carlo (MCMC) method. The analysis explores the kinematic behavior of the model, including the transition from deceleration to acceleration. The results indicate that the Universe is currently in an accelerated phase. Furthermore, we apply the obtained parameter values to constrain f(Q) gravity models and compare them with observations. This study provides valuable insights into the accelerating Universe and underscores the importance of employing a model-independent approach in cosmological investigations.

Ricci solitons (RS) have an extensive background in modern physics and are extensively used in cosmology and general relativity. The focus of this work is to investigate Ricci almost solitons (RAS) on Lorentzian manifolds with a special metric connection called a semi-symmetric metric $\mathbf{u}$-connection ($\mathcal{SSM}$-connection). First, we show that any quasi-Einstein Lorentzian manifold having a $\mathcal{SSM}$-connection, whose metric is RS, is Einstein manifold. A similar conclusion also holds for a Lorentzian manifold with $\mathcal{SSM}$-connection admitting RS whose soliton vector Z is parallel to the vector $\mathbf{u}$. Finally, we examine the gradient Ricci almost soliton (GRAS) on Lorentzian manifold admitting $\mathcal{SSM}$-connection.