Prasanta Sahoo

Recent DESI BAO measurements suggest dark energy may be dynamical rather thanstrictly constant. To investigate this, we introduce the δ-CDM framework, a controlleddeformation of ΛCDM in which deviations from a cosmological constant are encoded in aredshift-dependent function δ(z) defined by wde(z) = -1 + δ(z). We aim to reconstruct δ(z)from an underlying physical model. As a concrete example, we study thawing dark energyrealized by a scalar field. Within a unified setup, we include both quintessence and phantomfields via a switching parameter, so the formalism covers thawing quintessence and thawingphantom scenarios. We find that, within the thawing approximation, the reconstructed δ(z)is independent of the specific scalar-field realization. This illustrates that the effective fluiddescription in the δ-CDM framework can capture richer dark energy phenomenology. UsingPlanck CMB-SPA, DESI DR2 BAO, and Pantheon+ supernova data, we find this model isnot preferred over standard ΛCDM; hence current data do not statistically favor thawingdark energy. However, the ˜w0 ˜wa parametrization is still preferred over ΛCDM, hinting atmodels that allow the equation of state to cross the phantom divide rather than those thatforbid it. Overall, δ-CDM provides a way to implement minimal deviations from ΛCDMwhile remaining consistent with a broad class of physically motivated dark energy models.

In this work the dynamical role of the phantom divide is investigated within minimally coupled single-clock cosmological effective descriptions. Under general conditions including Einstein gravity, covariant energy–momentum conservation, autonomous closure of the background dynamics, and perturbative stability, the hypersurface Σ = ρ + P = 0 ( w = − 1 ) is shown to define an invariant manifold of the cosmological flow provided the effective clock is non-transversal at the phantom divide . In this case, the phase space vector field is tangent to the hypersurface, implying that the sign of Σ is preserved along dynamical trajectories and that continuous evolution between quintessence and phantom regimes is dynamically excluded in finite cosmic time within this subclass. The phantom divide is thereby identified as a dynamical separatrix of the background flow. The invariant manifold structure is conditional and not universal. In transversal realizations, for which the effective clock does not degenerate at the phantom divide ( c ˙ ≠ 0 at c = 0 ), the tangency condition is violated and continuous phantom crossing may occur within a regular effective field theory description. The results therefore provide a geometric characterization of phantom crossing in terms of phase space structure, yield a dynamical classification of single-clock cosmologies based on transversality, and offer a theoretical framework for interpreting observational constraints on the dark energy equation of state.

A string-motivated interacting dark sector cosmological model generated by scalar-coupled, particle-like string loops is investigated using dynamical systems techniques and observational constraints. The scalar dependence of the loop energy density naturally induces an interaction between the scalar field and the loop sector, providing a mechanism for energy exchange and establishing a connection between string-inspired microphysics and late-time cosmology. The dynamics are reformulated as an autonomous system and analyzed for both constant and dynamically evolving slope realizations of the scalar potential and loop coupling. The resulting phase space exhibits interacting loop--scalar scaling solutions together with a self-regulating interaction mechanism associated with a dynamically invariant hypersurface. The model is constrained using Pantheon+ SNe Ia, DES Y5, DESI DR2 BAO, and compressed CMB data. Both realizations are consistent with current observations while allowing controlled departures from the standard $\Lambda$CDM scenario. The constant-coupling realization provides the strongest overall fit to the combined datasets, whereas the dynamical-coupling realization remains observationally viable and exhibits a richer dynamical structure. These results demonstrate that scalar-coupled particle-like string loops constitute a viable and well motivated interacting dark sector framework.

In this work, a global dynamical analysis of spatially flat Friedmann–Lemaître–Robertson–Walker (FLRW) cosmologies driven by a canonical scalar field minimally coupled to gravity is presented. Under suitable regularity and asymptotic assumptions on the scalar field potential, it is shown that the Einstein–scalar evolution admits no forward trajectory along which the potential steepness becomes asymptotically unbounded. This establishes forward boundedness of the scalar sector and yields the existence of a compact absorbing set for the induced cosmological flow. Using techniques from invariant manifold theory and dissipative dynamical systems, the evolution is shown to admit a compact global attractor governing the late–time dynamics of all physically admissible solutions. The asymptotic behavior is further characterized by convergence toward a scalar field dominated invariant manifold, leading to a reduction in effective dynamical dimensionality. In particular, the late–time dynamics is governed by at most two independent degrees of freedom, with further reduction to one dimension for asymptotically exponential potentials. The resulting asymptotic structure is shown to be normally hyperbolic and structurally stable under smooth perturbations of the scalar field potential. A topological classification of the asymptotic dynamics is obtained using the Conley index, identifying universality classes corresponding to one and two dimensional invariant sets. These results provide a global characterization of late–time scalar field cosmologies and establish a model independent dynamical mechanism for asymptotic trapping in Einstein–scalar systems.

A class of dissipative dynamical systems evolving on smooth constraint hypersurfaces endowed with degenerate induced bilinear forms is studied. The intrinsic evolution is generated by constraint--preserving vector fields on manifolds whose tangent bundles admit a nontrivial null distribution associated with the degeneracy of the induced structure. In this indefinite setting, the absence of coercive Lyapunov functionals prevents the direct application of classical attractor theory developed for Riemannian phase spaces. Dissipation is instead characterized relative to functionals that are compatible with the null distribution and exhibit decay exclusively in directions transverse to the associated foliation. Under suitable involutivity and regularity assumptions on the null distribution, all bounded trajectories are shown to be asymptotically confined to invariant leaves of the corresponding foliation. Asymptotic compactness of the intrinsic evolution is then established without coercivity by reducing the dynamics to a projected semiflow on the quotient manifold determined by the characteristic distribution. In the presence of a bounded absorbing set and continuous semiflow structure, the intrinsic evolution admits a compact global attractor saturated by the null leaves, whose effective asymptotic dynamics are governed by a compact invariant subset of the reduced phase space. Furthermore, when the compatible functional satisfies a Morse--type nondegeneracy condition in directions transverse to the null distribution and the induced transversal linearization admits no center spectrum, the projected semiflow possesses no neutral directions. The resulting framework provides a mechanism by which constraint--induced degeneracy enforces effective dimensional reduction in dissipative geometric evolution systems.