Quadratic Dark Energy Phase-Space Dynamics and Analysis

We present a comprehensive phase-space analysis of a quadratic dark energy model where the pressure includes a nonlinear term proportional to the square of the energy density. This minimal extension beyond the $\Lambda$CDM framework introduces a dynamical parameter $\eta(z)$ that governs transitions between different cosmological regimes. Through dynamical systems theory, we identify critical points and their stability properties, revealing that negative $\eta$ values drive the system toward stable phantom attractors (sinks), while positive values correspond to unstable repellers (sources). The model exhibits a distinctive asymptotic approach to the phantom divide ($w_{\rm eff}=-1$) from both quintessence and phantom sides without actual crossing, providing a non-crossing alternative to the phantom-crossing behavior preferred by recent DESI DR2 constraints. Our analysis shows that stable phantom attractors produce enhanced Hubble expansion rates and more pronounced late-time acceleration, features that can be compared with recent DESI observations suggesting evolving dark energy.