Phase-Space Analysis of Quadratic Dark Energy: Stable Attractors and Their Observational Signatures
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 attractors (sinks) when $w_{\text{eff}} < 0$, which includes both quintessence and phantom regimes. Positive $\eta$ values correspond to unstable repellers (sources) when $w_{\text{eff}} > 0$. 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 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.