Analytical $polyΛ$CDM dynamics

Abstract

We develop a novel analytical dynamical analysis to derive precise energy density ratio evolutions for the $ϕ$CDM and $polyΛ$CDM models, comparing them to the standard $Λ$CDM model and validating against numerical solutions. Analytical solutions for the quintessence, i.e. $ϕ$CDM, show sub percent agreement with $Λ$CDM with greater reliability than numerical integration of stiff systems. The $polyΛ$CDM model, a phenomenological modified gravity framework, captures radiation, matter, dark energy, and exotic epochs, offering a streamlined yet comprehensive alternative to existing studies. Its dynamics reveal a global transition from a dark energy-dark matter exchange reflector, through saddle points of matter, radiation, curvature, and modified gravity, to an SVT modified gravity attractor-saddle, and finally to a cosmological constant attractor in the far future, with saddle transitions between modified gravity components. The $polyΛ$CDM model integrates modified gravity models, using dynamical analysis to distinguish observationally viable critical points and differentiate gravity epochs. All three models align with observed cosmic evolution, but $polyΛ$CDM richer phenomenology provides deeper insights into modified gravity dynamics. Code available at GitHub.