Algebraic Phantom Crossing in a Two-Sector Effective Fluid: Cosmological Signatures and the B=0 Baseline
Year: 2026
Type: preprint
Abstract: Recent DESI DR2 CPL analysis suggests a dynamical dark energy equation of state that crosses the phantom divide (w = -1). We present a two-sector effective-fluid model in which this crossing occurs algebraically: the crossing redshift z_c = (x_0/(1-x_0))^{1/\gamma}-1 is a direct geometric byproduct of the logistic trajectory. We adopt the B=0 baseline, guaranteeing \rho_{DE} \geq 0 for all redshifts and a smooth saturation w \to -1-\gamma/3. Using DESI DR1 BAO distances and Planck 2018 distance priors, without reference to the DESI CPL summary, we obtain z_c = 0.400 (68% CL [0.22, 0.53]), consistent within 1\sigma with the DESI DR2 CPL-implied value. The model yields \Omega_m = 0.316 \pm 0.016 and H_0 = 67.31 \pm 1.69 km s^{-1} Mpc^{-1}, in excellent agreement with Planck 2018, and reproduces CPL-matched parameters w_0 = -0.796 and w_a = -0.555. An information-criterion comparison gives \Delta AIC = -1.74 and \Delta BIC = -0.19 relative to \LambdaCDM, indicating the model is statistically indistinguishable from the standard cosmology at current precision. A CAMB/PPF analysis with c_s^2=1 shows the phantom crossing is perturbatively silent (\Delta P/P \approx +1.8%), and CMB TT acoustic peaks are preserved exactly. Derived \sigma_8 = 0.800 and S_8 = 0.821 are Planck-consistent.
Author: Sandaruwan, M. Uditha
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W7224079685