Comparative Study of Structure Formation, Recombination, and Matter--Radiation Equality in \textbf{$f(R, L_m)$} Gravity and \textbf{$Λ$}CDM Cosmology

Abstract: We present a comprehensive comparative analysis of three pivotal epochs in cosmic history: structure formation, recombination, and matter-radiation equality, within the framework of $f(R, L_m)$ gravity and the standard $\Lambda$CDM cosmology. Using a nonlinear evolution equation for density perturbations, we determine the collapse redshift ($z_c$), showing that cosmic structures form earlier in $f(R, L_m)$ gravity due to enhanced effective gravitational coupling. Recombination is studied via the visibility function $g(z)$, peaking at $z_{\mathrm{rec}} \approx 1092.6$ in both models, consistent with Planck 2018 data. We compute the full width at half maximum (FWHM) of $g(z)$ and find a slightly extended decoupling duration in $f(R, L_m)$. Finally, we analyze the evolution of energy densities and determine the matter-radiation equality redshift as $z_{\mathrm{eq}} \approx 4203$ in $f(R, L_m)$ and $z_{\mathrm{eq}} \approx 2779$ in $\Lambda$CDM, with corresponding cosmic times of approximately $67{,}756$ and $67{,}232$ years, respectively. These results confirm that $f(R, L_m)$ gravity retains key cosmological landmarks while introducing testable deviations from the standard paradigm.