Joint Cosmological Parameter Estimation Using Type Ia Supernovae, Cosmic Microwave Background, And Baryon Acoustic Oscillations

Joint parameter estimation has become central to precision cosmology because no single observable constrains the late-time expansion history, matter content, and dark energy sector with equal power. Type Ia supernovae trace the luminosity distance relation at low and intermediate redshift, the cosmic microwave background fixes the early-Universe acoustic scale and physical densities, and baryon acoustic oscillations provide a robust standard ruler across a wide redshift interval. The research problem addressed in this paper is how these probes, when analyzed together, alter both parameter precision and model interpretation in the current observational landscape. This empirical conference paper develops a comparative synthesis of published constraints from Pantheon+, Planck 2018, the completed SDSS/eBOSS analysis, DESI Year 1 BAO, and the SH0ES local H0 determination. The method harmonizes reported constraints within flat LambdaCDM, wCDM, and w0waCDM and evaluates three issues: degeneracy breaking, constraint tightening, and residual probe tension. The analysis shows that joint constraints significantly reduce uncertainty in Omega_m and w0 relative to supernova-only inference and preserve strong support for flat LambdaCDM in the highest-precision CMB plus BAO combinations. At the same time, the H0 tension remains unresolved, and recent DESI combinations suggest mild but not decisive instability in the dark energy sector when time variation is allowed. The paper contributes a concise empirical framework for interpreting why multi-probe cosmology is simultaneously a precision success and a site of continuing theoretical pressure.