Global Likelihood Analysis of the Dimensional Transit Model: High-ℓ CMB Improvement and Parameter Constraints from Planck, BAO, and Pantheon+

This work presents the first global likelihood evaluation of the baseline (static) Dimensional Transit Model (DTM) using standard cosmological datasets, including Planck 2018 CMB (TT, TE, EE, low-ℓ), SDSS DR12 BAO, and the Pantheon Type Ia supernova sample.

The model is implemented directly within the CLASS Boltzmann solver and evaluated using a full bobyqa maximum-likelihood minimisation with identical methodology to ΛCDM. The DTM introduces a fixed additional energy density component (Ω₍dtm₎ = 0.045) without increasing the number of free parameters relative to ΛCDM.

The analysis finds a modest improvement in fit (Δχ² = −1.25) driven primarily by high-ℓ CMB data, with small compensating degradations in BAO and supernova constraints. Parameter shifts are physically interpretable and reflect the presence of an additional smooth energy component affecting early-time dynamics.

These results demonstrate that the baseline DTM is statistically consistent with current observational data and provide a controlled foundation for subsequent transient extensions and full MCMC posterior analysis.