Coordinate Frame Acceleration: Fitting Cosmological Observations Without Dark Energy

The speed of light, c, remains constant everywhere along the geodesic within an expanding Universe, suggesting that coordinate frames accelerate from an emitting galaxy across a cosmological velocity gradient relative to the detector. A modified metric for photon propagation across expanding space is therefore proposed to formalise this effect. The model is consistent with special and general relativity and in the early Universe the model merges with ΛCDM, but before the era of recombination the models diverge sufficiently for H₀ to differ from the Planck Collaboration value derived with ΛCDM. The Cosmic Microwave Background-derived Hubble parameter predicted by the model is H₀=71.81±0.5 km s⁻¹ Mpc⁻¹ which is higher than the Planck value but closer to recent late-Universe determinations of H₀. In the late Universe, the model closely fits the range of SNe Ia data without requiring dark energy or additional arbitrary parameters; it matches the weighted w′CDM model with root-mean-square (RMS) errors of 0.0102 μ_B mag for each model and it closely fits the baryon acoustic oscillation (BAO) data with residual RMS errors of 0.0303 and 0.0378 Mpc for the modified model and w′CDM respectively. The model also well fits galaxy number-count data over a wide range of apparent magnitudes, requiring only modest luminosity evolution over the redshift range 0.5 ≤ z ≤ 2.5.