Cosmic Acceleration without Dark Energy from Momentum-Coherent Frame Evolution

Observations of distant Type Ia supernovae indicate an apparent late-time acceleration in the cosmic expansion, conventionally interpreted as evidence for a dark energy component. In this work, we present an alternative explanation in which the observed acceleration emerges as an observational effect associated with the evolution of the spacetime reference frame itself, rather than from any additional dynamical energy density. Within the Momentum Inertial / Non-Inertial Dynamics (MIND) framework, we show that large-scale momentum organization leads to a gradual relaxation of spacetime geometry as cosmic matter becomes increasingly dispersed. This progressive frame evolution modifies distance inferences made along null geodesics, producing an apparent acceleration when observations are interpreted under the assumption of a rigid spacetime background. No new forces, fields, or energy components are introduced. In this interpretation, cosmic expansion proceeds within a spacetime whose effective geometric density evolves continuously, yielding cumulative observational effects analogous to measuring motion through a medium whose scale properties change along the propagation path. Apparent acceleration arises without local energy injection, modification of gravity, or violation of conservation laws. We derive the formal conditions under which this evolving-frame description reproduces the observed expansion history and demonstrate its consistency with standard cosmological observables. General relativity remains intact, and the framework preserves all known dynamical laws, reassigning the origin of acceleration from cosmological dynamics to frame-dependent inference. This work extends previous empirical and theoretical studies of momentum coherence to cosmological scales, positioning MIND as a minimal geometric framework capable of accounting for late-time cosmic acceleration without invoking dark energy