Present-day coasting in a flat FLRW cosmology with a transient acceleration pocket

ABSTRACT

We revisit whether cosmological data require present-day acceleration. We introduce a compact phenomenological parametrization for the equation of state of a separately conserved smooth component, with independently adjustable low- and high-redshift limits and a localized pressure feature of variable location, width, and amplitude. This provides a flexible alternative to CPL-like forms, allowing intermediate-redshift structure in the expansion history to be modelled without tying it rigidly to the high-redshift limit. Embedding this component in a flat Friedmann–Lemaître–Robertson–Walker (FLRW) model with pressureless matter, we test exact present-day coasting and a corresponding free-$q_0$ extension. We fit DESI DR2 BAO, the Planck CMB distance prior, Pantheon + SH0ES Type Ia supernovae, and $f\sigma _8$, using both the original SN sample and the progenitor-age-corrected form proposed by Son et al. (2025). We find that exact coasting provides a competitive fit once the Son et al. correction is applied and freeing $q_0$ gives only a marginal improvement, so present-day acceleration is not required by this combined data set.