Mohit Yadav, Pankaj Chavan, Tapomoy Guha Sarkar

Cosmographic reconstructions provide a model-agnostic approach towards constraining cosmic evolution. In this work, we develop a semi-cosmographic framework that adopts a Padé-rational fraction parametrization of the Luminosity distance, but also invokes a phenomenology-motivated two-body decaying dark matter (DDM) sector. In this approach, we do not assume any model for the dark energy. However, we consider the dark matter sector to comprise a non-relativistic parent particle that decays into a massless and a massive daughter. Assuming a cosmographic expansion history and the DDM background evolution, a semi-cosmographic dark energy equation of state is inferred. The various cosmological observables, hence computed, are fitted to the data. We use DESI DR2 BAO data and with a forecasted 21\,-cm intensity-mapping power spectrum at $z\simeq 1.75$ with a SKA1-Mid-like instrument. Posterior constraints on the Padé and DDM parameters are obtained using Markov Chain Monte Carlo (MCMC) analysis. This allows us to reconstruct the equations of state of the massive daughter and dark energy.