Semi-cosmographic constraints on decaying dark matter and dynamical dark energy: DESI DR2 BAO and 21\, cm intensity-mapping forecasts

Abstract

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.

Figures (7)

• Figure 1: Left: Massive daughter EoS $w_2(a)$, Right: Redshift evolution of the rescaled densities $\rho(z)/(1+z)^3$ for the parent dark matter, massless daughter, massive daughter, and total matter components in the two-body decaying dark matter model.
• Figure 2: The schematic flowchart showing the semi-cosmographic method for constraining cosmologies in a model-independent way (without assuming any specific dark energy model) while also incorporating a specific two body decaying dark matter scenario.
• Figure 3: The cosmological evolution in $(x,p)$ phase space. Left: Here we assume the dark energy to be a cosmological constant and study the phase trajectories for different (coloured curves) decay parameters $(\epsilon,\tau)$ for a 2-body DDM scenario. Right: The dark matter is considered cold, and trajectories are shown for different dark-energy models: Chevallier--Polarski--Linder (CPL) and thawing quintessence (TQ). In both panels, the red-coloured trajectory represents the Planck18 $\Lambda$CDM model. The coloured ellipses show the DESI DR2 BAO projections mapped onto the $(x,p)$ plane (68%, 95% and 99.7% confidence regions). The blue dashed lines are constant-redshift consistency lines implied by spatial flatness, $x+(1+z)p = 1/E(z)$, shown at $z=0.51,\,0.706,\,0.93,\,1.32,\,1.484$ and $2.33$. The intersection of the horizontal dashed line $p=0$ with the trajectories corresponds to the maxima of $D_A(z)$.
• Figure 4: Baseline distribution function $\rho(U)$ for the adopted SKA1-MID array configuration. The inset shows the actual antenna locations in the $X$--$Y$ plane.
• Figure 5: Marginalized posterior distributions for the Padé + two-body decaying dark matter model. Left: DESI DR2 BAO only. Right: joint DESI DR2 BAO + mock $21$ cm power spectrum at $z=1.75$. Contours show the $68\%$, $95\%$, and $99.7\%$ credible regions. Numbers above the diagonal panels show posterior means and central $68\%$ credible intervals.