Beyond $\Lambda$CDM with HI intensity mapping: robustness of cosmological constraints in the presence of astrophysics

Mapping the unresolved intensity of the 21-cm emission of neutral hydrogen (HI) is now regarded as one the most promising tools for cosmological investigation in the coming decades. Here, we investigate, for the first time, extensions of the standard cosmological model, such as modified gravity and primordial non-Gaussianity, taking self-consistently into account the present constraints on the astrophysics of HI clustering in the treatment of the brightness temperature fluctuations. To understand the boundaries within which results thus obtained can be considered reliable, we examine the robustness of cosmological parameter estimation performed via studies of 21-cm intensity mapping, against our knowledge of the astrophysical processes leading to HI clustering. Modelling of astrophysical effects affects cosmological observables through the relation linking the overall HI mass in a bound object, to the mass of the underlying dark matter halo that hosts it. We quantify the biases in estimates of standard cosmological parameters and those describing modified gravity and primordial non-Gaussianity, that are obtained if one misconceives the slope of the relation between HI mass and halo mass, or the lower virial velocity cut-off for a dark matter halo to be able to host HI. Remarkably, we find that astrophysical uncertainties will not affect searches for primordial non-Gaussianity - one of the strongest science cases for HI intensity mapping - despite the signal being deeply linked to the HI bias.