Abstract
Line intensity mapping opens up a new and exciting window for probing cosmology and fundamental physics during the Epoch of Reionisation, extending to redshifts previously untested by galaxy surveys. The power spectra of these line fluctuations are a promising tool to test gravity over a large range of scales and redshifts. We simulate cosmological volumes of 21cm fluctuations in general parametrisations of modified gravity, in order to calculate the corresponding power spectra, where additional parameters are the initial condition of matter perturbations α and the scale-dependent modified gravity parameter Y (also known as Geff) that measures deviations from GR in the Poisson equation. We show the impact of these model-independent modifications of gravity, to either delay or expedite reionisation. For the 21cm intensity mapping survey to be performed by the SKA mission, we forecast the ability of line intensity mapping to constrain the parameters Y and α at redshifts z=6−11, where Y is assumed constant during this epoch (but without requiring constancy at all times). In our most conservative scenario, the Y parameter can be constrained at the tens of percent level, while for improved modelling of foregrounds as well as of the (mildly) non-linear regime, up to sub-percent level constraints are attainable. We show the impact of jointly estimating reionisation model parameters and corresponding parameter correlations, as well as of foreground removal. We note, that tomography is crucial to break degeneracies and for constraints not to degrade significantly when adding reionisation model parameters, with most constraining power coming from the redshift bins z=7−10 where the shape of the 21cm power spectrum is evolving fastest.