I will start my talk with an overview of constraining cosmological parameters with different astrophysical probes, and illustrate why 21cm tomography will be one of the most promising emerging cosmological tools that can improve the parameter measurement to better accuracy. Cosmological parameters quantify the standard cosmological model that assumes Einstein's general relativity. So in what follows, I will discuss testing gravity with ongoing gravitational experiments; as an example, I will discuss constraining torsion gravity with Gravity Probe B. I will outlook the prospects of constraining new gravitational parameters in a more general fashion, e.g. with upcoming cosmological probes