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We explore constraints on dark energy and modified gravity with forecast 21cm intensity mapping measurements using the Effective Field Theory approach. We construct a realistic mock data set forecasting a low redshift 21cm signal power spectrum $P_{21}(z,k)$ measurement from the MeerKAT radio-telescope. We compute constraints on cosmological and model parameters through Monte Carlo Markov chain techniques, testing both the constraining power of $P_{21}(k)$ alone and its effect when combined with the latest Planck 2018 CMB data. We complement our analysis by testing the effects of tomography from an ideal mock data set of observations in multiple redshift bins. We conduct our analysis numerically with the codes EFTCAMB/EFTCosmoMC, which we extend by implementing a likelihood module fully integrated with original codes. We find that adding $P_{21}(k)$ to CMB data provides significantly tighter constraints on $��_ch^2$ and $H_0$, with a reduction of the error with respect to Planck results at the level of more than $60\%$. For the parameters describing beyond $��$CDM theories, we observe a reduction in the error with respect to the Planck constraints at the level of $\lesssim 10\%$. The improvement increases up to $\sim 35\%$ when we constrain the parameters using ideal, tomographic mock observations. We conclude that the power spectrum of the 21cm signal is sensitive to variations of the parameters describing the examined beyond $��$CDM models and, thus, $P_{21}(k)$ observations could help to constrain dark energy. The constraining power on such theories is improved significantly by tomography., 38 pages, 16 figures, 12 tables, prepared for submission to JCAP

In this talk I will present a comprehensive analysis of atomic hydrogen (HI) properties using a semi-analytical model of galaxy formation and N-body simulations covering a large cosmological volume at high resolution. I will examine the HI mass function and the HI density, characterizing both their redshift evolution and their dependence on hosting halo mass. I will discuss the HI content of dark matter haloes in the local Universe and up to redshift z=5, analyzing the contribution of different galaxy properties. I will present results on HI clustering relevant for future 21cm Intensity Mapping (IM) experiments, including the HI bias and the shot noise level. I will detail the contribution from various galaxy properties on the HI power spectrum and their relation to the halo bias. Finally, I will present the 21cm signal in redshift space, a fundamental prediction to be tested against data from future radio telescopes such as SKA.