We investigate the possibility of performing cosmological studies in the redshift range 2.5 < z < 5 through suitable extensions of existing and upcoming radio-telescopes like CHIME , HIRAX and FAST .
We use the Fisher matrix technique to forecast the bounds that those instruments can place on the growth rate , the BAO distance scale parameters , the sum of the neutrino masses and the number of relativistic degrees of freedom at decoupling , N _ { eff } .
We point out that quantities that depend on the amplitude of the 21cm power spectrum , like f \sigma _ { 8 } , are completely degenerate with \Omega _ { HI } and b _ { HI } , and propose several strategies to independently constrain them through cross-correlations with other probes .
Assuming 5 \% priors on \Omega _ { HI } and b _ { HI } , k _ { max } = 0.2 ~ { } h { Mpc } ^ { -1 } and the primary beam wedge , we find that a HIRAX extension can constrain , within bins of \Delta z = 0.1 : 1 ) the value of f \sigma _ { 8 } at \simeq 4 \% , 2 ) the value of D _ { A } and H at \simeq 1 \% .
In combination with data from Euclid-like galaxy surveys and CMB S4 , the sum of the neutrino masses can be constrained with an error equal to 23 meV ( 1 \sigma ) , while N _ { eff } can be constrained within 0.02 ( 1 \sigma ) .
We derive similar constraints for the extensions of the other instruments .
We study in detail the dependence of our results on the instrument , amplitude of the HI bias , the foreground wedge coverage , the nonlinear scale used in the analysis , uncertainties in the theoretical modeling and the priors on b _ { HI } and \Omega _ { HI } .
We conclude that 21cm intensity mapping surveys operating in this redshift range can provide extremely competitive constraints on key cosmological parameters .