n Effective Bias Expansion for 21 cm Cosmology in Redshift Space

near-future detection of the 21cm signal from the epoch of reionizationwill provide unique opportunities to probe the underlying cosmology, providedthat such cosmological information can be extracted with precision. To thisend, we further develop effective field theory (EFT) inspired techniques forthe 21cm brightness temperature field during the epoch of reionization,incorporating renormalized bias and a treatment of redshift space distortions.Notably, we confirm that in redshift space, measures of the 21cm brightness,e.g the power spectrum, should have irreducible contributions that lack a biascoefficient and therefore contain direct, astrophysics-free information aboutthe cosmological density field; in this work, we study this effect beyondlinear order. To validate our theoretical treatment, we fit the predicted EFTFourier-space shapes to the THESAN suite of hydrodynamical simulations ofreionization at the field level, where the considerable number of modesprevents overfitting. We find agreement at the level of a few percent betweenthe 21cm power spectrum from the EFT fits and simulations over the wavenumberrange $k \lesssim 0.8$ h/Mpc and neutral fraction $x_\mathrm{HI} \gtrsim 0.4$,which is imminently measurable by the Hydrogen Epoch of Reionization Array(HERA) and future experiments. The ability of the EFT to describe the 21cmsignal extends to simulations that have different astrophysical prescriptionsfor reionization as well as simulations with interacting dark matter.