  Next generation telescopes such as the James Webb Space Telescope (JWST) and
the Nancy Grace Roman Space Telescope (NGRST) will enable us to study the first
billion years of our Universe in unprecedented detail. In this work we use the
ASTRAEUS (semi-numerical rAdiative tranSfer coupling of galaxy formaTion and
Reionization in N-body dArk mattEr simUlationS) framework, that couples galaxy
formation and reionization (for a wide range of reionization feedback models),
to estimate the cosmic variance expected in the UV Luminosity Function (UV LF)
and the Stellar Mass Function (SMF) in JWST surveys. We find that different
reionization scenarios play a minor role in the cosmic variance. Most of the
cosmic variance is completely driven by the underlying density field and
increases above $100\%$ for ${\rm M}_{\rm UV} \sim -17.5 (-20)$ at $z =12 (6)$
for the JADES-deep survey (the deep JWST Advanced Extragalactic Survey with an
area of 46 arcmin$^2$); the cosmic variance decreases with an increasing survey
area roughly independently of redshift. We find that the faint-end (${\rm
M}_{\rm UV} > -17$) slope of the Lyman Break Galaxies (LBGs) UV LF becomes
increasingly shallower with increasing reionization feedback and show how JWST
observations will be able to distinguish between different models of
reionization feedback at $z>9$, even accounting for cosmic variance. We also
show the environments (in terms of density and ionization fields)of Lyman Break
Galaxies during the EoR, finding that the underlying over-density and
ionization fraction scale positively with the UV luminosity. Finally, we also
provide a public software tool to allow interested readers to compute cosmic
variance for different redshifts and survey areas.
