  Observations with JWST have revealed an unexpected high abundance of bright
z>10 galaxy candidates. We explore whether a stellar initial mass function
(IMF) that becomes increasingly top-heavy towards higher redshifts and lower
gas-phase metallicities results in a higher abundance of bright objects in the
early universe and how it affects the evolution of galaxy properties compared
to a constant IMF. We incorporate such an evolving IMF into the Astraeus
framework that couples galaxy evolution and reionisation in the first billion
years. Our implementation accounts for the IMF dependence of supernova
feedback, metal enrichment, ionising and ultraviolet radiation emission. We
conduct two simulations: one with a Salpeter IMF and one with the evolving IMF.
Compared to a constant Salpeter IMF, we find that (i) the higher abundance of
massive stars in the evolving IMF results in more light per unit stellar mass,
a slower build-up of stellar mass and lower stellar-to-halo mass ratio; (ii)
due to the self-similar growth of the underlying dark matter halos, the
evolving IMF's star formation main sequence hardly deviates from that of the
Salpeter IMF; (iii) the evolving IMF's stellar mass-metallicity relation shifts
to higher metallicities while its halo mass-metallicity relation remains
unchanged; (iv) the evolving IMF's median dust-to-metal mass ratio is lower due
to its stronger SN feedback; (v) the evolving IMF requires lower values of the
escape fraction of ionising photons and exhibits a flatter median relation and
smaller scatter between the ionising photons emerging from galaxies and the
halo mass. Yet, the topology of the ionised regions hardly changes compared to
the Salpeter IMF. These results suggest that a top-heavier IMF alone is
unlikely to explain the higher abundance of bright z>10 sources, since the
lower mass-to-light ratio is counteracted by the stronger stellar feedback.
