  At peak, long-duration gamma-ray bursts are the most luminous sources of
electromagnetic radiation known. Since their progenitors are massive stars,
they provide a tracer of star formation and star-forming galaxies over the
whole of cosmic history. Their bright power-law afterglows provide ideal
backlights for absorption studies of the interstellar and intergalactic medium
back to the reionization era. The proposed THESEUS mission is designed to
detect large samples of GRBs at $z>6$ in the 2030s, at a time when supporting
observations with major next generation facilities will be possible, thus
enabling a range of transformative science. THESEUS will allow us to explore
the faint end of the luminosity function of galaxies and the star formation
rate density to high redshifts; constrain the progress of re-ionisation beyond
$z\gtrsim6$; study in detail early chemical enrichment from stellar explosions,
including signatures of Population III stars; and potentially characterize the
dark energy equation of state at the highest redshifts.
