We study the observability of the reionization epoch through the 21 cm hyperfine transition of neutral hydrogen .
We use a high-resolution cosmological simulation ( including hydrodynamics ) together with a fast radiative transfer algorithm to compute the evolution of 21 cm emission from the intergalactic medium ( IGM ) in several different models of reionization .
We show that the mean brightness temperature of the IGM drops from \delta T _ { b } \sim 25 \mbox { mK } to \delta T _ { b } \sim 10 ^ { -2 } \mbox { mK } during overlap ( over a frequency interval \Delta \nu \sim 25 \mbox { MHz } ) , while the root mean square fluctuations on small scales drop abruptly from \langle \delta T _ { b } ^ { 2 } \rangle ^ { 1 / 2 } \sim 10 \mbox { mK } to \langle \delta T _ { b } ^ { 2 } \rangle ^ { 1 / 2 } \sim 10 ^ { -1 } \mbox { mK } at the end of overlap .
We show that 21 cm observations can efficiently discriminate models with a single early reionization epoch from models with two distinct reionization episodes .