Probing large scale filaments with HI and $^3$HeII
  We explore the observability of the neutral hydrogen (HI) and the
singly-ionized isotope helium-3 ($^3$HeII) in the intergalactic medium (IGM)
from the Epoch of Reionization down to the local Universe. The hyperfine
transition of $^3$HeII, which is not as well known as the HI transition, has
energy splitting corresponding to 8 cm. It also has a larger spontaneous decay
rate than that of neutral hydrogen, whereas its primordial abundance is much
smaller. Although both species are mostly ionized in the IGM, the balance
between ionization and recombination in moderately high density regions renders
them abundant enough to be observed. We estimate the emission signal of both
hyperfine transitions from large scale filamentary structures and discuss the
prospects for observing them with current and future radio telescopes. We
conclude that HI in filaments is possibly observable even with current
telescopes after 100 hours of observation. On the other hand, $^3$HeII is only
detectable with future telescopes, such as SKA, after the same amount of time.
