CHIME will use Intensity Mapping of the 21cm line of neutral hydrogen to map
large-scale structure between redshifts of 0.8 and 2.5. By measuring Baryon
Acoustic Oscillations (BAO) we will place constraints on the dark energy
equation of state as it begins to dominate the expansion of the Universe,
particularly at redshifts poorly probed by current BAO surveys.
In this talk I will introduce CHIME, a transit radio interferometer designed
specifically for this purpose. I will discuss the promise and pitfalls of
Intensity Mapping and describe how we plan to confront the many challenges of
such observations, in particular removal of astrophysical foregrounds which are
six orders of magnitude larger than the 21cm signal. CHIME started operating at
the DRAO in Penticton, BC at the end of 2018 and I will report on current
progress and lessons already learned.