Research: Canadian Hydrogen Intensity Mapping Experiment

Photograph of the CHIME Pathfinder.
The CHIME Pathfinder. (Credit: Andre Recnik)

One of the biggest puzzles in contemporary cosmology is why the universe is accelerating in its expansion. For almost a century it has been evident that there is indeed cosmic expansion, but given the attractive force of gravity, it was long assumed that the rate of growth must be slowing down. However, since the late 1990's, it has become clear that expansion has been speeding up for the last few billion years. Something is counteracting gravity's attraction. Typically, this mysterious force is called ‘dark energy’, but this is little more than a name for our ignorance. While Einstein's theory of gravity allows for a ‘cosmological constant’ that could explain the accelerated expansion, observations to-date do not allow us to distinguish between this hypothesis and the possibility that new, previously undiscovered physics is responsible.

The Canadian Hydrogen Intensity Mapping Experiment (CHIME) is a new radio observatory in the Okanagan Valley of British Columbia that is designed to shed light on the nature of dark energy. In its final form, it will consist of four, 100-metre-long and 20-metre-wide cylinders, populated with a total of 1024 dual-polarisation radio antennas. Using this huge collecting area—ten thousand square metres—CHIME should be able to measure the radio waves emitted by billions of years ago by the clouds of hydrogen dispersed throughout distant galaxies. This signal is an excellent proxy for the distribution of matter at a given epoch in cosmological history: in particular, it should be possible to detect a higher-than-average density of matter on physical scales of about 500 million light years. We know this length scale quite precisely, since it was fixed very early on in the universe's history and can be calculated from the well-understood physics of the sound waves in the dense plasma which emerged from the big bang. However, the observed scale on the sky depends on the history of cosmic expansion between us and the emitting hydrogen. Thus, it is in principle possible for CHIME to measure how the rate of expansion has been evolving over a significant fraction of the universe's history.

Such a measurement is, however, difficult. The size of the signal emitted by cosmic hydrogen is so small that the amount of energy it will impart to CHIME in one year is roughly comparable to the energy it takes to lift a paperclip one millimetre off the ground. Our own galaxy emits radio waves that can be confused with those we are interested in measuring, requiring careful modelling and removal of this source of ‘contamination’. And CHIME will push current computational power to the brink, since it will be digitising and processing two trillion samples per second. In order to figure out how to address these and other challenges, we have built a ‘pathfinder’, consisting of two cylinders, each 37 metres in length and populated by a total of 128 antennas. Not is the pathfinder helping us to develop hardware, software and know-how to operate full CHIME, but it itself should be capable of making cutting-edge maps of cosmic structure which can begin refining our knowledge of the expansion history of the universe.

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