The Canadian Hydrogen Intensity Mapping Experiment (CHIME) is an interferometric radio telescope under construction at the Dominion Radio Astrophysical Observatory in British Columbia, Canada which will consist of four 100 x 20 metre semi-cylinders (roughly the size and shape of snowboarding half-pipes) populated with 1024 radio receivers sensitive at 400–800 MHz. The telescope's low-noise amplifiers are being built with components adapted from the cellphone industry and its data will be processed using a 1000-processor high-performance GPGPU cluster. The telescope has no moving parts and observes half of the sky each day as the Earth turns. CHIME is a partnership between the University of British Columbia, McGill University, the University of Toronto and the Dominion Radio Astrophysical Observatory.
One of the biggest puzzles in contemporary cosmology is why the expansion of the Universe is accelerating. About seventy percent of the Universe today consists of so-called dark energy that counteracts gravity's attractive force and causes this acceleration. Very little is known about what dark energy is. CHIME will make precise measurements of the acceleration of the Universe to improve the knowledge of how dark energy behaves. The experiment is designed to observe the period in the Universe's history during which the standard ΛCDM model predicts that dark energy began to dominate the energy density of the Universe and when decelerated expansion transitioned to acceleration.
In addition to its main, cosmological purpose, CHIME will also be well-equipped for other astrophysical science. It will be used for discovering and monitoring pulsars and other radio transients, a specialised instrument is being developed for these science objectives. CHIME's daily survey of the sky will enable study of our own Milky Way galaxy, in radio frequencies, and is expected to improve the understanding of galactic magnetic fields.
CHIME could also detect the mysterious extragalactic fast radio bursts (FRB) that last just milliseconds and have no known astrophysical explanation. It will also help other experiments to calibrate measurements of radio waves from rapidly spinning neutron stars, which researchers hope to use to detect gravitational waves.
The instrument is a hybrid semi-cylindrical interferometer designed to measure the large scale neutral hydrogen power spectrum across the redshift range 0.8 to 2.5. The power spectrum will be used to measure the baryon acoustic oscillation (BAO) scale across this redshift range where dark energy becomes a significant contributor to the evolution of the Universe.
CHIME is sensitive to the 21 cm radio light emitted by clouds of neutral hydrogen in distant galaxies. By measuring the distribution of the hydrogen in the Universe—a technique known as intensity mapping—CHIME will make a 3D map of the large-scale structure of the Universe between redshifts of 0.8 and 2.5, when the Universe was between about 2.5 and 7 billion years old. CHIME will thus map over 3% of the total observable volume of the Universe, substantially more than has been achieved by large-scale structure surveys to date, during an epoch when the Universe is largely unobserved. Maps of large-scale structure can be used to measure the expansion history of the Universe because sound waves in the early Universe, or baryon acoustic oscillations (BAO), have left slight overdensities in the distribution of matter on scales of about 500 million light-years. This characteristic BAO scale has been well-measured by experiments like Planck and can therefore be used as a ‘standard ruler’ to determine the size of the Universe as a function of time, thereby indicating the expansion rate.
BAO measurements to date have been made by observing the distribution of galaxies on the sky. While future experiments, like The Dark Energy Survey, Euclid and the Dark Energy Spectroscopic Instrument (DESI), will continue using this technique, CHIME is a pioneer in using the radio emission of hydrogen rather than the starlight as a tracer of structure for detecting BAO. Although CHIME cannot be used for the same auxiliary science that galaxy surveys excel at, for BAO measurement CHIME represents a very cost-effective alternative as individual galaxies do not need to be observed.
The choice to use a few elongated reflectors rather than many circular dishes is unusual but not original to CHIME: other examples of semi-cylindrical telescopes are the Molonglo Observatory Synthesis Telescope in Australia and the Northern Cross Radio Telescope in Italy. This design was chosen for CHIME as a cost-effective way of arranging close-packed radio antennas so that the telescope can observe the sky at a wide range of angular scales. Using multiple, parallel semi-cylinders gives comparable resolution along both axes of the telescope.
The antennas are custom-designed for CHIME to have good response in the 400 to 800 MHz range in two linear polarisations. Signal from the antennas are amplified in two stages that make use of technology developed by the cell-phone industry. This allows CHIME to keep the analogue chain at relatively low noise while still being affordable.
CHIME is operated as a correlator, meaning that the inputs from all the antennas are combined so that the entire system operates as one system. This requires considerable computing power. The analogue signals are digitised at 800 MHz and processed using a combination of custom-programmed field-programmable gate arrays (FPGA) and graphics processing units (GPU). The Pathfinder has a fully functional correlator made from these units, and has demonstrated that consumer-grade GPU technology provides sufficient processing power for CHIME at a fraction of the price of other radio correlators.
In 2013, the CHIME Pathfinder telescope was built, also at DRAO. It is a smaller-scale version of the full instrument, consisting of two, 36 x 20 metre semi-cylinders populated by 128 antennas, and is currently being used as a testbed for CHIME technology and observing techniques. Additionally, the Pathfinder will also be capable of making a competitive measurement of the baryon acoustic oscillations (BAO) and will become a useful telescope in its own right.
Construction of CHIME began in 2015 at the Dominion Radio Astrophysical Observatory (DRAO) near Penticton, British Columbia, Canada. In November 2015, CHIME was reported to be "nearly operational", requiring the installation of receivers, and construction of the super-computer. In March 2016 the contract for the processing chips was placed.