Canadian Hydrogen Intensity Mapping Experiment

Science goals

One of cosmology’s biggest open questions is why the universe’s expansion is speeding up. Roughly seventy percent of the cosmos today is made of dark energy, a force that works against gravity’s pull and drives this acceleration. Scientists know very little about what dark energy actually is. The Canadian Hydrogen Intensity Mapping Experiment, or CHIME, is working to measure the universe’s acceleration with high precision, aiming to better understand how dark energy behaves. The telescope is built to study the era when, according to the standard ΛCDM model, dark energy started to dominate the universe’s energy density and the expansion shifted from slowing down to speeding up.

Beyond its main cosmological goal, CHIME carries out other work. Its daily sky survey lets researchers study the Milky Way in radio frequencies, which should improve our grasp of galactic magnetic fields. The telescope also helps other experiments calibrate their measurements of radio waves from rapidly spinning neutron stars, which scientists hope to use for detecting gravitational waves. CHIME actively discovers and monitors pulsars and other radio transients; a dedicated instrument was built for these tasks. It keeps watch on ten pulsars at a time, around the clock, looking for tiny changes in their timing that could signal a passing gravitational wave. The telescope can also detect fast radio bursts, brief extragalactic flashes lasting just milliseconds that still lack a solid astrophysical explanation.

Method

CHIME is a specialized radio telescope that combines features of a cylinder-shaped dish and an interferometer. Its primary goal is to map the large-scale distribution of neutral hydrogen in the universe by measuring its power spectrum at redshifts between 0.8 and 2.5. This measurement will allow scientists to study the baryon acoustic oscillation (BAO) scale during an era when dark energy began to significantly influence cosmic expansion. The instrument detects the 21 cm radio waves emitted by neutral hydrogen clouds in distant galaxies, including those that have been stretched to longer wavelengths by the expansion of space. Using a technique called intensity mapping, CHIME will create a three-dimensional map of the universe’s large-scale structure as it appeared when it was between roughly 2.5 and 7 billion years old. This map will cover more than 3% of the entire observable universe, a volume far larger than what previous large-scale structure surveys have achieved, and it will explore a period that remains largely unobserved.

These maps help measure the universe’s expansion history because sound waves from the early universe, known as baryon acoustic oscillations, left subtle overdensities in matter distribution on a scale of about 500 million light-years. This characteristic scale has been precisely measured by experiments like Planck, making it a reliable “standard ruler” for determining how the universe’s size has changed over time and thus its expansion rate. While most BAO measurements have come from mapping the positions of individual galaxies—a method continued by future projects like the Dark Energy Survey, Euclid, and DESI—CHIME takes a different approach. Instead of using starlight, it uses radio emissions from hydrogen as a tracer of cosmic structure. Although CHIME cannot perform the same wide range of auxiliary science that galaxy surveys can, it offers a highly cost-effective way to measure BAO because it does not need to detect individual galaxies.

Technology

Instead of the more common array of many circular dishes, CHIME uses a smaller number of long, trough-shaped reflectors. This semi-cylindrical design is not unique—similar telescopes include Australia's Molonglo Observatory Synthesis Telescope and Italy's Northern Cross Radio Telescope. CHIME adopted this approach because it is a cost-effective way to pack radio antennas closely together, allowing the telescope to observe the sky across a broad range of angular scales. The use of multiple parallel half-pipes also provides comparable resolution in both directions of the telescope's view. The antennas themselves were built specifically for CHIME to work well in the 400 to 800 MHz frequency range, picking up two linear polarisations. These antennas are Teflon-based printed circuit boards shaped like cloverleaf petals, placed along the focal line of each wire-mesh half-pipe reflector. Baluns combine the differential signals from two adjacent petals into a single-ended signal. Each antenna has four petals, producing two analogue outputs. With 256 antennas per reflector and four reflectors total, the telescope generates 2,048 analogue outputs for processing. The signals are amplified in two stages using technology borrowed from the cell-phone industry, which keeps the analogue chain low-noise and affordable. Each antenna output first goes through a co-located low-noise amplifier. From there, the amplified signals travel 60 meters along coaxial cables to shielded containers called F-engines for processing. CHIME operates as a correlator, meaning all antenna inputs are combined so the entire system functions as one. This requires substantial computing power. The analogue signals are digitised at 800 MHz and processed using custom-built field-programmable gate array (FPGA) circuit boards.

History

Before the main telescope was built, a smaller test version called the CHIME Pathfinder was assembled at the same observatory in 2013. This Pathfinder consists of two semi-cylindrical structures, each 36 by 20 metres, fitted with 128 dual-polarization antennas. It serves as a testing ground for the technology and observing methods that would later be used in the full instrument, and it can also make an initial measurement of baryon acoustic oscillations using intensity mapping, making it a useful telescope on its own. Construction of the full Canadian Hydrogen Intensity Mapping Experiment started in 2015 at the Dominion Radio Astrophysical Observatory near Penticton, British Columbia. By November of that year, the project was described as nearly operational, with receivers still to be installed and a supercomputer to be built. A contract for the processing chips was placed in March 2016. Building work finished in August 2017, and a first light ceremony with federal Minister of Science Kirsty Duncan took place on 7 September 2017, marking the start of the commissioning phase. Science operations began in late September 2018, and within its first week the telescope detected several events. One of the early finds by the CHIME/Fast Radio Burst Project was the second repeating fast radio burst ever observed, designated FRB 180814. The project also discovered the first FRB that repeats at regular intervals: FRB 180916.J0158+65, which has a periodicity of 16.35 days and, at a distance of 500 million light years, is the closest FRB known. CHIME is sensitive enough that it was expected to detect dozens of FRBs each day. Its first catalog, covering July 2018 to 2019, reported 536 FRBs. A major milestone came on 28 April 2020 with the detection of FRB 200428, the first FRB for which emissions beyond radio waves were recorded, the first found within the Milky Way, and the first linked to a magnetar. In 2022, funding was approved to build three outrigger sites to better pinpoint where FRBs originate. In January 2025, astronomers announced that CHIME had detected multiple FRBs coming from a galaxy roughly 2 billion light years away, a galaxy over 11 billion years old that had long been considered dead.