The CHIME radio telescope, located at the Dominion Radio Astrophysical Observatory near Penticton BC, is a novel radio telescope with no moving parts. It has been explicitly designed to study the mysterious Dark Energy that fills the Universe.
PHOTO BY: Mark Halpern for the CHIME Collaboration
CHIME: The Canadian Hydrogen Intensity Mapping Experiment
CHIME is a new Canadian radio telescope, designed to study Dark Energy by measuring its effect on the distribution of gas over a very large volume of the Universe.
The Universe is expanding, and the rate of this expansion is accelerating. This acceleration is thought to be due to “Dark Energy”, about which we know almost nothing. Indeed the nature of Dark Energy is one of the greatest mysteries in modern science.
One very promising way of trying to understand Dark Energy is to determine how the Universe has expanded and accelerated in the distant past. We can do this by using acoustic patterns (sound waves) in the large-scale distribution of hydrogen gas throughout the cosmos. Shortly after the Universe began, there were slight variations in density in the gas that filled space. These density fluctuations rippled through space, in the form of sound waves: this process is called “baryon acoustic oscillations” (BAOs).
However, these waves froze to a halt when the Universe cooled and became transparent, about 380,000 years after the Big Bang. Ever since, the fossil imprint of these sound waves has left the Universe filled with spherical shells of excess gas, whose size today (13.8 billion years later) is approximately 450 million light years. As one looks out in space and back in time to earlier points in the Universe’s history, these shells have shrunk or expanded as the Universe itself has changed size, meaning that they act a natural yardstick, or “standard ruler”, for studying the Universe’s expansion and acceleration. The gas in these shells glows very faintly in the light of radio waves. Using the CHIME radio telescope, we aim to measure the size and shape of these relic shells at a series of different stages in the Universe’s distant past. Using these data, one can then infer the geometry and history of the Universe, and thereby test different models of Dark Energy.
CHIME is located at the Dominion Radio Astrophysical Observatory, near Penticton, British Columbia. This is a site that has special protection from the terrestrial radio signals that would otherwise completely drown out the faint radio emission from the heavens. CHIME is a completely new sort of radio telescope: it doesn’t steer or point, and has no moving parts. CHIME consists of four curved radio panels, each 20 metres wide and 100 metres long. The panels always stare straight up, and as the Earth rotates, the entire sky passes overhead every 24 hours.
CHIME targets times in the Universe’s history ranging from 7 to 11 billion years ago, a period when Dark Energy emerged as the dominant force in the Universe. Radio waves from this distant era reflect off the mesh, and are brought to a focus at a series of antennas sitting above the surface. The combined data rate of all these signals is larger than that of the entire North American telephone system, and requires a dedicated custom supercomputer located on site to process it all.
In addition to probing Dark Energy, CHIME is also simultaneously studying a very different sort of celestial radio signal. CHIME is able to detect the very regular radio pulses from spinning neutron stars, or “pulsars”, which serve as ultra-precise natural clocks. By accurately timing the signals from hundreds of pulsars every day, CHIME is searching for the minuscule stretching and squeezing of space-time expected from the gravitational waves predicted by Einstein’s Theory of General Relativity. In addition, CHIME will soon begin to be used to detect “fast radio bursts” (FRBs), a newly discovered cosmic puzzle, and to make a detailed map of radio emission from our own galaxy, the Milky Way.
CHIME has been built by the University of British Columbia, McGill University, the University of Toronto, and NRC’s Dominion Radio Astrophysical Observatory. While CHIME was funded independently of the Square Kilometre Array, it is playing a pivotal role as an SKA scientific and technical prototype. For example CHIME is performing very sensitive observations of faint distant structures in the sky, which has required the CHIME team to develop pioneering techniques for understanding and removing the very bright foreground signals from the Milky Way. This will be a major challenge for the SKA as well.
There are also many technical challenges that CHIME shares with the SKA. The radio signals that CHIME collects are combined in a specialised supercomputer called a “signal correlator”, which uses a mix of customized electronics and commercial graphical processing units (GPUs). This approach is a major step forward in lowering the cost of radio telescopes. The CHIME team has also developed a very effective and high-fidelity way of transporting radio signals using optical fibres, and has developed a new data compression algorithm that is crucial for being able to transfer the massive amounts of data produced by telescopes like CHIME and the SKA.