Cosmology
Since the time of Edwin Hubble, it has been known the Universe is expanding. More recently, observations have shown that this expansion is accelerating. The reason for this acceleration is unknown but it has been suggested that a “Dark Energy” is causing the expansion rate to increase. The origin and physics of this Dark Energy are presently unknown. One way to probe the nature of Dark Energy is through studying in detail the expansion itself.
CHIME will map the history of the expansion rate of the Universe by observing hydrogen gas in distant galaxies that were very strongly affected by Dark Energy. The experiment will measure the relic of Baryon Acoustic Oscillations (BAO), spherical shells of matter over-density in which galaxies and gas are more likely to be found today. The radius of these shells was established by conditions in the early Universe (up to ~400,000 years after the Big Bang), and in principle is still detectable in the distant distribution of hydrogen gas. What this means is that, for the past 13 billion years, this characteristic distance scale evolved solely due to the expansion of the Universe, and hence provides a standard ruler to measure the expansion rate.
The BAO scale has been measured before using galaxy surveys to map the distribution of matter. This is a long and difficult process that requires resolving each individual galaxy and has a limited redshift range. CHIME will map the distribution of matter using the 21 cm radio emission of intergalactic hydrogen at a resolution much lower than that of individual galaxies, but high enough to measure the BAO scale. This technique, known as Hydrogen Intensity (HI) Mapping, is much faster and will allow for a much larger survey volume than has ever been observed. It avoids reliance on the complicated physics of galaxy formation since galaxies are not resolved and counted, and is well suited to measuring structure on large physical scales.
Fast Radio Bursts
Fast Radio Bursts are brief (few millisecond) bursts of radio waves coming from far beyond our Milky Way galaxy. The phenomenon was first reported in 2007 and as of mid-2017, roughly two dozen have been reported. Their origin is unknown. However, they are ubiquitous: current best estimates suggest these events are arriving at Earth roughly a thousand times per day over the entire sky.
The CHIME telescope’s large collecting area, wide bandwidth and enormous field-of-view make it a superb detector of FRBs. As of mid 2020, CHIME has detected well over 1000 Fast Radio Burst sources. So high an event rate promises major progress on this puzzling new astrophysical phenomenon. Bright CHIME-discovered FRBs will be found in real time and reported immediately to the worldwide astrophysical community for multi-wavelength follow up.
The CHIME Fast Radio Burst team has recently received a grant from the Gordon and Betty Moore Foundation to design CHIME Outrigger telescopes – smaller versions of CHIME to be located at great distances from CHIME itself. These Outriggers will enable CHIME-detected Fast Radio Bursts to be precisely localized on the sky so that astronomers can pinpoint the exact galaxy from which the event emerged.
Pulsars
Radio pulsars are rapidly rotating, highly magnetized neutron stars that can act as exquisitely precise cosmic clocks. Discovered in 1967 by Jocelyn Bell, radio pulsars are like cosmic light houses: they emit beams of radio light from their magnetic poles, which are misaligned with the neutron star rotation axis. Nearly 3000 pulsars are known today.
Pulsar timing involves monitoring a pulsar over weeks to years and keeping count of every single pulse emitted. In this way pulsars are very useful for a wide variety of precision astrophysical measurements ranging from unique tests of relativistic theories of gravity using strong-field binary pulsars to the detection of a cosmic background of gravitational waves having periods of many years, predicted to exist due to mergers of supermassive black holes in galaxy-galaxy interactions. The latter is accomplished through a “Pulsar Timing Array” in which radio telescopes worldwide monitor dozens of very rapidly rotating pulsars on a regular basis over many years.
The CHIME telescope, with its wide bandwidth and large collecting area, is excellent for pulsar timing. Its wide field of view and novel software-steered beams allow scientists to time up to 10 pulsars at any time, 24 hrs per day, 7 days per week. This enables unique studies of highly variable pulsars such as nullers and mode changers, as well as daily observation of Pulsar Timing Array sources. The relatively low radio frequencies detected by the CHIME telescope facilitate the removal of the effects of the variable interstellar medium on the radio pulses, one known important source of noise for gravitational wave experiments. This is improving the precision of the experiments carried out by Pulsar Timing Array projects like NANOGrav and the International Pulsar Timing Array, in which several CHIME/Pulsar team members are involved.
The CHIME telescope will also soon be able to search for radio pulsars. A large-scale slow pulsar search is currently under design, and will make use of CHIME’s daily observations of the full Northern sky to find, in particular nulling and intermittent pulsars, or pulsars in eclipsing binary systems. This is part of the CIRADA Project.
