T. Jarrett (IPAC/Caltech); B. Saxton, NRAO/AUI/NSF

Neutral hydrogen emits radiation a characteristic wavelength of 21 centimeters, or about 1,420 megahertz (MHz). Expansion of the universe stretches the wavelength of this line out, which is why CHIME is optimized for the 400- to 800-MHz range, corresponding to wavelengths of about 75 to 38 cm, allowing it to search for this emission at great distances. With this sensitivity, CHIME can map the hydrogen density between redshifts of 0.8 and 2.5, when the universe was between about 2.5 and 7 billion years old (and when dark energy began to play an important role).“We’re comparing the distances we infer from the size of these spheres [near ones bigger, far ones smaller] with the distances we calculate from their redshift velocities to give us an expansion history,” says Halpern. And the outcome of that comparison could mean a lot. “A smoothly accelerating universe would yield a graph of a particular shape. Other models of cosmic acceleration would yield other shapes.”Although BAO scales have been observed in the past, it has been via indirect measurements of the distributions of faraway galaxies, a challenging and painstaking process. The CHIME hydrogen intensity approach will be establishing new science by observing BAOs faster and at higher resolution. CHIME’s measurements will also rely on less complicated physics, simplifying the process for better results. “When we make that first measurement and we see the BAOs and characterize their scale,” says Dobbs, “we’ll have done the first measurement of a direct signal with 21-centimeter intensity mapping ever.”Additional uses for the telescope were identified early in its development. “It was actually an historical accident,” says Dobbs. “The telescope and cosmology team was already in the midst of constructing CHIME when the topic of fast radio bursts became hot. While interacting with the FRB community, however, particularly at CIfAR (Canadian Institute for Advanced Research) meetings, we realized that we had the world’s best instrument already designed to detect them. So, we partnered with world leaders in fast radio bursts and put in a new grant request to build the FRB back end.”With these new back end instruments, the telescope will study fast radio bursts (FRBs) and pulsars. Fast radio bursts are emissions lasting only milliseconds. Astronomers theorize that they’re caused by rapidly spinning neutron stars or black holes, but they remain poorly understood. While only a few dozen have been discovered so far (including one repeating FRB), some astronomers believe that up to 10,000 bursts might be occurring each day across the entire sky – but no telescope can currently track such a wide expanse. CHIME, however, can provide the kind of comprehensive, persistent gaze needed to characterize FRB distributions and behavior.The frequency components of radio pulses like FRBs can “disperse,” traveling at different speeds as they move through the interstellar medium. Brief pulses of only a single millisecond at their source can be stretched slightly by the time they reach a radio telescope. CHIME processing has the capability to reconstruct the original signal shapes, however, which means that the telescope can better localize the source of each FRB.CHIME data are first organized by its main processing engines and sent to the FRB back end instrument. Candidate FRBs identified at this stage are then processed further to determine their locations, distances and characteristics. The CHIME system is designed to issue an automatic alert to the worldwide astrophysical community within seconds of an identified FRB event, to enable astronomers to follow up with other telescopes.In addition to FRB science, CHIME can also contribute to gravitational wave detection. Pulsars are rapidly rotating, highly magnetized neutron stars that emit radio energy from their magnetic poles (which are misaligned with the star’s rotation axis). They act as extremely precise cosmic clocks. Radio telescopes around the world monitor large numbers of rotating pulsars and catalog their performance over time as part of the Pulsar Timing Array (PTA), and changes to these timing patterns can be used to indicate the passage of gravitational waves . CHIME will be able to effectively monitor pulsars in the northern sky, generating data to increase the precision and sensitivity of studies that use the PTA.