Fathoming the Large-Scale Structure of the Universe

Shining galaxies, resplendent and chockful of stars, naturally draw much of the attention. Although conspicuous, galaxies are but small pieces in the overall puzzle that researchers refer to as the large-scale structure of the universe. This structure consists of concentrations of matter and dark matter on the vastest of scales, dwarfing any individual galaxy. Unanswered questions about the origin, evolution, and fate of the universe are all tied up in ascertaining the large-scale structure, an effort that is a focus for today's cosmologists and astrophysicists.

Kiyoshi Masui is a leader in this endeavor through his research with a powerful instrument dubbed CHIME, the Canadian Hydrogen Intensity Mapping Experiment. With CHIME, Masui and colleagues are mapping the radio waves emitted by otherwise invisible clouds of hydrogen associated with the large-scale structure. The researchers are also innovatively probing the large-scale structure through brilliant, brief flashes of radio waves called fast radio bursts.

Overall, Masui is helping bring the phantom scaffolding of the cosmos to light, and with it, potential elucidations of long-confounding phenomena.

"CHIME is unprecedented in its so-called 'mapping speed,' the combination of having high sensitivity to individual spots on the sky and being able to see a lot of the sky at once," says Masui, the Principal Investigator of the Masui Synoptic Radio Lab at the Massachusetts Institute of Technology's (MIT) Kavli Institute for Astrophysics and Space Research (MKI). "With CHIME, we're making substantial progress in revealing the large-scale structure and delivering on CHIME's promise of advancing our understanding of the universe."

Masui's involvement and interest in CHIME date back to the instrument's construction infancy a decade ago, when Masui earned his PhD in 2013 and completed his thesis work on large-scale structure surveys. CHIME intrigued Masui then, as it does now, for its ability to reveal the intrinsic workings of nature. "I had always wanted to understand how the universe works at a fundamental level," says Masui.

Initially, Masui planned to pursue this understanding by going into particle physics and dealing with the nitty-gritty of nature on the smallest of scales. But he soon realized that the fastest progress in fundamental physics was actually being made through the macroscales of astronomy and cosmology. His PhD advisor, Professor Ue-Li Pen at the University of Toronto, convinced Masui "that radio observations of the large-scale structure would ultimately be the most powerful probe of the universe," he says. One thing led to another, with Masui joining MKI in 2018 as an Assistant Professor of Physics.

Masui opened his own Synoptic Radio Lab at MKI barely a year after CHIME had likewise opened its proverbial doors for scientific business in British Columbia. CHIME consists of four curved radio antennae running about 100 meters long and 20 meters wide, often compared visually to the halfpipes enjoyed by skiers and snowboarders. The cylinders of CHIME capture a faint emission line of hydrogen that, according to the emissions' intensity, sketches out a map of where hydrogen is clumped up throughout the vast volumes of cosmic space and time.

Getting the most out of CHIME is a major part of the Masui Synoptic Radio Lab. The technique just described of hydrogen intensity mapping presents numerous technical and data-analysis challenges that Masui's group is seeking to overcome. "Making these methods work is a key long-term priority for us," Masui says.

To this end, the group is developing algorithms for analyzing and understanding collected observational data. Gleaning how the large-scale structure changes over time will move scientists closer to nailing down why the universe's expansion is accelerating, while also helping trace cosmic evolution back to its earliest phases.

"The large-scale structure survey that CHIME was built for will be used to precisely measure the expansion of the universe to better understand what is driving it," says Masui. "But beyond that, the same observational methods could one day be used to better understand how the universe began, which is a question I've been interested in nearly my whole life."

Fast radio bursts could prove to be tremendously valuable in this grand endeavor. CHIME is uniquely well-suited as a finder of these so-called FRBs, standing as the only instrument of its kind that can nab the enigmatic bursts by the hundreds every year. However, CHIME lacks the resolution to tell where these FRBs are located in space. Doing so, though, would allow researchers to leverage FRBs to probe a different component of the large-scale structure—the wispy gas outside of galaxies.

Masui and colleagues are working on bringing this capability to bear through the CHIME/FRB Outriggers program. The program envisions installing three CHIME-like telescopes across North America. Capturing signals from the same FRB at the differing, distant sites of the mini-CHIMEs would enable researchers to infer the true source locations of the radio signals. The three observatories are nearly complete and Masui is busily working on getting them working to their full potential.

"We are starting to become very excited about using FRBs as probes of large-scale structure," Masui explains. "FRBs have this special property that they are distorted as they propagate through the very diffuse gas that permeates the space between galaxies. This allows us to measure precisely how much gas is between us and the burst source, which in turn lets us measure the large-scale distribution of this gas. It turns out that this gas is essentially impossible to observe by other means."

In the years ahead, Masui and his colleagues have much to look forward to as CHIME continues bringing in a treasure trove of cosmological data. Collaborating with his lab partners has been richly rewarding in its own right for Masui.

"Working with the members of my lab is an immense privilege as they are a truly special group of people," Masui says. "Not only are they immensely talented in research and academics, but every one of them puts a huge amount of effort into building and serving the MKI community. I'm so grateful that they've chosen to work in our lab."