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03/25/2020 - Detailed Cosmic Map to Reveal Dark Energy's Sway

By Adam Hadhazy

a photo of an observatory against a star-streaked sky DESI, a bold celestial mapmaking effort, will help unravel the mystery of the universe's accelerating expansion rate. Here, star trails take shape around the 14-story Mayall Telescope dome in this long-exposure image. The Dark Energy Spectroscopic Instrument will reside within this dome. (Credit: P. Marenfeld and NOAO/AURA/NSF)

Compared to cosmic mapmakers, terrestrial cartographers have it relatively easy. Map generation here on Earth generally deals with just two dimensions—the latitude and longitude of where portions of landmasses and oceans all stitch together, forming the surface of our planet. 

Out in space, though, the third dimension reigns.

After all, space is not called "space" for nothing; the sheer amount of volume out there through which galaxies are strewn is staggering. In order to understand the universe's development, researchers need to chart its great expanse by accurately gauging distances between clusters of galaxies. Like on Earth, where the location of landmasses today speaks to eons of geophysical evolution, continental drift, tectonics, and so on, so, too, do the locations of galaxies speak to the dynamic history of the cosmos. 

The geo-analogy is only so apt, however, because looking deep into space is also looking deep into time; the farther away something is, the farther back in the universe's chronology we are glimpsing it. As a result, not only does a cosmic map capture where things are in relation to each other, it also captures when things are in relation to each other. 

Tackling this time-and-space challenge anew is the Dark Energy Spectroscopic Instrument (DESI). Now mounted on a telescope in Arizona, DESI will soon begin making the most detailed 3D map of the universe to date. More than 500 researchers, hailing from 75 institutions in 13 countries, are part of the DESI project, including researchers at the Kavli Institute for Particle Astrophysics and Cosmology (KIPAC), a joint project of Stanford University and the SLAC National Accelerator Laboratory.

Numbers-wise, over a five-year observing run that starts in 2020, DESI will look at a third of the sky, accurately mapping the distances to approximately 35 million galaxies and 2.4 million quasars. (Quasars are ultra-bright galaxies whose central supermassive black holes are gorging on matter.) That tidy galactic sum represents about 20 times more objects than any previous such effort. 

"We'll know in 3D space where the galaxies are with DESI, and with that, we can look at how galaxies are arranged in the universe," says Aaron Roodman, who worked on image analysis elements of the Guiding, Focusing and Alignment (GFA) subsystem for DESI. Roodman is a professor and chair of the Particle Physics & Astrophysics department at SLAC and a member of KIPAC.

DESI's mapmaking is not for astronaut traveling purposes, of course. (Heck, we humans will be lucky if we get back to the Moon in the next decade, after a 50-year hiatus.) Instead, the goal for DESI is to help us better understand the history and eventual fate of the universe by nailing down the workings of dark energy. Right there in DESI's name, "dark energy" refers to an enigmatic force that theoretically comprises most of reality.

"This thing we call dark energy makes up 70 of the matter-energy density of the universe, but we don’t know what it is," says Roodman.  

Dark energy's presence looms large, invoked to explain why the universe's rate of acceleration is increasing over history. The overall situation is thus: Matter, the substance that comprises everything we deal with every day, registers at just 5% of the total contents of the cosmos. A second enigmatic ingredient, dark matter, takes up the remaining 25% percent (once dark energy's dominance – clocking in at nearly 70% – is counted). Dark matter interacts with normal matter only through gravity, so far as we know, acting like a universe-spanning scaffold upon which normal matter is hung. That normal matter interacts with itself through the forces of nature to give us galaxies, stars, planets, elephants, paramecia, you name it. 

Researchers want to bring this overview picture into sharper focus. "With DESI, we will be able to independently measure the universe’s expansion rate and how fast its structure of matter and dark matter grow, both of which are influenced by dark energy. Then when you compare those measurements, you get a precise test of the physics governing the universe," said Risa Wechsler, the current Director of KIPAC and former spokesperson of the DESI collaboration, in comments to The Kavli Foundation in 2017

The other part of DESI's name, "spectroscopic," refers to how DESI will get the all-important distance measurements to millions of galaxies. DESI contains 5,000 robotic "eyes," made of human-hair-thin, cosmic-light-gathering fiber optic cables, held in particular orientations by robotic positioners. Each "eye" is pointed at a bright galaxy to collect enough of its stars' collective light to perform spectroscopy—breaking apart the light into different wavelengths for study. "Whatever light comes down the fiber, we spread out the whole rainbow," says Roodman. The signature astronomers look for in this light is called redshift, caused by the universe's expansion, and which can be used to determine a galaxy's distance. 

With precise distance measurements to an unprecedentedly large swath of galaxies, the DESI researchers will plot the course of dark energy's influence as never before, better constraining its properties. Roodman, for one, can't wait. 

"What is causing the expansion of the universe to accelerate is a question I'm very interested in," says Roodman. "DESI will really teach us something about dark energy."

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