To study dark energy’s effects over the past 11 billion years, the Dark Energy Spectroscopic Instrument (DESI) has created the largest 3D map of our cosmos ever constructed, with the most precise measurements to date. This is the first time astronomers have measured the expansion history of the young Universe with a precision better than 1%, giving us our best view yet of how the Universe evolved.
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A slice of the 3D map of galaxies collected in the first year of the DESI survey with annotations identifying key features in the map. Earth is at the tip, with the furthest galaxies plotted at distances of 11 billion light-years. Each point represents one galaxy. This version of the DESI map includes 600,000 galaxies — less than 0.1% of the survey’s full volume. Image credit: DESI Collaboration / NOIRLab / NSF / AURA / R. Proctor.
Since beginning its survey of the sky in 2021, the DESI instrument has observed a new set of 5,000 galaxies every 20 minutes, totaling more than 100,000 galaxies per night, in its quest to create the largest 3D map of the Universe ever.
To map the cosmos, DESI collects light from millions of galaxies across more than a third of the entire sky.
By breaking down the light from each galaxy into its spectrum of colors, DESI can determine how much the light has been redshifted, or stretched to a longer wavelength, by the expansion of the Universe during the billions of years it traveled before reaching Earth. In general, the higher the redshift the further away the galaxy is.
Equipped with 5,000 tiny robotic ‘eyes,’ DESI is able to perform this measurement at an unprecedented rate.
In its first year alone DESI surpassed all previous surveys of its kind in terms of quantity and quality.
With incredible depth and precision, DESI has brought new insight to one of the biggest mysteries in physics: dark energy — the unknown ingredient causing the expansion of our Universe to accelerate.
“The DESI instrument has transformed NSF’s Mayall Telescope into the world’s premier cosmic cartography machine,” said Dr. Pat McCarthy, director of NOIRLab.
“The DESI team has set a new standard for studies of large-scale structure in the Universe.”
“These first-year data are only the beginning of DESI’s quest to unravel the expansion history of the Universe and they hint at the extraordinary science to come.”
DESI’s first-year data have allowed astronomers to measure the expansion rate of the Universe out to 11 billion years in the past, when the Universe was only a quarter of its current age, using a feature of the large-scale structure of the Universe called Baryon Acoustic Oscillations (BAO).
BAO are the leftover imprint of pressure waves that permeated the early Universe when it was nothing but a hot, dense soup of subatomic particles.
As the Universe expanded and cooled the waves stagnated, freezing the ripples in place and seeding future galaxies in the dense areas.
This pattern, resembling the rippling surface of a pond after a handful of pebbles is tossed in, can be seen in DESI’s detailed map, which shows strands of galaxies clustered together, separated by voids where there are fewer objects.
At a certain distance, the BAO pattern becomes too faint to detect using typical galaxies.
So instead astronomers look at the ‘shadow’ of the pattern as it’s backlit by extremely distant, bright galactic cores known as quasars.
As the quasars’ light travels across the cosmos it gets absorbed by intergalactic clouds of gas, allowing astronomers to map the pockets of dense matter.
To implement this technique, the researchers used 450,000 quasars — the largest set ever collected for this type of study.
With DESI’s unique ability to map millions of objects both near and far, the BAO pattern can be used as a cosmic ruler.
By mapping nearby galaxies and distant quasars, astronomers can measure the spread of the ripples across several periods of cosmic history to see how dark energy has stretched the scale over time.
“We’re incredibly proud of the data, which have produced world-leading cosmology results,” said Dr. Michael Levi, director of DESI and researcher at the Lawrence Berkeley National Laboratory.
“So far we’re seeing basic agreement with our best model of the Universe, but we’re also seeing some potentially interesting differences that could indicate dark energy is evolving with time.”
While the expansion history of the Universe may be more complex than previously imagined, confirmation of this must await the completion of the DESI project.
By the end of its five-year survey DESI plans to map over 3 million quasars and 37 million galaxies. As more data are released, astronomers will further improve their results.
“This project is addressing some of the biggest questions in astronomy, like the nature of the mysterious dark energy that drives the expansion of the Universe,” said Dr. Chris Davis, NSF program director for NOIRLab.
“The exceptional and continuing results yielded by NSF’s Mayall Telescope with DESI will undoubtedly drive cosmology research for many years to come.”
“We are delighted to see cosmology results from DESI’s first year of operations,” said Dr. Gina Rameika, associate director for high energy physics at the Department of Energy.
“DESI continues to amaze us with its stellar performance and how it is shaping our understanding of dark energy in the Universe.”
