Largest-ever map of dark matter

The Dark Energy Survey collaboration has just released 30 new papers from the first three years of their observing run, including the largest-ever map of the distribution of dark matter.

We know that ordinary matter makes up just 5% of the universe, while dark energy (which forces the universe to expand at ever-increasing speed) comprises 70%, and dark matter (which holds galaxies together) comprises 25%. Both of these “dark” substances are invisible – but can be detected through their gravitational influence on other objects.

The goal of Dark Energy Survey (DES) is to understand dark matter and dark energy by studying how they shape the large-scale structure of the universe. The survey involved hundreds of nights of observations from the Víctor M. Blanco Telescope in Chile, where a powerful digital camera surveyed a large chunk of the sky and catalogued hundreds of millions of objects over six years from 2013 to 2019.

To study the distribution and amount of dark matter and dark energy, DES relied on two cosmic phenomena.

The dark energy survey camera (decam) at the sidet clean room.
The Dark Energy Survey camera (DECam) at the SiDet clean room. Credit: DOE/FNAL/DECam/R. Hahn/CTIO/NOIRLab/NSF/AURA

Firstly, it mapped the “cosmic web” – the web-like structure of galaxies, which reveal regions where dark matter is concentrated.

Secondly, it detected the signature of dark matter via weak gravitational lensing, where the gravity of a foreground object can distort the light of another object behind; from this, astronomers can infer the distribution of dark matter, or how “clumpy” it is.

The map that was just released is based on the first three years of the six-year observing run. It covers an eighth of the whole sky and uses measurements of 226 million galaxies, providing the most accurate measurements to date of what the universe is made of and how it evolved. It also peered seven billion light-years out into the universe, meaning it spans seven billion years of time.

Such a map provides an excellent test of our current models of how the universe evolved.

The standard model of cosmology tells us that the universe started with a bang, then rapidly expanded and matter evolved. How this matter is distributed and shaped depends on Einstein’s theory of general relativity (which describes gravity).

The team compared their results with the European Space Agency’s orbiting Planck observatory, which studies the cosmic microwave background as a test of cosmological models, and found that they were generally consistent.

“DES has obtained limits that rival and complement those from the cosmic microwave background,” says Brian Yanny, scientist from Fermilab, US, who coordinates DES data.

Ten areas in the sky were selected as "deep fields" that the dark energy camera imaged several times during the survey, providing a glimpse of distant galaxies.
Ten areas in the sky were selected as “deep fields” that the Dark Energy Camera imaged several times during the survey, providing a glimpse of distant galaxies and helping determine their 3D distribution in the cosmos. Credit: Dark Energy Survey/DOE/FNAL/DECam/CTIO/NOIRLab/NSF/AURA Acknowledgments: T.A. Rector (University of Alaska Anchorage/NSF’s NOIRLab), M. Zamani (NSF’s NOIRLab) & D. de Martin (NSF’s NOIRLab)

“It’s exciting to have precise measurements of what’s out there and a better understanding of how the universe has changed from its infancy through to today.”

However, something still isn’t quite right – although these results are consistent with models, previous results have suggested that the universe is a few percent less “clumpy” than predicted. Perhaps these previous measurements were slightly off, perhaps there is a problem with the model, or perhaps there is something wrong with Einstein’s theory.

Either way, it will be interesting to see the upcoming results from analysing the final three years of DES observations, taken from 2016 to 2019. This will be a formidable task, given the massive amounts of data produced.

“These analyses are truly state-of-the-art, requiring artificial intelligence and high-performance computing super-charged by the smartest young scientists around,” says Scott Dodelson, a physicist at Carnegie Mellon University, US, and co-leader of the DES Science Committee.

According to Michael Troxel, a physicist at Duke University, US, “The real legacy of DES will be the leaps forward we’ve had to make that were essential for this key result, and which will be critical for the next generation of cosmological experiments starting soon.

“With these instruments we’ve built to stare into the dark, we are working to solve universal mysteries.”

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