Astronomers map the universe through analysis of dark energy distribution in galaxies

Scientists continue to explore the nature of dark matter and dark energy in the universe.
By Lliane Hunter | Apr 04, 2018
In its first year of data, the Dark Energy Survey (DES) telescope experiment has gauged the amount of dark energy and dark matter in the universe by measuring the clumpiness of galaxies, writes Natalie Wolchover for Quanta Magazine. Posted on DES's website, the analysis is based on observations of 26 million galaxies in a large swath of the southern sky. It charts the universe as 74 percent dark energy and 21 percent dark matter, with galaxies and all other visible matter filling the remaining 5 percent.

"As the universe evolves, the gravity of dark matter is making it more clumpy, but dark energy makes it less clumpy because it's pushing galaxies away from each other," said Joshua Frieman, co-founder and director of DES and an astrophysicist at Fermi National Accelerator Laboratory (Fermilab) and the University of Chicago.

The telescope's first observing season began in August 2013 and lasted six months. It will begin its fifth and final planned observing season this month. Researchers are focused on studying the presence of dark energy, and they have already incrementally improved the measurement of a key quantity that will reveal what dark energy is.

The terms dark energy and dark matter are designations for unknown physics, Wolchover writes.Dark energy refers to whatever is causing the expansion of the universe to acceleratea discovery thatastronomers first made in 1998.Dark matter has been inferred after 80 years of observations of its apparent gravitational effect on visible matterhowever what this dark matter consists of (actual particles, or something else), scientists have yet to determine.

Previously, the cosmic microwave background was the premierway in which scientists mapped the universe. It extends from the farthest point that can be seen in every direction. Planck's map (a high-resolution map of the cosmic microwave background), shows an extremely homogenous young universe with subtle density variations that grew into the galaxies and voids that fill the universe today, writes Wolchover.

This method provides a 2D picture of the universe, whereas the study of galaxies offers a richerpicture of the universe's governing laws since they span the full three-dimensional volume of space.Thus, the DES team scrutinized a section of the universe spanning an area 1,300 square degrees wide in the sky, and stretching back 8 billion years. They examined the distortion in the galaxies' apparent shapes (an effect known as "weak gravitational lensing") that indicates how much space-warping dark matter lies between the galaxies and Earth.

Galaxy clustering and weak lensing are two of the four approaches that DES will eventually use to inventory the cosmos. The team's recent results compared and contrasted DES's measurement with the older estimate from Planck. Planck pegged matter at 33 percent of the cosmos today, while DES's plot placed the matter measurement at 26 percent.

This displays only a slight tension between the two, and not enough of a discrepancy to claim a discovery, explains Wolchover. "We saw they didn't quite overlap," said Gary Bernstein an astrophysicist at the University of Pennsylvania and co-chair of the DES science committee. He adds, however,that the results were still exciting.

Whether this minor discrepancy strengthens or vanishes as more data is collected will be revealed as the DES team embarks on its next analysis.

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