Does Dark Matter And Dark Energy Make Up The Cosmos?

Researchers from The Ohio State University and their colleagues from the Dark Energy Survey (DES) collaboration have reached a new milestone mapping the growth of the universe from its infancy to present day. It comes exactly 100 years after Einstein confronted the idea of an expanding universe in his general theory of relativity.

The new results released last Thursday confirm the surprisingly simple but puzzling theory that the present universe is comprised of only 4% ordinary matter, 26% mysterious dark matter, and the remaining 70% in the form of mysterious dark energy, which causes the accelerating expansion of the universe.

The findings are based on data collected during the DES first year, which covers over 1300 square degrees of the sky or about the area of 6,000 full moons. DES uses the Dark Energy Camera mounted on the Blanco 4m telescope at the Cerro Tololo Inter-American Observatory high in the Chilean Andes.

“We had to construct the most powerful instrument of its kind. It is sensitive enough to collect light from galaxies 8 billion light years away,” said Klaus Honscheid, professor of physics and leader of the Ohio State DES group. Key components of the 570mp camera were built at Ohio State.

Paradoxically, it is easier to measure the structure of the universe in the distant past than it is to measure it today. In the first 400,000 years following the Big Bang, the universe was filled with a glowing gas, the light from which survives to this day.

This cosmic microwave background (CMB) radiation gives us a snapshot of the universe at that very early time. Since then, the gravity of dark matter has pulled mass together and made the universe clumpier over time. But dark energy has been fighting back, pushing matter apart. Using the CMB as a start, cosmologists can calculate precisely how this battle plays out over 14 billion years.

“With the new results, we are able for the first time to see the current structure of the universe with a similar level of clarity as we can see its infancy. Dark energy is needed to explain how the infant Universe evolved to what we observe now,” said Niall MacCrann, postdoctoral fellow at Ohio State’s Center for Cosmology and Astro-Particle Physics (CCAPP) and major contributor to the analysis.

DES scientists used two methods to measure dark matter. First, they created maps of galaxy positions as tracers, and second, they precisely measured the shapes of 26 million galaxies to directly map the patterns of dark matter over billions of light years, using a technique called gravitational lensing.

Ashley Ross of CCAPP and leader of the DES large scale structure working group said: “For the first time we were able to perform these studies with data from the same experiment allowing us to obtain the most accurate results to date.”

To make these ultra-precise measurements, the DES team developed new ways to detect the tiny lensing distortions of galaxy images, an effect not even visible to the eye, enabling revolutionary advances in understanding these cosmic signals.

In the process, they created the largest guide to spotting dark matter in the cosmos ever drawn. The new dark matter map is ten times the size of the one DES released in 2015 and will eventually be three times larger than it is now.

A large scientific team achieved these results working in seven countries across three continents. “Successful collaboration at this scale represents many years of deep commitment, collective vision and sustained effort,” said Ami Choi, CCAPP postdoctoral fellow who worked on the galaxy shape measurements.

Michael Troxel, CCAPP postdoctoral fellow and leader of the weak gravitational lensing analysis added: “These results are based on unprecedented statistical power and detailed understanding of the telescope and potential biases in the analysis. Crucially, we performed a ‘blind’ analysis, in which we finalized all aspects of the analysis before we knew the results, thereby avoiding confirmation biases.”

The DES measurements of the present universe agree with the results obtained by the Planck satellite that studied the cosmic microwave background radiation from a time when the universe was just 400,000 years old.“The moment we realized that our measurement matched the Planck result within 7% was thrilling for the entire collaboration,” said Honscheid, “and this is just the beginning for DES with more data already observed. With one more observing season to go we expect to ultimately use five times more data to learn more about the enigmatic dark sector of the Universe.”

The new results from the Dark Energy Survey will be presented by Kavli fellow Elisabeth Krause at the TeV Particle Astrophysics Conference in Columbus, Ohio, on August 9, and by CCAPP’s Troxel at the International Symposium on Lepton Photon Interactions at High Energies in Guanzhou, China, on August 10.

The publications can be accessed on the Dark Energy Survey website.

Ohio State University is an institutional member of the Dark Energy Survey collaboration. Funding for this research coms in part from the Ohio State’s Center for Cosmology and Astro-Particle Physics. The Ohio Supercomputer Center provided a portion of the computing power for this project.

The Ohio State DES team includes Honscheid; Paul Martini and David Weinberg, both professors of astronomy; Choi, Ross, MacCrann and Troxel, all postdoctoral fellows at CCAPP; and doctoral students Su-Jeong Lee and Hui Kong.