Einstein’s general theory of relativity has helped an international team of researchers measure the lumpiness of dark matter in our universe today by analyzing images of 10 million distant galaxies, and further use it to understand dark energy.
Dark matter is responsible for the formation of galaxies in the universe, while dark energy is responsible for the acceleration in the universe’s ongoing expansion. Together dark matter and dark energy make up 95 percent of our universe, but why this number is so big remains a mystery. To explain the behavior and the fate of the universe, scientists must find out what dark matter and dark energy are.

Studies to measure the amount of cosmic structure have been carried out in the past, especially when the universe was very young, such as the European Space Agency Planck satellite. Dark energy can affect the growth rate of these structures at a later time, closer to today. Scientists are just starting to measure the growth of these structures by studying dark matter in the later Universe and thus using it to learn about dark energy.

“Fluctuations measured by Planck are like a precise arrow shot from the early Universe, and we have measured where the arrow landed with Subaru Hyper Suprime-Cam,” said paper author and Inter-University Center for Astronomy & Astrophysics Associate Professor Surhud More.

The Hyper Suprime-Cam Survey, which uses the 820 mega-pixel Hyper Suprime-Cam camera attached to the 8.2 meter Subaru telescope at the summit of Maunakea, allowed researchers to study galaxies billions of light-years away. These galaxies existed billions of years ago but their light only reached Earth today.

Albert Einstein predicted gravitational lensing in his theory of general relativity, where gravity can bend the path of light, making far away galaxies appear distorted to observers on Earth.

Since 2014, researchers from Japan, Taiwan, and the US, led by Kavli Institute for the Physics and Mathematics of the universe (Kavli IPMU) Project Assistant Professor Chiaki Hikage, studied these minute distortions caused by gravitational lensing to reconstruct where matter is distributed in the universe. Now the team can see how fluctuations of dark matter across the sky have changed over billions of years, and how dark energy has influenced this growth of structure.

“I had a long-cherished hope to undertake high precision cosmology research such as that enabled by WMAP and Planck. I am very excited to share the measurements of the growth of dark matter structures in the universe with great accuracy using Subaru HSC data,” said Hikage.

However, Hikage and his team knew their excitement could potentially bias their results, especially if they looked to confirm results from previous studies during their analysis. To ensure the results were sound, they performed a blind analysis, a technique well known in medical trials where no one knows which patients have been given a new treatment. Two fake data catalogs were created, and the true identity of each catalog was locked away in a box. Following an unveiling event on June 26, for the first time researchers found their analysis had revealed the Hyper Suprime-Cam Survey was indeed consistent with past gravitational lensing studies, but their result suggested cosmic structures might be evolving a tad bit slower in the universe today than that predicted by the Planck satellite in the concordance cosmological model.

This could be a statistical fluctuation due to a small data set, or it could indicate a breakdown in the standard model of the universe.

Luckily, there is more data for the team to analyze in the future. This result uses only 11 percent of the full survey, because the Hyper Suprime-Cam is still taking images, and is scheduled to finish around 2020.

“This is just a first step, and the completed data of Hyper Suprime-Cam survey promises to advance our understanding of dark matter and dark energy,” said Kavli IPMU Principal Investigator and paper author Masahiro Takada.

Reference: Cosmology from Cosmic Shear Power Spectra with Subaru Hyper Suprime-Cam First-Year Data,” Chiaki Hikage (1), Masamune Oguri (2,3,1), Takashi Hamana (4), Surhud More (1,5), Rachel Mandelbaum (6), Masahiro Takada (1) et al., 2018, submitted to Publications of the Astronomical Society of Japan [https://academic.oup.com/pasj, preprint: https://arxiv.org/abs/1809.09148]. Author affiliations: 1 Kavli Institute for the Physics and Mathematics of the universe (Kavli IPMU WPI), UT Institutes for Advanced Study, University of Tokyo, Kashiwa 277-8583, Japan; 2 Research Center for the Early Universe, University of Tokyo, Tokyo 113-0033, Japan; 3 Department of Physics, University of Tokyo 1130-0033, Japan; 4 National Astronomical Observatory of Japan, Mitaka, Tokyo 181-8588, Japan; 5 The Inter University Center for Astronomy and Astrophysics, Post Bag 4, Ganeshkhind, Pune, 411007, India; 6 McWilliams Center for Cosmology, Department of Physics, Carnegie Mellon University, Pittsburgh, PA 15213, USA