An article published in the journal “Physical Review Letters” reports the results of an analysis of dark matter distribution around 1.5 million primordial galaxies. A team of researchers led by Hironao Miyatake of the Japanese University of Nagoya used observations conducted with the Subaru telescope and analyzed data collected by the Planck Surveyor space probe to detect distortion of the cosmic microwave background (CMB).
The results of this study show fluctuations in dark matter distribution in the early universe that led to inhomogeneity in the aggregation of ordinary matter that formed galaxies. That aggregation is lower than predicted by the Lambda-CDM model, the one that currently best describes the observations. The uncertainty lies in the difficulty of obtaining precise results in examining very distant galaxies.
To obtain quality observations of primordial galaxies more than 12 billion light-years away from Earth, astronomers use gravitational lenses. The distortion generated by the force of gravity of a galaxy is one of the indirect ways of examining the dark matter within it. However, when the galaxy acting as a lens is very far away, it’s difficult to accurately assess the distortion it causes in the image of even more distant objects and this, in turn, makes it difficult to assess the amount of dark matter inside it.
To try to circumvent this difficulty in examining very distant galaxies, Hironao Miyatake’s team tried to use the data about the cosmic microwave background. ESA’s Planck Surveyor space probe measured what is considered the fossil radiation of the Big Bang allowing to obtain various increasingly precise maps up to the final one published by ESA in July 2018.
The Subaru Hyper Suprime-Cam (HSC) Subaru Strategic Program (SSP) Survey is a cosmic census that released the first data in March 2017. For this study, it identified about 1.5 million primordial galaxies acting as gravitational lenses. They are so far away that there are no galaxies even farther away that allow to directly assess their gravitational effects and therefore the amount of dark matter inside them. For this reason, the researchers used data collected by the Planck Surveyor space probe to examine the distortion generated by those galaxies in the cosmic microwave background.
The image (Courtesy Reiko Matsushita (Nagoya University), all rights reserved) shows an artistic representation of the combination of the observations obtained in the HSC-SSP survey and the cosmic microwave background data collected by the Planck Surveyor space probe.
According to the Lambda-CDM model, subtle fluctuations in the cosmic microwave background form clumps of high-density matter by attracting surrounding matter with their own force of gravity. This generates inhomogeneous clumps that form stars and entire galaxies in these dense regions.
The results obtained in this study suggest that measurements of the clumping tendency of matter are lower than predicted by the Lambda-CDM model. Further assessments will be needed to understand whether this is a limitation of the model or of the results of this study. Uncertainty calls for further studies that could provide new insights into the nature of dark matter. Other models could also be tested, including alternative ones that don’t predict the existence of dark matter.
Hironao Miyatake admitted the uncertainty of the discovery and the need for more precise measurements of the distribution of dark matter. The pandemic slowed the construction work of the Vera C. Rubin Observatory in Chile, where the cosmological Legacy Survey of Space and Time (LSST) is already planned, which could also provide valuable data in studies concerning dark matter. In short, it’s a type of study that will see new advances in the coming years to uncover some of the secrets of the cosmos.
