An Unexpected Twist in the Cosmos: The Dark Matter Distribution Dilemma

Figure 1: Sample image obtained through HSC-SSP. Credit: HSC-SSP project & NAOJ

The “clumpiness” or concentration of the Universe’s dark matter, determined to be 0.76 by astrophysicists, contradicts the value of 0.83 derived from the Cosmic Microwave Background, implying potential miscalculations or an insufficient cosmological model. The Hyper Suprime-Cam Subaru Strategic Program’s data was employed in the study, which will continue to probe this intriguing discrepancy.

Numerous researchers from international institutes, including the NAOJ and the Kavli Institute for the Physics and Mathematics of the Universe, have submitted a collection of five papers. These papers detail the measurement of the Universe’s dark matter “clumpiness” – a factor known as S8 in cosmology. The reported value of 0.76 agrees with previous gravitational lensing surveys, but not with the Cosmic Microwave Background value of 0.83, derived from the Universe’s early history – around 380,000 years old. The slight difference in these values seems non-random, suggesting potential errors in measurements or deficiencies in the standard cosmological model.

The conventional model of the Universe is characterized by a few parameters: the Universe’s expansion rate, the dark matter’s clumpiness (S8), the relative contributions of the Universe’s constituents (matter, dark matter, and dark energy), the total density of the Universe, and a technical quantity correlating the Universe’s clumpiness on large and small scales. To test this model, cosmologists aim to refine these parameters using various methods, like studying fluctuations in the Cosmic Microwave Background, modeling the Universe’s expansion history, or measuring recent clumpiness of the Universe.

Figure 2: An example of a 3D distribution of dark matter derived from HSC-SSP. This map is obtained by using the first year’s data, but the present study examined an area on the sky about three times larger than that. Credit: University of Tokyo/NAOJ

Scientists from the Kavli IPMU, the University of Tokyo, Nagoya University, Princeton University, and astronomical societies of Japan and Taiwan, have spent the past year unlocking the secrets of the elusive dark matter. Using advanced computer simulations and three years’ worth of data from the Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP), the team made their observations with one of the most potent astronomical cameras globally, the Hyper Suprime-Cam (HSC), mounted on the Subaru Telescope. The utilized HSC-SSP data covers approximately 420 square degrees of the sky, nearly equivalent to 2000 full moons.

Dark matter clumps bend the light of far-off galaxies via weak gravitational lensing, as predicted by Einstein’s General Theory of Relativity. This distortion is minuscule and unnoticeable in a single galaxy but is precisely measurable when the data from 25 million faint galaxies billions of light-years away are combined. Consequently, the team was able to determine the clumpiness of the Universe in its current state (Figure 3).

Figure3: The measurement results of S8 parameter from HSC-SSP Year 3 data. The chart shows the results from four different methods, which used different parts of the HSC-SSP Year 3 data or combined the HSC-SSP Year 3 data with other data. For comparison, “Planck CMB” shows the measurement result for S8 from the cosmic microwave background data from the Planck satellite. “Other weak lensing results” shows the results from similar weak lensing measurements based on the Dark Energy Survey (DES) and Kilo-Degree Survey (KiDS) data. Credit: Kavli IPMU

The minute discrepancy between the S8 values from the HSC-SSP and the Planck satellite data raises questions. The research team is confident that the measurement was conducted thoroughly and accurately. The statistical analysis indicates a mere 5% probability that the difference is a fluke. While this is compelling, it isn’t definitive, prompting the team to further explore this inconsistency using the entire HSC-SSP dataset and improved methods. These further investigations could potentially lead to new discoveries about our Universe, so stay tuned for more.

For additional information on this research, refer to ‘Measuring Dark Matter With Hyper Suprime-Cam Reveals Discrepancy.’

References:

(Relevant references)

This research was made possible by the support of numerous organizations, including the National Science Foundation Graduate Research Fellowship Program, the National Astronomical Observatory of Japan, the Kavli Institute for the Physics and Mathematics of the Universe, the University of Tokyo, the High Energy Accelerator Research Organization (KEK), the Academia Sinica Institute for Astronomy and Astrophysics in Taiwan, Princeton University, the Japanese Cabinet Office’s FIRST program, the Ministry of Education, Culture, Sports, Science and Technology (MEXT), the Japan Society for the Promotion of Science, the Japan Science and Technology Agency, the Toray Science Foundation, and the Vera C. Rubin Observatory.

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