The Paradoxical Dark Matter Distribution: A Curveball from the Universe

Figure 1: An example of an image obtained with HSC-SSP. Credit: HSC-SSP project & NAOJ

Astrophysicists have discovered that the “clumpiness” of dark matter in the Universe is 0.76, which conflicts with the Cosmic Microwave Background (CMB) value of 0.83. This discrepancy indicates potential errors or an incomplete cosmological model. The research used data from the Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP) and will further investigate this compelling discrepancy.

An international team of astrophysicists and cosmologists from various institutes, including NAOJ and the Kavli Institute for the Physics and Mathematics of the Universe, have submitted a set of five papers that measure the value for the “clumpiness” of dark matter, known as S8 to cosmologists. The reported value is 0.76, which aligns with other gravitational lensing surveys that looked at the relatively recent Universe. However, it does not align with the value of 0.83 derived from the Cosmic Microwave Background, which dates back to when the Universe was about 380,000 years old. The gap between these two values is small, but it doesn’t appear to be accidental. The possibilities are that there’s some as-yet unrecognized error or mistake in one of these two measurements or the standard cosmological model is incomplete in some interesting way.

The standard model of the Universe is defined by only a handful of numbers: the expansion rate of the Universe, a measure of how clumpy the dark matter is (S8), the relative contributions of the constituents of the Universe (matter, dark matter, and dark energy), the overall density of the Universe, and a technical quantity describing how the clumpiness of the Universe on large scales relates to that on small scales. Cosmologists are eager to test this model by constraining these numbers in various ways, such as by observing the fluctuations in the Cosmic Microwave Background, modeling the expansion history of the Universe or measuring the clumpiness of the Universe in the relatively recent past.

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

A team of astronomers led by Kavli IPMU, the University of Tokyo, Nagoya University, Princeton University, and the astronomical communities of Japan and Taiwan, spent the past year teasing out the secrets of dark matter, using sophisticated computer simulations and data from the first three years of the Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP). The observation program used Hyper Suprime-Cam (HSC), one of the most powerful astronomical cameras in the world, mounted on the Subaru Telescope. The research team used HSC-SSP data, covering about 420 square degrees of the sky, equivalent to 2000 full moons.

Clumps of dark matter distort the light of distant galaxies through weak gravitational lensing, a phenomenon predicted by Einstein’s General Theory of Relativity. This distortion is a really small effect; the shape of a single galaxy is distorted by an imperceptible amount. But the team measured the distortion with quite high precision by combining the measurements for 25 million faint galaxies that are billions of light-years away. Then, the team measured the clumpiness of the Universe today (Figure 3).

Figure3: The measurement results of the S8 parameter obtained 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 discrepancy between the HSC-SSP S8 values and the Planck satellite is very subtle. The team believes that the measurement was done correctly and carefully, and the statistics show that there’s only a one in 20 probability that the difference is just due to chance. The team will further pursue this compelling inconsistency using the full HSC-SSP dataset and refined methods. The team might discover something new about the Universe, so stay tuned.

“Hyper Suprime-Cam Year 3 Results: Cosmology from Galaxy Clustering and Weak Lensing with HSC and SDSS using the Emulator Based Halo Model” by Hironao Miyatake, Sunao Sugiyama, Masahiro Takada, Takahiro Nishimichi, Xiangchong Li, Masato Shirasaki, Surhud More, Yosuke Kobayashi, Atsushi J. Nishizawa, Markus M. Rau, Tianqing Zhang, Ryuichi Takahashi, Roohi Dalal, Rachel Mandelbaum, Michael A. Strauss, Takashi Hamana, Masamune Oguri, Ken Osato, Wentao Luo, Arun Kannawadi, Bau-Ching Hsieh, Robert Armstrong, Yutaka Komiyama, Robert H. Lupton, Nate B. Lust, Lauren A. MacArthur, Satoshi Miyazaki, Hitoshi Murayama, Yuki Okura, Paul A. Price, Tomomi Sunayama, Philip J. Tait, Masayuki Tanaka and Shiang-Yu Wang, 3 April 2023, Astrophysics > Cosmology and Nongalactic Astrophysics.arXiv:2304.00704

“Hyper Suprime-Cam Year 3 Results: Measurements of Clustering of SDSS-BOSS Galaxies, Galaxy-Galaxy Lensing and Cosmic Shear” by Surhud More, Sunao Sugiyama, Hironao Miyatake, Markus Michael Rau, Masato Shirasaki, Xiangchong Li, Atsushi J. Nishizawa, Ken Osato, Tianqing Zhang, Masahiro Takada, Takashi Hamana, Ryuichi Takahashi, Roohi Dalal, Rachel Mandelbaum, Michael A. Strauss, Yosuke Kobayashi, Takahiro Nishimichi, Masamune Oguri, Arun Kannawadi, Robert Armstrong, Yutaka Komiyama, Robert H. Lupton, Nate B. Lust, Satoshi Miyazaki, Hitoshi Murayama, Yuki Okura, Paul A. Price, Philip J. Tait, Masayuki Tanaka and Shiang-Yu Wang, 3 April 2023, Astrophysics > Cosmology and Nongalactic Astrophysics.arXiv:2304.00703

“Hyper Suprime-Cam Year 3 Results: Cosmology from Galaxy Clustering and Weak Lensing with HSC and SDSS using the Minimal Bias Model” by Sunao Sugiyama, Hironao Miyatake, Surhud More, Xiangchong Li, Masato Shirasaki, Masahiro Takada, Yosuke Kobayashi, Ryuichi Takahashi, Takahiro Nishimichi, Atsushi J. Nishizawa, Markus M. Rau, Tianqing Zhang, Roohi Dalal, Rachel Mandelbaum, Michael A. Strauss, Takashi Hamana, Masamune Oguri, Ken Osato, Arun Kannawadi, Robert Armstrong, Yutaka Komiyama, Robert H. Lupton, Nate B. Lust, Satoshi Miyazaki, Hitoshi Murayama, Yuki Okura, Paul A. Price, Philip J. Tait, Masayuki Tanaka and Shiang-Yu Wang, 3 April 2023, Astrophysics > Cosmology and Nongalactic Astrophysics.arXiv:2304.00705

“Hyper Suprime-Cam Year 3 Results: Cosmology from Cosmic Shear Power Spectra” by Roohi Dalal, Xiangchong Li, Andrina Nicola, Joe Zuntz, Michael A. Strauss, Sunao Sugiyama, Tianqing Zhang, Markus M. Rau, Rachel Mandelbaum, Masahiro Takada, Surhud More, Hironao Miyatake, Arun Kannawadi, Masato Shirasaki, Takanori Taniguchi, Ryuichi Takahashi, Ken Osato, Takashi Hamana, Masamune Oguri, Atsushi J. Nishizawa, Andrés A. Plazas Malagón, Tomomi Sunayama, David Alonso, Anže Slosar, Robert Armstrong, James Bosch, Yutaka Komiyama, Robert H. Lupton, Nate B. Lust, Lauren A. MacArthur, Satoshi Miyazaki, Hitoshi Murayama, Takahiro Nishimichi, Yuki Okura, Paul A. Price, Philip J. Tait, Masayuki Tanaka and Shiang-Yu Wang, 3 April 2023, Astrophysics > Cosmology and Nongalactic Astrophysics.arXiv:2304.00701

“Hyper Suprime-Cam Year 3 Results: Cosmology from Cosmic Shear Two-point Correlation Functions” by Xiangchong Li, Tianqing Zhang, Sunao Sugiyama, Roohi Dalal, Markus M. Rau, Rachel Mandelbaum, Masahiro Takada, Surhud More, Michael A. Strauss, Hironao Miyatake, Masato Shirasaki, Takashi Hamana, Masamune Oguri, Wentao Luo, Atsushi J. Nishizawa, Ryuichi Takahashi, Andrina Nicola, Ken Osato, Arun Kannawadi, Tomomi Sunayama, Robert Armstrong, Yutaka Komiyama, Robert H. Lupton, Nate B. Lust, Satoshi Miyazaki, Hitoshi Murayama, Takahiro Nishimichi, Yuki Okura, Paul A. Price, Philip J. Tait, Masayuki Tanaka, Shiang-Yu Wang, 3 April 2023, Astrophysics > Cosmology and Nongalactic Astrophysics.arXiv:2304.00702