Discrepancy in Dark Matter: A Puzzle in the Standard Cosmological Model

A group of cosmologists and astrophysicists from around the world has presented five papers proposing a value of 0.76 for the “clumpiness” (S8 value) of dark matter in the universe. This finding contradicts the value of 0.83 derived from the analysis of the Cosmic Microwave Background, although it aligns with other surveys based on gravitational lensing.

Utilizing advanced techniques and the powerful Hyper Suprime-Cam, an international team of scientists has investigated the clumpiness of dark matter and determined an S8 value of 0.76. However, this result deviates from the value of 0.83 obtained from the study of the Cosmic Microwave Background. This inconsistency could indicate measurement inaccuracies or reveal an incomplete understanding of the standard cosmological model.

The team, comprised of cosmologists and astrophysicists affiliated with prestigious institutions such as the Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU), has submitted a series of five papers. These papers discuss the measurement of the universe’s dark matter clumpiness, referred to as S8, which corresponds with values reported by other surveys based on gravitational lensing conducted in the relatively recent past. However, it does not correspond to the value derived from the analysis of the Cosmic Microwave Background, which provides insights into the universe’s early stages, approximately 380,000 years after its origin. The team has uploaded their results as pre-print papers on arXiv.

Although the difference between these two values is small, the increasing number of studies confirming each value suggests that it is not a mere coincidence. Possible explanations for this discrepancy include measurement errors in one of the two studies or the presence of intriguing gaps in the standard cosmological model.

Dark energy and dark matter constitute a significant portion, approximately 95%, of our observable universe. However, our understanding of their nature and their evolution throughout cosmic history remains limited. Clusters of dark matter cause subtle distortions in the light emitted by distant galaxies, a phenomenon predicted by Einstein’s General Theory of Relativity.

Professor Masahiro Takada from Kavli IPMU explains that this distortion is an extremely minute effect, as the shape of an individual galaxy is distorted by an imperceptible amount. Nevertheless, by combining measurements from millions of galaxies, scientists can precisely quantify this distortion.

The standard cosmological model relies on a few key parameters, including the universe’s expansion rate, the clumpiness of dark matter (S8), the contributions of different components (matter, dark matter, and dark energy), the overall density of the universe, and a technical quantity that relates the clumpiness on large and small scales.

Cosmologists are eager to scrutinize this model by constraining and refining these parameters through various approaches, such as studying fluctuations in the Cosmic Microwave Background, modeling the universe’s expansion history, and measuring the clumpiness of the universe in the more recent past.

Over the past year, a team led by astronomers from Kavli IPMU, the University of Tokyo, Nagoya University, Princeton University, and various astronomical communities in Japan and Taiwan have meticulously investigated the elusive substance known as dark matter. They employed sophisticated computer simulations and utilized data from the initial three years of the Hyper Suprime-Cam survey. The survey conducted observations using one of the world’s most potent astronomical cameras, the Hyper Suprime-Cam (HSC), mounted on the Subaru Telescope situated atop Maunakea in Hawaii.

The HSC serves as the largest camera on a telescope of its size globally. The research team’s survey covered approximately 420 square degrees of the sky, equivalent to around 2000 full moons. It consisted of six distinct portions, each about the size of a person’s outstretched fist. The surveyed galaxies, totaling 25 million, are so far away that the HSC captured their state billions of years in the past.

Despite each of these galaxies radiating the luminosity of billions of suns, their extreme distance renders them exceedingly faint, up to 25 million times fainter than the faintest stars visible to the naked eye.

For further details on this research, please refer to the article “Measuring Dark Matter With Hyper Suprime-Cam Reveals Discrepancy.”

References:

1. Hironao Miyatake, et al. “Hyper Suprime-Cam Year 3 Results: Cosmology from Galaxy Clustering and Weak Lensing with HSC and SDSS using the Emulator Based Halo Model.” arXiv:2304.00704.

2. Surhud More, et al. “Hyper Suprime-Cam Year 3 Results: Measurements of Clustering of SDSS-BOSS Galaxies, Galaxy-Galaxy Lensing and Cosmic Shear.” arXiv:2304.00703.

3. Sunao Sugiyama, et al. “Hyper Suprime-Cam Year 3 Results: Cosmology from Galaxy Clustering and Weak Lensing with HSC and SDSS using the Minimal Bias Model.” arXiv:2304.00705.

4. Roohi Dalal, et al. “Hyper Suprime-Cam Year 3 Results: Cosmology from Cosmic Shear Power Spectra.” arXiv:2304.00701.

5. Xiangchong Li, et al. “Hyper Suprime-Cam Year 3 Results: Cosmology from Cosmic Shear Two-point Correlation Functions.” arXiv:2304.00702.

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More about Dark Matter Discrepancy

• Measuring Dark Matter With Hyper Suprime-Cam Reveals Discrepancy
• Hyper Suprime-Cam Year 3 Results: Cosmology from Galaxy Clustering and Weak Lensing with HSC and SDSS using the Emulator Based Halo Model
• Hyper Suprime-Cam Year 3 Results: Measurements of Clustering of SDSS-BOSS Galaxies, Galaxy-Galaxy Lensing and Cosmic Shear
• Hyper Suprime-Cam Year 3 Results: Cosmology from Galaxy Clustering and Weak Lensing with HSC and SDSS using the Minimal Bias Model
• Hyper Suprime-Cam Year 3 Results: Cosmology from Cosmic Shear Power Spectra
• Hyper Suprime-Cam Year 3 Results: Cosmology from Cosmic Shear Two-point Correlation Functions