Investigating Dark Matter with Hyper Suprime-Cam Reveals Discrepancy

A group of astrophysicists and cosmologists from around the world has spent the past year studying dark matter using the Hyper Suprime-Cam (HSC), an exceptionally powerful astronomical camera. By analyzing data from the first three years of the HSC sky survey, led by Princeton University, they have discovered a discrepancy in the distribution of dark matter, specifically in its “clumpiness” (referred to as the S8 value), between the present universe and the early universe. This finding could indicate either an unidentified error in measurement or an incomplete standard cosmological model.

While the challenge of observing something invisible might seem like a classical paradox, modern astronomers tackle this issue by studying the effects of dark matter on observable phenomena. In the case of dark matter, scientists observe how light from distant galaxies bends around it.

Over the past year, an international team of astrophysicists and cosmologists has dedicated their efforts to unraveling the mysteries of this elusive substance. They employed advanced computer simulations and the data obtained from the Hyper Suprime-Cam (HSC), one of the most powerful astronomical cameras worldwide. This team, comprising researchers from Princeton University, as well as the astronomical communities of Japan and Taiwan, utilized data from the first three years of the HSC sky survey. This survey involved wide-field imaging conducted with the 8.2-meter Subaru telescope situated atop Maunakea in Hawai’i. The National Astronomical Observatory of Japan operates the Subaru telescope, named after the Pleiades star cluster.

The team is currently preparing a set of five papers to document their findings.

Their main objective is to measure fundamental properties of our universe, as stated by Roohi Dalal, a graduate student in astrophysics at Princeton and the first author of one of the papers. Dalal highlights that although dark energy and dark matter constitute 95% of our universe, our understanding of their true nature and how they have evolved remains limited. The team accomplishes their investigation by examining how clumps of dark matter distort the light emitted by distant galaxies through weak gravitational lensing, a phenomenon predicted by Einstein’s General Theory of Relativity. Despite being an extremely subtle effect, where the shape of a single galaxy undergoes an imperceptible distortion, the team achieves highly precise measurements by observing this phenomenon in 25 million galaxies.

In their research, the team has measured a value of 0.776 for the clumpiness of dark matter (S8 parameter) in the current universe. This value aligns with those obtained by other gravitational lensing surveys conducted on the relatively recent universe. However, it deviates from the value of 0.83 derived from the Cosmic Microwave Background, which dates back to the origins of the universe.

Although the discrepancy between these two values is small, its consistency across multiple studies suggests that it is not accidental. Other possibilities include the presence of an unrecognized error or flaw in one of the measurements or an interesting incompleteness in the standard cosmological model.

Michael Strauss, chair of Princeton’s Department of Astrophysical Sciences and one of the leaders of the HSC team, emphasizes their cautious approach. They do not claim to have disproven modern cosmology. However, as more experiments confirm the same conclusion, it is possible that their findings will provide evidence for a new understanding of the universe.

To avoid biases, the team conducted a blinded analysis, effectively hiding their results from themselves and their colleagues for months. They even introduced an additional layer of obfuscation by running their analyses on three different galaxy catalogs, one real and two with randomly offset numerical values. This way, even accidental exposure to the values would not indicate which catalog was genuine.

The team finally revealed their findings during a Zoom conference, experiencing a collective sense of relief and celebration. The Hyper Suprime-Cam (HSC) is currently the largest camera on a telescope of its size globally, and it will maintain this distinction until the Vera C. Rubin Observatory’s Legacy Survey of Space and Time (LSST) commences in late 2024. The LSST will utilize the same software pipelines designed for processing HSC’s raw data. The HSC survey used by the research team covers approximately 420 square degrees of the sky, equivalent to about 2000 full moons. It comprises six distinct areas, each about the size of an outstretched fist. The galaxies surveyed are so distant that the observations capture their state billions of years ago.

The results obtained from the HSC collaboration provide an exciting glimpse into what can be expected from the future Rubin Observatory. Alexandra Amon, a cosmologist from Cambridge University, describes the deep survey conducted by HSC as producing remarkable data. The consistency between HSC and other independent weak lensing surveys, all indicating a low value for S8, serves as essential validation. These tensions and trends provoke a pause for reflection on what the data reveals about our universe.

The standard model of cosmology is surprisingly simple, consisting of ordinary matter, dark matter, dark energy, and photons as the fundamental constituents. According to this model, the universe has expanded since the Big Bang 13.8 billion years ago, leading to the formation of structures such as galaxies enveloped in clumps of dark matter. The current composition of the universe comprises approximately 5% ordinary matter, 25% dark matter, 70% dark energy, and a minor contribution from photons.

This model relies on a small set of parameters, including the expansion rate of the universe, the clumpiness of dark matter (S8), and the relative contributions of the constituents. The standard model has been widely accepted since the early 2000s.

Scientists aim to test this model by refining and constraining these parameters through various approaches, such as studying fluctuations in the Cosmic Microwave Background, analyzing the universe’s expansion history, and measuring the clumpiness of the universe in different epochs. The team’s research supports the growing belief in the scientific community that a genuine discrepancy exists between measurements of clumping in the early universe (derived from the Cosmic Microwave Background) and those obtained from galaxies about 9 billion years ago.

The five papers being submitted jointly to Physical Review D provide a comprehensive analysis of the research conducted. They cover Fourier-space and real-space analyses, as well as a 3×2-point analysis that combines weak lensing measurements with galaxy clustering. The team coordinated their efforts and conducted the analyses using blinded data to ensure accuracy and minimize potential methodological errors. The fact that the results of the different analyses align reinforces their confidence in the validity of their findings.

This research has been made possible through the support of various 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, and Princeton University, among others.

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More about dark matter discrepancy

• Hyper Suprime-Cam (HSC)
• Weak Gravitational Lensing
• Cosmic Microwave Background (CMB)
• Standard Cosmological Model