Subaru Telescope 2.0

Science Goal > To probe the nature of dark matter and dark energy, and determine the neutrino mass

Science Goal #1: To probe the nature of dark matter and dark energy, and determine the neutrino mass

dark matter and dark energy

The Universe is filled with dark matter that boosts structure formation, and dark energy, which causes the acceleration of the expanding Universe. However, their nature remains unknown. About 68% of the Universe’s average energy density is dark energy, and approximately a quarter (27%) is dark matter. In comparison, only 5% is made out of the ordinary matter we are familiar with, called baryons, which consist of atoms. Identifying the nature of dark matter and dark energy is the highest priority in physics and astronomy.

At present, survey observation of galaxies over extensive sky coverage is the only way to probe dark energy. The structure formation of the Universe is governed by dark matter’s gravity and the acceleration of cosmic expansion by dark energy. By thoroughly looking into structure formation in the Universe, Subaru Telescope 2.0 aims to determine the nature of dark matter and dark energy and stand at the forefront of refining the neutrino mass with the greatest reliability in the world.

image

(Left) 3D distribution of dark matter and galaxies revealed by the Subaru Telescope’s Hyper Suprime-Cam (HSC). This is the largest-scale map as of 2022. The dark matter distribution is estimated by the weak gravitational lensing technique combined with the redshift estimates of the background galaxies. (Credit: University of Tokyo / NAOJ)

(Right) Example of the galaxy distribution where clumps of dark matter are located. The image was taken in the direction of the Virgo constellation with HSC. The apparent shapes of galaxies in the image, which are distorted by the gravitational lens effect of the dark matter mass, give us information on how dark matter is distributed in the Universe. (Credit: NAOJ)

Observation Target

About 10 billion years ago to the present Universe

Subaru Telescope’s Objectives

(1) Measure the shape distortion of hundreds of millions of galaxies in images captured by HSC-SSP, covering an extensive area of about 1,200 square degrees. This measurement will enable us to create a 3D map of dark matter and galaxies covering the widest area yet. It will be eight times larger than the above map produced in 2018. At the same time, we will search for primordial black holes, a potential source of dark matter.

(2) By using Prime Focus Spectrograph (PFS), Subaru Telescope will conduct spectral observation of numerous galaxies captured in HSC's images, including distant galaxies, to be able precisely to measure redshifts, which correspond to distances of galaxies. The redshifts will allow us to determine cosmological parameters with unprecedented precision, which will pave the way to identify dark energy's nature. This is the key to understanding the Universe's expansion and will also lead to a high-reliability measurement of the mass of neutrinos, which are part of dark matter.

Dark matter and dark energy are known to be fundamental components of the standard model for structure formation in the Universe, but they are yet unexplained. Understanding their nature is the highest priority in physics and astronomy.

Ever since the discovery in 1998 of the Universe's accelerating expansion, it is believed that dark matter and dark energy make up about 95% of the mass of the existing Universe. The structure formation of the Universe by dark matter's gravity and the acceleration of cosmic expansion by dark energy are closely interlinked. The entire history of the Universe, where the Universe is expanding while density fluctuations develop and then galaxies and galaxy clusters are formed, should be impacted by the nature of dark matter and dark energy. Looking into the history of structure formation in the Universe through wide-field surveys will open up the possibility of revealing their nature. Currently, wide-field surveys of galaxies are the only way to look into dark energy.

Data from the survey covers an area of 160 square degrees by the Subaru Telescope’s HSC produced a 3D distribution of dark matter and galaxies (the Subaru Telescope’s observation result, March 1, 2018). The survey provided the deepest view and widest area of the Universe at that time, which led to the precision cosmological parameter that determines the formation of structure in the Universe. As a result, it is found that they may significantly contradict the values obtained from data for the early Universe (the Subaru Telescope’s observation result, September 26, 2018). Through the expansion of the survey area by eight times (1,200 square degrees), Subaru Telescope 2.0 will investigate the history of structure formation induced by dark matter, with an aim to verify this contradiction about the cosmological parameter that was obtained on a 2-sigma level (95 percent) by raising the reliability to more than a 3-sigma level (99.7 percent). This expanded survey will tell whether this contradiction is simply due to statistical uncertainty derived from the limited amount of data or any flaw in the standard model of the Universe based on General Relativity and the universal constant.

In addition, based on the data from the HSC’s wide-field surveys, PFS’s multi-object spectroscopy accurately measures distances to galaxies. We aim to measure the speed of structure formation from approximately 10 billion years ago to the present, with a few-percent error by substantially improving the precision of the 3D map of dark matter. As explained above, because structure formation in the Universe is determined by opposite effects of the balance between dark matter and dark energy, this will provide information on the nature of dark energy. Past measurements, which have contained dozens of percent errors, are far from being able to verify various proposed models of dark matter, but ultra-wide field surveys in combination with HSC and PFS will give us a whole new look.

Neutrinos are considered to play a major role in structure formation on a galaxy-cluster scale. The more the neutrinos weigh, the more likely structure formation is prevented. Conversely, if the neutrino weighs little, galaxy-cluster-scale structures can be formed more easily. Close investigation into the history of structure formation through PFS will enable us to measure the neutrino mass with an unprecedented level of accuracy. Because the current ground-based experiment cannot reach the absolute value of the neutrino mass, this survey will significantly impact the area of particle physics.

img

The cosmological constraints on the fractional contribution of matter to the energy budget of the Universe (the rest of it corresponds to dark energy), and the clumpiness of the matter distribution today (S8), as inferred from the analysis of the 3D dark matter map. The comparison among the results from the Subaru Telescope’s HSC observations (in red), U.S.-based measurement (in green), and Europe-based measurement (in gray) by using weak gravitational lensing effects on galaxies in the nearby Universe, and the results from the cosmic microwave background observations during the Universe’s infancy obtained by the Planck satellite (in blue). The thinner colors represent the cosmological parameter with the 1-sigma (68 percent) reliability, and the darker colors represent the parameter with 2-sigma (95 percent) reliability.

MORE