Hyper Suprime-Cam Survey (HSC)


	Cosmological constraints from the first-year Subaru Hyper Suprime-Cam survey

Using the powerful Japanese Subaru telescope, the Hyper Suprime-Cam survey (HSC) collaboration team has made and analyzed the deepest wide field map of the three-dimensional distribution of matter in the Universe. Led by Chiaki Hikage (Project Assistant Professor, Kavli IPMU) and a collaboration co-chaired by Surhud More (Associate Professor, IUCAA), a team of scientists primarily from Japan, Taiwan and Princeton University has used the gravitational distortion of images of about 10 million galaxies to make a precise measurement of the lumpiness of matter in the Universe. By combining this measurement with the European Space Agency Planck satellite’s observations of the cosmic microwave background and other cosmological experiments, the team has been able to further learn the properties of the “dark energy” that dominates the energy density of the Universe.

Although dark matter cannot be directly seen, its gravitational effects, predicted by Albert Einstein’s general theory of relativity, cause stretching and squeezing of the light from distant galaxies as they travel across the cosmos, to be detected by the Subaru telescope. They are witness to the growth of cosmic structure (Figure-1, left) and can be used to unlock the mysteries of dark energy. The simplest model for dark energy was introduced by Einstein, termed the “Cosmological Constant”. This model can explain all existing observations, including those of HSC.

The gravitational lensing effect from the distribution of dark matter in the universe is quite weak, but results in small but measurable distortions in the images of the galaxies in the HSC images. Like a pointillist painting (Figure-1, right), the distorted images of millions of galaxies located at a range of distances paint a three-dimensional picture of the distribution of matter in the Universe (Figure-1, right and Movie-1). The HSC research team has precisely characterized the fluctuations or lumpiness of the distribution of dark matter, and the change in that lumpiness over billions of years — from its adolescence to adulthood. This lumpiness is a key parameter that describes how structure in the universe grew from its initial smooth beginnings after the Big Bang to the galaxies, stars and planets we see today.

Figure 1: Left panel: The 3-dimensional dark matter map of the Universe inferred from one of the six HSC observation areas is shown in the background with various shades of blue (brighter areas have more dark matter). The map was inferred from the distortions of shapes of galaxies in the HSC data which are indicated by white sticks. The stick lengths represent the amount of distortion and the angle of the stick corresponds to the direction of the distortion. Right panel: The measurements are enabled by the light from distant galaxies that travels through the Universe and gets deflected by matter at different epochs in the Universe, before reaching the Subaru telescope.

With the high-precision HSC data, the team measured the lumpiness with a precision of 3.6% (Figure-2), which is similar to the precision with which it has been measured by other lensing surveys. These other surveys, including the Dark Energy Survey carried out on the Victor Blanco Telescope in Chile, surveyed brighter and thus nearer-by galaxies than did the HSC; the consistency of results at different distances and thus cosmic epochs gives confidence in the robustness of the results.

Figure 2: The cosmological constraints on the clumpiness of the Universe today (S8) predicted using observations at different times in the Universe. The HSC measurement of the clumpiness of the Universe is shown with the red symbol and are among the farthest measurements using weak gravitational lensing. These should be compared with the Planck results obtained from observations of the cosmic microwave background in the very early Universe and other contemporary weak lensing experiments KiDS and DES.

When compared to the fluctuations expected from those seen in the Universe’s infancy by the Planck satellite, the HSC measurements offer a consistent picture of the cosmological model (Figure-3). The Universe today is dominated by dark matter and dark energy, and that dark energy behaves like Einstein’s cosmological constant (Figure-4).

Figure 3: The cosmological constraints on the fractional contribution of matter to the energy budget of the Universe (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 results of the clumpiness of the matter distribution from HSC observations of the distant Universe using weak gravitational lensing are consistent with results from other similar observations (Dark Energy Survey and the Kilo Degree Survey) of slightly nearby Universe. The results from the cosmic microwave background observations during the Universe’s infancy obtained by the Planck satellite are shown in blue.

Figure 4: Cosmological constraints on the dark energy equation of state (blue contours alone from HSC), red contours corresponds to constraints after combining with cosmological results from the Planck CMB satellite and other contemporary cosmological measurements using Supernovae and Baryon acoustic oscillations.

However, taken together the results from weak lensing surveys prefer a slightly smaller value of fluctuations than that predicted by the Planck satellite (Figure-5). This could just be a statistical fluctuation due to the limited amount of data available, or might be a signature of the breakdown of the standard model of the Universe, based on General Relativity and the cosmological constant.

Figure 5: The weak lensing surveys such as HSC prefer a slightly less clumpy Universe than that predicted by Planck. The pictures show the slight but noticeable difference as expected from large computer simulations. Is this difference a statistical fluctuation? Astronomers all around the world continue to collect more and more data to answer this question.

This is just the beginning, the HSC continues taking more data and will continue mapping roughly 10 times larger area than used in this work. IUCAA is also involved with the Large Synoptic Survey Telescope which will survey a 100 times more galaxies in a 100 times bigger area to reveal the hidden dark matter and use it to learn the secrets of dark energy.

The research paper has now been submitted to the journal Publications of the Astronomical Society of Japan and will undergo rigorous peer review by the scientific community.

Preprint manuscript of this paper

Paper title: Cosmology from cosmic shear power spectra with Subaru Hyper Suprime-Cam first-year data
Authors: Chiaki Hikage⁽¹⁾, Masamune Oguri^(2,3,1), Takashi Hamana⁽⁴⁾, Surhud More^(5,1), Rachel Mandelbaum⁽⁶⁾, Masahiro Takada⁽¹⁾ et al.
Author affiliations:

Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU WPI), UT Institutes for Advanced Study, University of Tokyo, Kashiwa 277-8583, Japan
Research Center for the Early Universe, University of Tokyo, Tokyo 113-0033, Japan
Department of Physics, University of Tokyo 1130-0033, Japan
National Astronomical Observatory of Japan, Mitaka, Tokyo 181-8588, Japan
The Inter-University Center for Astronomy and Astrophysics, Post Bag 4, Ganeshkhind, Pune 411007, India
McWilliams Center for Cosmology, Department of Physics, Carnegie Mellon University, Pittsburgh, PA 15213, USA

This paper manuscript will be uploaded to the preprint server (https://arxiv.org/abs/1809.09148 ), and will be submitted to the Publication of Astronomical Society of Japan (PASJ).

Research contacts:

	Chiaki Hikage Project Assistant Professor, Kavli IPMU, The University of Tokyo Tel.: +81-04-7136-6532 E-mail: chiaki.hikage_at_ipmu.jp * please change _at_ to @
	Surhud More Associate Professor, The Inter University Center for Astronomy and Astrophysics, Tel.: +91-20-25604-120 E-mail: surhud_at_iucaa.in * please change _at_ to @

Glossary

Dark Matter:

More than 80% of matter in the Universe is believed to be dark matter, a mysterious form of matter not made of atoms. It is believed that dark matter is our mother, namely that it was responsible for building structures of the Universe such as stars, galaxies, and clusters of galaxies. Weak lensing measurements such as those presented here reveal the distribution of dark matter and hence how much structure was indeed built.

Weak lensing:

Gravitational lensing is an effect caused by dark matter that bends light due to its gravitational pull. Galaxies and clusters of galaxies therefore act like a magnifying lens, making the images of far-away galaxies stretched and bent, an effect called shear. Over the vast majority of the area of the HSC survey, the lensing effect is weak, meaning that the shear effect represents only a one percent distortion in the shape of galaxies.

Dark Energy:

Gravity only pulls, never pushes. After the Big Bang dispersed everything apart from each other, gravitational pull is expected to put a brake on the motion of galaxies and slow down the expansion of the Universe. Yet it was discovered in 1998 that the Universe is expanding at faster and faster rate. We term the mysterious force responsible for “pushing” the expansion dark energy. If dark energy picks up momentum, the expansion rate becomes eventually infinitely fast, making the Universe “end” in a Big Rip. It is therefore believed to hold the key to the fate of the Universe.

Cosmic Structure:

The COBE, WMAP, and Planck satellites have looked to the earliest light of the universe, the Cosmic Microwave Background. This light comes from a time when the Universe was only about 130,000 years old and as hot as 4000 degrees Celsius. They show that the Universe was very bland and boring, with only tiny variations in densities of dark matter at the level of one part in a hundred thousand. The gravitational pull of dark matter is believed to pull more dark matter into the slightly dense area, making the density higher, then the gravity stronger, pulling more dark matter, and so on. This is how the structures of the Universe such as stars, galaxies, and clusters of galaxies are built. On the other hand, dark energy speeds up the expansion, effectively pulling apart the structure to counteract dark matter. The growth of cosmic structures therefore depends on a competition between dark matter and dark energy. Precision measurements of cosmic structure reveal this dance and how much exactly dark matter built the structure and dark energy is stripping it away.

Blind analysis:

Movie

Movie 1: Movie of the 3D dark matter map as observed by the Hyper Suprime-Cam Survey can be used to measure the lumpiness of the dark matter distribution. The measurement sheds light on dark matter and dark energy - the mysterious components that dominate the Universe.