Observational Cosmology

Understanding the nature and origin of large-scale structure in the Universe is one of most compelling issues in observational cosmology. The currently most conventional scenario is given by the cold dark matter (CDM) dominated model, where gravitational instability mainly driven by spatial inhomogeneities of CDM distribution amplifies the seed density perturbations to form the present-day hierarchical structures. Therefore revealing distribution and amount of CDM is crucial to understanding the formation of large-scale structure. In addition the presence of dark energy drives the accelerating cosmic expansion, and therefore affects the growth of structure formation. The dark matter distribution and the nature of dark energy can be explored from massive galaxy surveys.

We have been actively working both on the measurements using currently available telescope facilities and on the planning of future instruments. The two powerful investigative tools are the gravitational lensing effect and the baryon acoustic oscillation.

Gravitational lensing effect:

The path of light ray emitted by a distant galaxy is bent by gravitational force of intervening large-scale structure during the propagation, causing the image to be distorted—the so-called weak lensing shear. Conversely, measuring the coherent shear signals between galaxy images allows us to reconstruct the distribution of invisible dark matter. Moreover, since the weak lensing shear deals with the light propagation on cosmological distance scales, the lensing strengths depend on the cosmic expansion history that is sensitive to the nature of dark energy. Thus weak lensing based observables offer a powerful way for studying the nature of invisible components, dark matter and dark energy. We are carrying out observational and theoretical studies of weak lensing phenomena using our own Subaru data sets as well as simulations of large-scale structure.

Baryon acoustic oscillation:

To measure properties of dark energy, one needs to measure the expansion history of the universe precisely. Because light travels at a finite speed, one can measure the expansion rate of the past by looking far. Comparing the expansion rate at varying distances would reveal the expansion history. The expansion itself is relatively easy to measure. The light emitted by a distant galaxy is stretched by the expansion of space and becomes redder, which can be measured by any decent spectrograph.

To measure the expansion history, however, we also need to know how far back in time the light was emitted from the galaxy, or equivalently, how far away it is. Measuring precise distances in cosmological scales is very challenging. Clustering of baryonic matter at a certain characteristic scale that is imprinted by baryon acoustic oscillation (BAO), or propagation of acoustic waves, in the early universe serves as a “standard ruler” for cosmological observations. This technique requires to study millions of galaxies in a wide field of view, and map the spatial distribution of luminous galaxies to detect the characteristic scale.

Hyper Suprime-Cam (HSC):

The HSC is the project to replace the prime focus camera of Subaru Telescope (8.2 meter optical-infrared telescope at the summit of 4,200m-Mauna Kea, Hawaii) with a new camera that has wider field-of-view than the previous one by a factor of 10. Fully utilizing the unique capabilities of HSC, its survey speed and excellent image quality, we conduct a massive galaxy survey that covers an area of a few thousands square degrees and reaches to the depth to probe the Universe up to redshifts of a few. In fact these data sets will provide us an ideal data sets for exploring the nature of dark matter and dark energy via measurements of cosmological observables available from the data, weak lensing and galaxy clustering statistics. We, Kavli IPMU members, are actively involved in this HSC project, and working on the designing and planning of HSC galaxy survey and development of data analysis pipeline.

Sloan Digital Sky Survey III:

In January 2011, the SDSS-III collaboration released the largest digital color image of the sky ever made. The image has been put together over the last decade from millions of 2.8-megapixel images taken at the 2.5-meter telescope at the Apache Point Observatory in New Mexico, thus creating a color image of more than a trillion pixels. This new SDSS-III data release, along with the previous SDSS-I and SDSS-II data releases that it builds upon, gives astronomers the most comprehensive view of the night sky ever made. SDSS data have already been used to discover nearly half a billion astronomical objects, including asteroids, stars, galaxies and distant quasars.

Kavli IPMU has been a part of the SDSS-III and involved in the study of these rich images. But our focus has been to conduct a new survey to a deeper universe with the improved spectrograph. This survey, the Baryon Oscillation Spectroscopic Survey (BOSS), maps the spatial distribution of luminous galaxies and quasars to detect the characteristic scale imprinted by baryon acoustic oscillations in the early universe. Using the acoustic scale as a standard ruler, we can infer the angular diameter distance to the galaxy redshift. The BOSS has started to take data in 2009 and will continue until 2014. Its goal is to precisely measure how Dark Energy has changed over the recent history of the Universe.

PrimeFocusSpectrograph (PFS):

The PFS project that mounts a next generation spectrograph on the Subaru Telescope and is planned to start data taking later this decade was overwelmingly endorsed at the Subaru Users meeting of January 2011. Using a wide angle view of Subaru Telescope and the PFS, we can study several thousand galaxies at the same time and use the baryon acoustic oscillation technique.

In addition to BAO, there are a number of other measurements to constrain the properties of dark energy using this instrument. Furthermore, this type of spectrograph with a large field of view and a massive multi-object capability will be unique among the largest telescopes in the world, allowing for unprecedented studies of formation and evolution of galaxies, as well as the assembly history of our own Milky Way Galaxy.

The strength of this project comes from exploiting the data using the HSC. The combination of imaging using HSC and spectroscopy using PFS is dubbed SuMIRe, Subaru Measurement of Images and Redshifts. The SuMIRe project is expected to repeat and exceed the tremendous success of Sloan Digital Sky Survey (SDSS), but with a much deeper view of the Universe back to the era that formed early stars and supermassive black holes.

Group Members