The present-day cosmic large-scale structure, seen from the distribution of galaxies, is believed to originate from the primordial seed fluctuations generated at the beginning of the universe, Big Bang. The tight coupling between photons and baryon in the hot thermal plasma, which is the remnant left over from the Big Bang, imprints a characteristic length scale onto the distribution of galaxies – the so-called baryon acoustic oscillation (BAO) scale. The BAO scale is now very precisely constrained by the cosmic microwave background (CMB) experiments such as the satellite WMAP experiment, telling us that the BAO scale should be about 500 million light years (about 150Mpc) today. By using the BAO scale inferred from the measured galaxy distribution as a standard ruler, we are able to determine the cosmological distances to redshift of the galaxies, which are determined by the cosmic expansion history in Einstein gravity theory. Thus the BAO experiment gives us a geometrical probe of the universe with which we can explore the nature of the cosmic acceleration – the most tantalizing mystery in particle physics and cosmology.
We will use the Prime Focus Spectrograph (PFS) on the 8.2m Subaru Telescope in order to map the spatial distribution of galaxies for more than a few millions of galaxies. The PFS/Subaru enables us to carry out a very efficient spectroscopic survey of galaxies over a wide-area coverage of the sky and up to the unique redshift range 0.8<z<2.4, thanks to its unique capabilities, high multiplicity (a measurement of more than 2000 galaxies’ redshifts at once), wide field-of-view, photon collecting power and wide wavelength coverage (from optical to near-infrared). We can then use the measured galaxy distribution to carry out the high-precision BAO measurements. When combined with the existing and current other BAO experiments, we can explore the cosmic expansion history over a long period of the cosmic expansion history, from today to 11 billion years ago in the cosmic age of 13.7 billion years, which can well cover the transition regime from the accelerating cosmic expansion phase to the decelerating expansion, the transition of dark energy dominance in the cosmic energy budget.
Further, we plan to have a full overlap of the PFS survey region on the sky with that of the imaging survey of galaxies carried out using the Hyper SuprimeCam, which is another next-generation prime focus instrument of Subaru Telescope and will come online before the PFS. The HSC project is also another important component of the SuMIRe project. The Hyper SurpimeCam imaging survey enables us to reconstruct the distribution of invisible dark matter in the universe via measurements of gravitational lensing distortion effects on images of galaxies. By combining the PFS and HSC data sets, we can know how the PFS galaxies are distributed in the dark matter distribution. Thus, with such a unique combination of the PFS and HSC surveys, we can very robustly address the nature of the mysterious cosmic acceleration, the nature of dark energy or perhaps whether or not the cosmic acceleration is due to the breakdown of Einstein gravity on cosmological scales.