Probing the Origin of the Cosmic Acceleration with the Subaru/FMOS Cosmological Redshift Survey

We propose the first large-scale cosmological redshift survey of the z > 1 Universe using the newly commissioned FMOS wide-field spectrograph in order to test for predicted gravitational departures from General Relativity in the early Universe. Spectroscopic redshifts will be secured using the Hα emission line for 20,000 galaxies with 0.7 < z < 1.6 over 30 deg by the target selection method that we have already established in pre-existing fields from FMOS commissioning data. A precise measurement of the two dimensional clustering of galaxies both along and across the line-ofsight will allow us to measure the gravitational growth rate of large-scale structure at z > 1, when the universe is still decelerating, at an accuracy sufficient to discriminate the popular scenarios (dark energy or modified gravity) for the origin of cosmic acceleration. Such a discovery would offer a significant breakthrough shedding light on the most fundamental problem in modern physics. To carry out this critical test we ask for 120 SSP nights of Subaru time over 2 years. 1 Scientific Background 1.1 The Dark Energy Problem and Galaxy Redshift Surveys The most significant mystery concerning our current picture of the Universe is the accelerating nature of the cosmic expansion. This is well-described by the ‘ΛCDM’ model which supplements dark matter with a repulsive cosmological constant. However this has severe conceptual problems, in that while it is consistent with all current observations (see Frieman et al. 2008, ARA&A, 46, 385 for a recent review) it is an entirely empirical model without a fundamental physical basis. The natural interpretation of Λ as representing the zero-point energy of the vacuum fails by 122 orders of magnitude and an extreme finetuning is required for it to appear now in the long history of the universe. This has motivated numerous ideas for new physics of ‘dark energy’ or ‘modified gravity’ to explain the large scale cosmic dynamics of spacetime (e.g., Caldwell & Kamionkowski 2009, Ann. Rev. Nucl. Part. Sci. 59, 397 for theoretical reviews.) Large scale redshift surveys of galaxies are now widely recognized as one of the most powerful approaches to tackle this important and difficult problem. This is because they are able to simultaneously measure the effects of dark energy fields (which modify the expansion) and modifications of gravity (which also affect the growth of structures in the Universe). Low redshift surveys such as 2dFGRS and SDSS have delivered a wealth of cosmological results this way but the results remain consistent with ΛCDM and have, frustratingly, not provided new clues to the underlying physics. The new instrument, FMOS, at the Subaru Telescope, has the potential to make a significant breakthrough in our understanding of the accelerating Universe by undertaking the first systematic cosmological redshift survey at z > 1, probing the early epoch of the Universe for the first time, due to the