Scientific Background

The Bigbang theory of cosmology is based on General Relativity, which is the standard theory of gravity established by A. Einstein. It is surprizingly successful to quantitatively explain a lot of observations, and is now widely accepted as the basic framework of scientific cosmology. However, there are still several problems that must be answered, and a variety of scientific efforts are being made by scientists all over the world. Among them, 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 Lambda-CDM model, which is based on General Relativity but includes an additional cosmological constant that works as a repulsive force in the Einstein equation.

However this model has severe conceptual problems, in that it is an entirely empirical model without a fundamental physical basis. The natural interpretation of the cosmological constant as representing the zero-point energy of the vacuum fails by 122 orders of magnitude and an extreme fine-tuning 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.

Therefore, testing the gravity theory on the cosmological scale should provide important clues for the dark energy problem. A wide-area spectroscopic galaxy survey give such a test by measuring redshift (shift of galaxy spectra toward longer (redder) wavelength by cosmic expansion) of many galaxies. The redshifts are related to the distances to the galaxies by the uniform expansion of the universe, but a 3-D galaxy map made by redshift is slightly distorted because redshifts are also affected by peculiar motions of galaxies with respect to the rest-frame of the universe. If you measure this redshift space distortion (RSD) effect, it can be translated into the speed of the structure growth in the universe, because the large scale peculiar velocity field is a direct measure of structure growth. Since the structure growth occurs by the gravity, measurements of RSD give an important test for the gravity thoery on cosmological scales.

Past surveys such as 2dF and SDSS already provided RSD measurements at relatively local universe (redshift z << 1, where wavelength shift is by a factor of (1+z) than the original wavelength). However, it is also important to measure RSD at various redshifts, to constrain the structure growth rate at various epoch of the cosmic history. A large and wide-field telescope is required to perform such a survey at high redshift. This is the purpose of the FastSound project, aiming to deliver the first RSD measurement at z > 1, where galaxy wavelength is reddened by a factor of more than two, corresponding to a cosmic age of less than 6 Gyr compared with the current age of the universe, 14 Gyr.