One of the most important discoveries of the last decades is the fact that we live in a spatially flat Universe whose expansion rate is accelerating. This acceleration is consistent with the effect of a cosmological constant, but it may also be caused by the presence of a dynamical energy component with negative pressure, now termed dark energy (DE). This accelerated expansion might also point to a fundamental modification of the laws of gravity. Presently, dark energy seems to be the dominant component of the Universe, significantly more important than either barionic matter (atoms), radiation or dark matter. The final explanation for this component will open a new window into the nature of gravity, particle physics and the fate of the Universe as a whole.
As recognized by the U.S. Dark Energy Task Force (DETF, Albrecht et al. 2006, arxiv.org/abs/astro-ph/0609591):
"We strongly recommend that there be an aggressive program to explore dark energy as fully as possible, since it challenges our understanding of fundamental physical laws and the nature of the cosmos."
J-PAS is a large step in that direction, at a fraction of the cost of other similar efforts.
Probably the most powerful tool to study dark energy is the feature of the distribution of galaxies known as baryon acoustic oscillations (BAOs). These oscillations are very subtle ripples in the distribution of galaxies. J-PAS will be able not just to measure the angular component of BAOs, but also its radial (line-of-sight) component.
The filter system of J-PAS will allow to measure many parameters which are relevant for the study of the evolution of galaxies: direct stellar temperatures, stellar masses, distribution of stellar ages, metallicity, dust extinction, and interstellar gas emission.
Galaxy clusters are the largest collapsed structures in the universe, containing up to hundreds or thousands of individual galaxies. J-PAS will provide a new window for accurate optical cluster detection and selection, based on the combination of photometric colors and good photo-z precision.
Supernovae are another area of impact of the J-PAS survey. Due to the broad spectral features of supernovae, the filter system of J-PAS is ideal not only to discover them, but also to measure their light curves, to characterize their types and to extract their redshifts.
Weak lensing is sensitive to both the distance and the growth factor as a function of redshift. The lensing effect can be measured using either the shear or the magnification. Despite not being specifically designed for this task, the J-PAS camera is expected to achieve high quality measurements of galaxy ellipticities, which will be combined with our ultra-precise photometric redshifts to form an extremely powerful weak lensing survey.
QSOs are another area where J-PAS will have a major impact. The narrow-band filter system is ideal to detect the broad emission lines of type-1 quasars, and we expect to identify and measure with high accuracy the redshifts of more than 3 million of these objects, up to redshifts of z~6.
By selecting the JPCam filters appropriately, several stellar parameters observed in J-PAS should be measurable, such as effective temperature, surface gravity, iron abundance, and α/Fe. The photometric data obtained by J-PAS reproduce the spectral energy distribution (SED) of the stars.
The distribution and chemical composition of asteroids is one of the most significant measurements of their formation and evolutionary histories, but it is also one of the most difficult quantities to be measured due the selection effects. The system of 56 filters of J-PAS camera will allow to extend this study enormously, basically getting entire spectra of each asteroid, leading the exploration of this new scientific window to a new level.
J-PAS's redshift accuracy will be sufficiently good to identify individual not only the galaxies, but the structures themselves (clusters, filaments, walls and voids), both in the angular and in the radial (line-of-sight) directions. Accurate measurements of the redshift-space power spectrum will be possible in the linear and mildly nonlinear regime, and a detailed comparison with theoretical predictions will be done in conjunction with the measurement of BAOs.