Baryonic Accoustic Oscillations
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, with a length scale of approximately 150 Mpc (or about 450 million light-years). These ripples were generated at a very early epoch, when the Universe was less than one million years old (presently, its age is approximately 13.7 billion years old), and it was so hot and dense that radiation and matter constituted a single, tightly coupled fluid. The sound waves of this fluid can be seen today as the BAOs (see an artist's conception in the figure to the right).
The signature of the BAOs is a slight preference for galaxies to be found at a distance of about 150 Mpc from another galaxy. This distance can be on the angular direction (easy to measure), or in the radial direction (hard to measure, since the direction along the line-of-sight is difficult to measure accurately for such distant galaxies). Because of its vastly improved quality of photometric redshifts of galaxies, J-PAS will be able not just to measure the angular component of BAOs, but also its radial (line-of-sight) component. This will provide independent and more precise measurements of the DE equation of state parameter w.