Planck Collaboration, Ade, P. A. R., Aghanim, N., Arnaud, M., Ashdown, M., Aumont, J., et al. (2016). Planck 2015 results - XIV. Dark energy and modified gravity. Astronomy & Astrophysics, 594: A14, pp. 1-31. doi:10.1051/0004-6361/201525814.