That is, we modify the right-hand side of the Einstein equations.
Essentially, dark energy models modify the stress-energy content of the Universe, adding another component with equation of state w≃−1. A convenient classification scheme has emerged in the field of cosmology, separating different physical scenarios of accelerating cosmologies into the categories of dark energy and modified gravity. All current observations are consistent with a cosmological constant (CC) whereas the CC is in some sense the most economical possibility, it has its own theoretical and naturalness problems ( 3, 4), so it is worthwhile to consider alternatives. Elucidating the physics behind the acceleration is one of the main science drivers for a major experimental effort using large-scale structure (LSS) surveys. The accelerated expansion of the Universe, discovered in 1998 ( 1, 2), has raised fascinating questions for cosmology and physics as a whole.
