Understand
We analyse the role, on large cosmological scales and laboratory experiments, of the leading curvature squared contributions to the low energy effective action of gravity.
- We argue for a natural relationship $c_0\lambda^2\simeq 1$ at low-energy between the ${\cal R}^2$ coefficients $c_0$ of the Ricci scalar squared term in this expansion and the dark energy scale $\Lambda=(\lambda M_{\rm Pl})^4$ in four dimensional Planck mass units.
- We show how the compatibility between the acceleration of the expansion rate of the Universe, local tests of gravity and the quantum stability of the model all converge to select such a relationship up to a coefficient which should be determined experimentally.
- When embedding this low energy theory of gravity into candidates for its ultraviolet completion, we find that the proposed relationship is guaranteed in string-inspired supergravity models with modulus stabilisation and supersymmetry breaking leading to de Sitter compactifications.