1. Linear Force Law: Back to Newton? Ofer Lahav University College London

2. Present: Dark Energy or Modified Gravity? Abate & OL, arXiv:0805.3160 From low-z SN Ia peculiar velocities LCDM DGP

3. Future: Dark Energy Surveys (2010-2020) one of five DES lensesCTIO 4m: the DES platform

4. “Dark Energy”: 10, 90 or 320 years old?

5. Past: back to Newton’s Principia? Weak field limit of GR: F = -GM/r 2 +  /3 r X * Lucy Calder & OL A&G Feb 08 issue http://www.star.ucl.ac.uk/~lahav/CLrev.pdf http://www.star.ucl.ac.uk/~lahav/CLrev.pdf (revised)

6. Newton, Principa (1687, Book I, Proposition 77, Theorem 37)

7. Forces inside and outside a spherical shell CL 08

8. The Mach connection Newton Einstein Can we infer ?!

9. Open Questions Should be Newton (& Hooke) be credited for the  Lambda-term (linear force)? Why didn’t Newton consider superposition of the inverse square and linear laws? Was Einstein aware of the linear force in Principia? Does the Machian connection between  and the total mass of the universe hold a clue for the origin of `Dark Energy’?

10. “Evidence” for Dark Energy Observational data Type Ia Supernovae Galaxy Clusters Cosmic Microwave Background Large Scale Structure Gravitational Lensing Physical effects: Geometry Growth of Structure Both depend on the Hubble expansion rate: H 2 (z) = H 2 0 [  M (1+z) 3 +  DE (1+z) 3 (1+w) ] (flat)