The exterior Cauchy problem is discussed for the fourth-order theories of gravity derived from the Lagrangian densities .2" = ,[=g (R + !aR 2 + bRf/N R /LV) -K.2" m' When b # 0, the Cauchy problem can be solved by the standard method already used in general relativity. When b = 0, the problem cannot be formulated as in the case where b #0, since the corresponding fourth-order theory is shown to be equivalent to a second-order scalar-tensor theory. This scalar-tensor theory is proved to coincide with one of the models of gravity proposed by O'Hanlon in order to present a covariant version of the massive dilaton theory suggested by Fujii. This result is generalized: The models of O'Hanlon are shown to be indistinguishable from the fourth-order theories derived from the Lagrangian densities.2" = Jg F (R ) -K.2" m' where F is any real function such that F "(R ) does not identically vanish.

No abstract

We summarise the scientific and technological aspects of the Search for Anomalous Gravitation using Atomic Sensors (SAGAS) project, submitted to ESA in June 2007 in response to the Cosmic Vision 2015-2025 call for proposals. The proposed mission aims at flying highly sensitive atomic sensors (optical clock, cold atom accelerometer, optical link) on a Solar System escape trajectory in the 2020 to 2030 time-frame. SAGAS has numerous science objectives in fundamental physics and Solar System science, for example numerous tests of general relativity and the exploration of the Kuiper belt. The combination of highly sensitive atomic sensors and of the laser link well adapted for large distances will allow measurements with unprecedented accuracy and on scales never reached before. We present the proposed mission in some detail, with particular emphasis on the science goals and associated measurements and technologies.