Progress in Lunar Laser Ranging Tests of Relativistic Gravity

TL;DR

This paper reports highly precise tests of relativistic gravity and the Equivalence Principle using Lunar Laser Ranging data, confirming general relativity's predictions and constraining possible deviations.

Contribution

It provides the most stringent Lunar Laser Ranging-based limits on EP violations, PPN parameters, geodetic precession, and gravitational constant variation to date.

Findings

EP violation parameter (-1.0  1.4) ^{-13}

SEP violation parameter (-2.0  2.0) ^{-13}

No evidence for variation of G ((4  9) ^{-13} yr^{-1})

Abstract

Analyses of laser ranges to the Moon provide increasingly stringent limits on any violation of the Equivalence Principle (EP); they also enable several very accurate tests of relativistic gravity. We report the results of our recent analysis of Lunar Laser Ranging (LLR) data giving an EP test of \Delta (M_G/M_I)_{EP} =(-1.0 +/- 1.4) x 10^{-13}. This result yields a Strong Equivalence Principle (SEP) test of \Delta (M_G/M_I)_{SEP} =(-2.0 +/- 2.0) x 10^{-13}. Also, the corresponding SEP violation parameter \eta is (4.4 +/- 4.5) x 10^{-4}, where \eta=4\beta-\gamma-3 and both \beta and \gamma are parametrized post-Newtonian (PPN) parameters. Using the recent Cassini result for the parameter \gamma, PPN parameter \beta is determined to be \beta-1=(1.2 +/- 1.1) x 10^{-4}. The geodetic precession test, expressed as a relative deviation from general relativity, is K_{gp}=-0.0019 +/- 0.0064. The search for a time variation in the gravitational constant results in \dot G/G=(4 +/- 9) x 10^{-13} yr^{-1}, consequently there is no evidence for local (~1AU) scale expansion of the solar system.