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European Reasearch Council
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\large{\textbf{ERC Synergy Grant 2012}}\\
\large{\textbf{Research Proposal (Part B1)}}\\
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\large{\textbf{GG (``Galileo Galilei'') and GG on Ground: a very high sensitive experiment to probe the foundations of
General Relativity}}\\
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\Large{\textbf{GGonGround}}
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Corresponding Principal Investigator: \textbf{Anna M. Nobili}\\
Principal Investigator: \textbf{Guido Zavattini}\\
Corresponding Host Institution: \textbf{EGO}\\
Proposal full title: \textbf{GG (``Galileo Galilei'') and GG on Ground: a very high sensitive experiment to probe the foundations of General Relativity}\\
Proposal short name: \textbf{GGonGround}\\
Proposal duration in months: \large{\textbf{72}}\\
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\large{\textbf{Proposal Summary}}\\
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General Relativity (GR) is the best theory of gravity to-date but attempts at merging gravity with the other forces of nature have failed and most of the mass of the universe is unexplained. GR is based on the hypothesis that the gravitational force is composition independent: in a gravitational field all bodies should fall with the same acceleration regardless of their mass and composition (Universality of Free Fall, UFF). UFF is unique to gravity and is a direct consequence of the Equivalence Principle (EP). Tests of UFF are unique tests of GR in that they address the assumed composition independence of gravity; this makes them the most deeply probing tests in the search for new physics beyond General Relativity and the current impasse. It is generally recognized that experimental evidence of a violation of UFF (hence of EP) would make for a scientific revolution. UFF has been tested to $10^{-13}$  but a radically new type of experiment is needed to improve this limit by several orders of magnitude. We propose an unconventional  sensor which can be run in space with 1e4 improvement by making the main critical issues disappear by design. A small satellite GG (``Galileo Galilei'') and the corresponding ground experiment GGG have been designed  and a pre-prototype has been built by solving many problems in a variety of fields. An agreement exists between JPL -the Jet Propulsion Laboratory of CalTech and NASA- and ASI (Agenzia Spaziale Italiana) to submit GG to the EXPLORER program of small size missions as a NASA led mission and the partnership of ASI. GG is a high precision physics experiment which can reach its final sensitivity and meet its outstanding science goal only in orbit, but that is just the final run of an experiment whose performance can and must be tested and demonstrated in the lab.  The EXPLORER program is the route to space for GG. GGonGround -by synergy between two highly dedicated groups with complementary skill- is the route for GG to selected for flight  within the EXPLORER competition.



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