Do solar system experiments constrain scalar-tensor gravity?

Area of Science:

• Theoretical physics
• Gravitational physics
• Cosmology

Background:

• Scalar-tensor theories, like Brans-Dicke gravity, are alternatives to Einstein's general relativity (GR).
• Experimental tests often assume Brans-Dicke gravity converges to GR for large coupling constants ($$).
• The parametrized post-Newtonian (PPN) formalism is crucial for analyzing these tests.

Purpose of the Study:

• To investigate the validity of the GR limit in Brans-Dicke gravity under the PPN formalism.
• To determine if the assumption of equivalence between large $$ and GR limits holds.
• To assess the implications for experimental tests of gravity, particularly concerning light deflection and Shapiro time delay.

Main Methods:

• Analysis of Brans-Dicke gravity within the linearized PPN approximation.
• Inclusion of second-order and higher-order terms in the PPN expansion.
• Examination of strong gravity regimes.

Main Results:

• The anomaly between Brans-Dicke gravity and GR disappears in the linearized PPN approximation.
• However, the anomaly persists at second-order and higher, and in strong gravity.
• Brans-Dicke gravity, even for large $$, does not reduce to GR when higher-order PPN terms are considered.

Conclusions:

• The assumption that Brans-Dicke gravity reduces to GR for large $$ is flawed beyond the linearized approximation.
• Experimental tests relying on the PPN formalism may need re-evaluation, especially those testing higher-order effects.
• Scalar-tensor gravity theories may exhibit distinct gravitational phenomena not captured by standard GR limits.