Solar System Constraints on Scalar Tensor Theories with Non-Standard Action

TL;DR

This paper calculates the Post-Newtonian parameter gamma for scalar-tensor theories with a non-standard Dirac-Born-Infeld action, analyzing how solar system observations constrain such models.

Contribution

It introduces a novel calculation of the PPN parameter gamma for scalar-tensor theories with DBI-type scalar actions and derives observational constraints from solar system data.

Findings

The PPN parameter gamma depends on the scalar field's effective mass.

Solar system measurements constrain the model parameters.

The results are consistent with Cassini mission data.

Abstract

We compute the Post-Newtonian parameter (PPN),$\gamma$, for scalar-tensor gravity theory when the action functional for the scalar field is a non-standard one, namely the Dirac-Born-Infeld (DBI) type action, used in the literature for a tachyon field. We investigate two different cases (Linear and conformal couplings) when the scalar field is non-minimally coupled to gravity via the scalar curvature. We find that the PPN parameter $\gamma$, which measures the amount of space curvature per unit rest mass, becomes a function of the effective mass of the scalar field. Using this PPN parameter, we calculate the time delay $\Delta\tau$ for the signal to travel the round trip distance bertween a ground based antenna and a reflector placed in a spacecraft which is produced due to the grvitational field of Sun. We use this $\Delta\tau$ to compare our result with that obtained by the Cassini mission and derive the constraints on the model parameters.