Does the Cosmological Expansion Change Local Dynamics?

Abstract

It is well a known fact that the Newtonian description of dynamics within Galaxies for its known matter content is in disagreement with the observations as the acceleration approaches $a_0 \approx 1.2 \times 10^{-10}m/s^2$ (slighter larger for clusters). Both the Dark Matter scenario and Modified Gravity Theories (MGT) fails to explain the existence of such an acceleration scale. Motivated by the closeness of this acceleration scale and $c H_0\approx 10^{-9} h$ $ m/s^2$, we analyse whether this coincidence might have a Cosmological origin for scalar-tensor and spinor-tensor theories, performing detailed calculations for perturbations that represent the local matter distribution on the top of the cosmological background. Then, we solve the field equations for these perturbations in a power series in the present value of the Hubble constant. As we shall see, for both theories the power expansion contains only even powers in the Hubble constant, a fact that renders the cosmological expansion irrelevant for the local dynamics. At last, we show what a difference a theory predicting linear terms in H makes in the local dynamics.