Constraining A String Gauge Field by Galaxy Rotation Curves and
  Perihelion Precession of Planets
  We discuss a cosmological model in which the string gauge field coupled
universally to matter gives rise to an extra centripetal force and will have
observable signatures on cosmological and astronomical observations. Several
tests are performed using data including galaxy rotation curves of 22 spiral
galaxies of varied luminosities and sizes, and perihelion precessions of
planets in the solar system.
  The rotation curves of the same group of galaxies are independently fit using
a dark matter model with the generalized Navarro--Frenk--White (NFW) profile
and the string model. A Remarkable fit of galaxy rotation curves is achieved
using the one-parameter string model as compared to the three-parameter dark
matter model with the NFW profile. The average $\chi^2$ value of the NFW fit is
9\% better than that of the string model at a price of two more free
parameters. Furthermore, from the string model, we can give a dynamical
explanation for the phenomenological Tully-Fisher relation. We are able to
derive a relation between field strength, galaxy size and luminosity, which can
be verified with data from the 22 galaxies.
  To further test the hypothesis of the universal existence of the string gauge
field, we apply our string model to the solar system. Constraint on the
magnitude of the string field in the solar system is deduced from the current
ranges for any anomalous perihelion precession of planets allowed by the latest
observations. The field distribution resembles a dipole field originating from
the Sun. The string field strength deduced from the solar system observations
is of a similar magnitude as the field strength needed to sustain the
rotational speed of the Sun inside the Milky Way. This hypothesis can be tested
further by future observations with higher precision.
