Feng Xu’s research while affiliated with University of Bonn and other places

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 effects on cosmological and astronomical observations. Several tests are performed using data including galaxy rotation curves of twenty-two spiral galaxies of varied luminosities and sizes, and perihelion precessions of planets in the solar system. Remarkable fit of galaxy rotation curves is achieved using the one-parameter string model as contrasted to the three-parameter model of dark matter model with the Navarro-Frenk-White profile. The rotation curves of the same group of galaxies are independently fit using dark matter model with the generalized Navarro-Frenk-White profile and using the string model. The average chi-squared of the NFW fit is 9% better than that of the string model at a price of two more free parameters. From the string model we give a dynamical explanation of Tully-Fisher relation. We are able to derive a relation between field strength, galaxy size and luminosity, which can be verified with data of the 22 galaxies. To test the hypothesis of the universal existence of string gauge field, the string model is also used to explain the anomalous perihelion precession of planets in the solar system. The required field strength is deduced from the data. The field distribution resembles a dipole field originated from the Sun. A refined model assumes that the string gauge field is partly contributed by the Sun and is partly contributed by the stellar matter in the Milky Way. The field strength needed to explain the excess precession is of similar magnitudes as the field strength needed to sustain the rotational speed of the sun inside the Milky Way.

Remarkable fit of galaxy rotation curves is achieved using a simple model from string theory. The rotation curves of the same group of galaxies are also fit using dark matter model with the generalized Navarro-Frenk-White profile for comparison. String model utilizes three free parameters vs five in the dark matter model. The average chi-squared of the string model fit is 1.649 while that of the dark matter model is 1.513. The generalized NFW profile fits marginally better at a price of two more free parameters.