arXiv subjects

Zhu Yi

Publications and source records attributed to Zhu Yi.

39 records · Page 3Linked to original sources

arXiv2016

PPN parameters in gravitational theory with nonminimally derivative coupling

The nonminimal coupling of the kinetic term to Einstein's tensor helps the implementation of inflationary models due to the gravitationally enhanced friction. We calculate the parametrized post-Newtonian (PPN) parameters for the scalar--tensor theory of gravity with nonminimally derivative coupling. We find that under experimental constraint from the orbits of millisecond pulsars in our galaxy, the theory deviates from Einstein's general relativity in the order of $10^{-20}$, and the effect of the nonminimal coupling is negligible if we take the scalar field as dynamical dark energy. With the assumed conditions that the background scalar field is spatially homogeneous and evolves only on cosmological timescales and the contribution to stress--energy in the solar system from the background scalar field is subdominant, the scalar field is required to be massless.

gr-qc↗

arXiv2015

Thermodynamics of scalar-tensor theory with non-minimally derivative coupling

With the usual definitions for the entropy and the temperature associated with the apparent horizon, we show that the unified first law on the apparent horizon is equivalent to the Friedmann equation for the scalar--tensor theory with non-minimally derivative coupling. The second law of thermodynamics on the apparent horizon is also satisfied. The results support a deep and fundamental connection between gravitation, thermodynamics, and quantum theory.

gr-qc↗

arXiv2013

Warping of accretion disk and launching of jet by a spinning black hole in NGC 4258

We fit the most updated broadband spectral energy distribution from radio to X-rays for NGC 4258 with a coupled accretion-jet model that surrounding a Kerr black hole (BH), where both the jet and the warped H_2O maser disk are assumed to be triggered by a spinning BH through Blandford-Znajek mechanism and Bardeen-Petterson effect respectively. The accretion flow consists with an inner radiatively inefficient accretion flow (RIAF) and an outer truncated standard thin disk, where the transition radius R_tr~3*10^3Rg for NGC 4258 based on the width and variability of its narrow Fe K$α$ line. The hybrid jet formation model, as a variant of Blandford-Znajek model, is used to model the jet power. Therefore, we can estimate the accretion rate and BH spin through the two observed quantities--X-ray emission and jet power, where the observed jet power is estimated from the low-frequency radio emission. Through this method, we find that the BH of NGC 4258 should be mildly spinning with dimensionless spin parameter a_*=0.7\pm0.2. The outer thin disk mainly radiates at near infrared waveband and the jet contributes predominantly at radio waveband. Using above estimated BH spin and the inferred accretion rate at the region of the maser disk based on the physical existence of the H_2 O maser, we find that the warp radius is ~8.6*10^4 R_g if it is driven by the Bardeen-Petterson effect, which is consistent with the observational result very well.

astro-ph.HE↗