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Kin-Wang Ng

Publications and source records attributed to Kin-Wang Ng.

100 records · Page 6Linked to original sources
arXiv1997

Dynamically Broken Scale Invariance and Cosmology

A new scalar-tensor theory of gravity induced by dynamically broken scale invariance is proposed, and its cosmological implications are discussed. It is found that the model admits an inflation via the Hawking-Moss bubbling, but the inflation rate remains undetermined due to the strong gravity limit. In light of this, scale-invariant metric perturbations having a dominant tensor component can be generated without slow-rollover. In addition, the deviation from the standard hot big-bang is vanishingly small after inflation.

hep-ph↗
arXiv1997

Generalized Scale Invariant Gravity

We generalize the scale invariant gravity by allowing a negative kinetic energy term for the classical scalar field. This gives birth to a new scalar-tensor theory of gravity, in which the scalar field is in fact an auxiliary field. For a pure gravity theory without matter, the scale symmetric phase represents an equivalent class of gravity theories, which the Einstein gravity plus a cosmological constant belongs to under a special gauge choice. The one-loop quantum correction of the theory is calculated by using the Vilkovisky-DeWitt's method. We find that the scale symmetry is broken dynamically, and that the Einstein gravity is the ground state of the broken phase. We also briefly discuss the consequent cosmological implications. It is shown that the time-delay experiment restricts the present universe to be very close to the ground state.

hep-th↗
arXiv1996

Stokes Parameters in Cosmic Microwave Background Measurements

We compute numerically the scalar- and tensor-mode induced Stokes parameters of the cosmic microwave background, by taking into account the basis rotation effects. It is found that the tensor contribution to the polarization power spectrum get enhanced and dominates over the scalar contribution for low multipoles in a universe with or without recombination. Furthermore, we show that all full-sky averaged two-point cross-correlation functions of the Stokes parameters vanish. We thus comment on the cross-correlation between the anisotropy and polarization of the CMB, and calculate the expected signal to noise ratio for the polarization experiment underway.

astro-ph↗
arXiv1995

Graviton Mode Function in Inflationary Cosmology

We consider the production of gravitons in an inflationary cosmology by approximating each epoch of change in the equation of state as sudden, from which a simple analytic graviton mode function has been derived. We use this mode function to compute the graviton spectral energy density and the tensor-induced cosmic microwave background anisotropy. The results are then compared to the numerical calculations which incorporate a smooth radiation-matter phase transition. We find that the sudden approximation is a fairly good method. Besides, in determining the frequency range and amplitude of the mode function, we introduce a pre-inflationary radiation-dominated epoch and use a physically sensible regularization method.

gr-qc↗
arXiv1995

Large-angle Polarization of the Cosmic Microwave Background Radiation and Reionization

We discuss the effect of matter reionization on the large-angular-scale anisotropy and polarization of the cosmic microwave background radiation (CMBR) in the standard CDM model. We separate three cases in which the anisotropy is induced by pure scalar, pure tensor, and mixed metric perturbations respectively. It is found that, if reionization occurs early enough, the polarization can reach a detectable level of sequentially $6\%$, $9\%$, and $6.5\%$ of the anisotropy. In general, a higher degree of polarization implies a dominant contribution from the tensor mode or reionization at high redshift. Since early reionization will suppress small-scale CMBR anisotropies and polarizations significantly, measuring the polarization on few degree scales can be a direct probe of the reionization history of the early universe.

astro-ph↗
arXiv1994

Random and Correlated Phases of Primordial Gravitaional Waves

The phases of primordial gravity waves is analysed in detail within a quantum mechanical context following the formalism developed by Grishchuk and Sidorov. It is found that for physically relevant wavelengths both the phase of each individual mode and the phase {\it difference} between modes are randomly distributed. The phase {\it sum} between modes with oppositely directed wave-vectors, however, is not random and takes on a definite value with no rms fluctuation. The conventional point of view that primordial gravity waves appear after inflation as a classical, random stochastic background is also addressed.

hep-ph↗
arXiv1994

Gravitational Wave Induced Large-scale Polarization of \\Cosmic Microwave Background Radiation

We discuss the contribution of gravitational wave to the cosmic microwave background radiation (CMBR) anisotropy and polarization. It is found that the large-scale polarization of CMBR is less than 1\% for a standard recombination universe. The effect of matter reionization will enhance the CMBR polarization to a 10\% level. We have computed the CMBR polarization for two extreme cases (not absolutely ruled out) and found that further enhancement of the ratio is possible. We conclude that measuring the polarization of CMBR on large-angular scales can probe the ionization history of the early universe, set constraints on baryon density and the spectral index of the gravitational waves.

astro-ph↗
arXiv1994

Large-scale Polarization of the Cosmic Microwave Background Radiation

The anisotropy and polarization of the cosmic microwave background radiation (CMBR) induced by the scalar and tensor metric perturbations are computed in the long-wavelength limit. It is found that the large-scale polarization of CMBR induced by the decaying tensor mode can reach a few percents. This is different from the scalar or inflation-induced tensor mode, whereas the polarization is at least two orders of magnitude lower. The effect of matter re-ionization on CMBR is also considered. We conclude that measuring the polarization of CMBR on large-angular scales can probe the ionization history of the early Universe, and test different cosmological models.

astro-ph↗
arXiv1993

Cosmological Bound on the Decay $π^0\rightarrowγX$

Using the upper bound on the effective number of light neutrino species during primordial nucleosynthesis and the cosmological pion-pole mechanism $γγ\rightarrow π^0\rightarrow γX$, we obtain an upper limit on the branching ratio for the decay BR$(π^0\rightarrow γX)<3\times 10^{-13}$, where $X$ is any long-lived weakly interacting neutral vector particle with mass smaller than the neutral pion mass.

hep-ph↗