arXiv subjects

Kin-Wang Ng

Publications and source records attributed to Kin-Wang Ng.

At least 91 records · Page 5Linked to original sources

arXiv2004

Full-sky correlation functions for CMB experiments with asymmetric window functions

We discuss full-sky convolution of the instrumental beam with the CMB sky signal in CMB single-dish and interferometry experiments, using the method (Challinor et al. 2000) that the measured temperature and polarization anisotropies are defined globally on the group manifold of the three-dimensional rotation by means of Wigner D-functions. We re-derive the anisotropy and polarization correlation functions incorporated with asymmetric window functions, which are then explicitly calculated for a single-dish elliptical Gaussian beam and an interferometric Gaussian beam.

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arXiv2004

Bound on the time variation of the fine structure constant driven by quintessence

The bound on the time variation of the fine structure constant ($α$) driven by the dynamics of quintessence scalar field which is coupled to electromagnetism is discussed using phenomenological quintessential models constrained by SNIa and CMB observations. We find that those models allowing early quintessence give the largest variation $Δα$ at the decoupling epoch. Furthermore, the fifth force experiments imply that $Δα/α$ is less than about 0.1%.

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arXiv2004

Decaying Superheavy Dark Matter and Subgalactic Structure of the Universe

The collisionless cold dark matter (CCDM) model predicts overly dense cores in dark matter halos and overly abundant subhalos. We show that the idea that CDM are decaying superheavy particles which produce ultra-high energy cosmic rays with energies beyond the Greisen-Zatsepin-Kuzmin cutoff may simultaneously solve the problem of subgalactic structure formation in CCDM model. In particular, the Kuzmin-Rubakov's decaying superheavy CDM model may give an explanation to the smallness of the cosmological constant and a new thought to the CDM experimental search.

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arXiv2004

E/B Separation in CMB Interferometry

We study the problem of separating E and B modes in interferometric observations of the polarization of the cosmic microwave background. The E and B band powers and their mixings are measured from both single-dish and interferometric mock observations using the quadratic estimator of the maximum likelihood analysis. We find that the interferometer can separate E and B modes in a single-pointing measurement and is thus well suited for detecting the faint lensing induced and gravity-wave induced B modes. In mosaicking observation, compared to the single dish, the interferometer is in general more efficient in separating E and B modes, and for high signal-to-noise per pixel it needs about three times fewer pixels to measure extremely blue polarization power spectra.

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arXiv2003

Constraining Evolution of Quintessence with CMB and SNIa Data

The equation of state of the hypothetical dark energy component, which constitutes about two thirds of the critical density of the universe, may be very different from that of a cosmological constant. Employing a phenomenological model, we investigate semi-analytically the constraints imposed on the scalar quintessence by supernovae observations, and by the acoustic scale extracted from recent CMB data. We show that a universe with a quintessence-dominated phase in the dark age is consistent with the current observational constraints.

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arXiv2003

Observational Strategies of CMB Temperature and Polarization Interferometry Experiments

We have simulated the interferometric observation of the Cosmic Microwave Background (CMB) temperature and polarization fluctuations. We have constructed data pipelines from the time-ordered raw visibility samples to the CMB power spectra which utilize the methods of data compression, maximum likelihood analysis, and optimal subspace filtering. They are customized for three observational strategies, such as the single pointing, the mosaicking, and the drift-scanning. For each strategy, derived are the optimal strategy parameters that yield band power estimates with minimum uncertainty. The results are general and can be applied to any close-packed array on a single platform such as the CBI and the forthcoming AMiBA experiments. We have also studied the effect of rotation of the array platform on the band power correlation by simulating the CBI single pointing observation. It is found that the band power anti-correlations can be reduced by rotating the platform and thus densely sampling the visibility plane. This enables us to increase the resolution of the power spectrum in the l-space down to the limit of the sampling theorem (Delta l = 226 = pi / theta), which is narrower by a factor of about sqrt{2} than the resolution limit (Delta l = 300) used in the recent CBI single pointing observation. The validity of this idea is demonstrated for a two-element interferometer that samples visibilities uniformly in the uv-annulus. From the fact that the visibilities are the Fourier modes of the CMB field convolved with the beam, a fast unbiased estimator (FUE) of the CMB power spectra is developed and tested. It is shown that the FUE gives results very close to those from the quadratic estimator method without requiring large computer resources even though uncertainties in the results increase.

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arXiv2002

Photon Production from Nonequilibrium Disoriented Chiral Condensates in a Spherical Expansion

We study the production of photons through the non-equilibrium relaxation of a disoriented chiral condensate formed in the expanding hot central region in ultra-relativistic heavy-ion collisions. It is found that the expansion smoothes out the resonances in the process of parametric amplification such that the non-equilibrium photons are dominant to the thermal photons over the range 0.2-2 GeV. We propose that to search for non-equilibrium photons in the direct photon measurements of heavy-ion collisions can be a potential test of the formation of disoriented chiral condensates.

hep-ph↗

arXiv2002

Primordial Magnetic Fields from Dark Energy

Evidences indicate that the dark energy constitutes about two thirds of the critical density of the universe. If the dark energy is an evolving pseudo scalar field that couples to electromagnetism, a cosmic magnetic seed field can be produced via spinoidal instability during the formation of large-scale structures.

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arXiv2001

Direct photons: a nonequilibrium signal of the expanding quark-gluon plasma

Direct photon production from a longitudinally expanding quark-gluon plasma (QGP) at Relativistic Heavy Ion Collider (RHIC) and Large Hadron Collider (LHC) energies is studied with a real-time kinetic description that is consistently incorporated with hydrodynamics. Within Bjorken's hydrodynamical model, energy nonconserving (anti)quark bremsstrahlung q(\bar{q})\to q(\bar{q})γand quark-antiquark annihilation q\bar{q}\to γare shown to be the dominant nonequilibrium effects during the transient lifetime of the QGP. For central collisions we find a significant excess of direct photons in the range of transverse momentum 1-2 \lesssim p_T \lesssim 5 GeV/c as compared to equilibrium results. The photon rapidity distribution exhibits a central plateau. The transverse momentum distribution at midrapidity falls off with a {\em power law} p^{-ν}_T with 2.5 \lesssim ν\lesssim 3 as a consequence of these energy nonconserving processes, providing a distinct experimental {\em nonequilibrium signature}. The power law exponent νincreases with the initial temperature of the QGP and hence with the total multiplicity rapidity distribution dN_π/dy.

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arXiv2001

AMiBA: Array for Microwave Background Anisotropy

As part of a 4-year Cosmology and Particle Astrophysics (CosPA) Research Excellence Initiative in Taiwan, AMiBA $-$ a 19-element dual-channel 85-105 GHz interferometer array is being specifically built to search for high redshift clusters of galaxies via the Sunyaev-Zeldovich Effect (SZE). In addition, AMiBA will have full polarization capabilities, in order to probe the polarization properties of the Cosmic Microwave Background. AMiBA, to be sited on Mauna Kea in Hawaii or in Chile, will reach a sensitivity of $\sim 1$ mJy or 7$μ$K in 1 hour. The project involves extensive international scientific and technical collaborations. The construction of AMiBA is scheduled to starting operating in early 2004.

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arXiv2000

Complex Visibilities of Cosmic Microwave Background Anisotropies

We study the complex visibilities of the cosmic microwave background anisotropies that are observables in interferometric observations of the cosmic microwave background, using the multipole expansion methods commonly adopted in analyzing single-dish experiments. This allows us to recover the properties of the visibilities that is obscured in the flat-sky approximation. Discussions of the window function, multipole resolution, instrumental noise, pixelization, and polarization are given.

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arXiv2000

Out-of-equilibrium Photon Production from Disoriented Chiral Condensates

We study the production of photons through the non-equilibrium relaxation of a disoriented chiral condensate. We propose that to search for non-equilibrium photons in the direct photon measurements of heavy-ion collisions can be a potential test of the formation of disoriented chiral condensates.

hep-ph↗

arXiv2000

The weak-field expansion for processes in a homogeneous background magnetic field

The weak-field expansion of the charged fermion propagator under a uniform magnetic field is studied. Starting from Schwinger's proper-time representation, we express the charged fermion propagator as an infinite series corresponding to different Landau levels. This infinite series is then reorganized according to the powers of the external field strength $B$. For illustration, we apply this expansion to $γ\to ν\barν$ and $ν\to νγ$ decays, which involve charged fermions in the internal loop. The leading and subleading magnetic-field effects to the above processes are computed.

hep-th↗

arXiv1999

Neutrino-photon scattering and its crossed processes in a background magnetic field

We study the neutrino-photon processes such as $γγ\to ν\barν$ and $νγ\to νγ$ in a background magnetic field smaller than the critical magnetic field $B_c\equiv m_e^2/e$. Using Schwinger's proper-time method, we extract leading magnetic-field contributions to the above processes. Our result is valid throughout the kinematic regime where both neutrino and photon energies are significantly smaller than $m_W$. We briefly discuss the astrophysical implications of our result.

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arXiv1999

Photon Production of Axionic Cold Dark Matter

Using the non-equilibrium quantum field theory, photon production from the coherently oscillating axion field in a flat Robertson-Walker cosmology is re-examined. First neglecting the Debye screening of the baryon plasma to photons, we find that the axions will dissipate into photons via spinodal instability in addition to parametric resonance. As a result of the pseudo-scalar nature of the axion-photon coupling, we observe a circular polarization asymmetry in the produced photons. However, these effects are suppressed to an insignificant level in the expanding universe. We then briefly discuss a systematic way of including the plasma effect which can further suppress the photon production. We note that the formalism of the problem can be applied to any pseudo-scalar field coupled to photon in a thermal background in a general curved spacetime.

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arXiv1999

Tensor-Induced CMB Temperature-Polarization Correlation in Reionized Universes

We reexamine the temperature-polarization correlation function of the cosmic microwave background induced by tensor mode with a scale-invariant spectrum in reionized standard cold dark matter models. It is found that the sign of the correlation function is positive on all angular scales even in a model with substantial reionization.

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arXiv1998

Nonequilibrium Photons as a Signature of Quark-hadron Phase Transition

We study the nonequilibrium photon production in the quark-hadron phase transition, using the Friedberg-Lee type solitons as a working model for quark-hadron physics. We propose that to search for nonequilibrium photons in the direct photon measurements of heavy-ion collisions may be a characteristic test of the transition from the quark-gluon to hadronic phases.

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arXiv1998

Correlation Functions of CMB Anisotropy and Polarization

We give a full analysis of the auto- and cross-correlations between the Stokes parameters of the cosmic microwave background. In particular, we derive the windowing function for an antenna with Gaussian response in polarization experiment, and construct correlation function estimators corrected for instrumental noise. They are applied to calculate the signal to noise ratios for future anisotropy and polarization measurements. While the small-angular-scale anisotropy-polarization correlation would be likely detected by the MAP satellite, the detection of electric and magnetic polarization would require higher experimental sensitivity. For large-angular-scale measurements such as the being planned SPOrt/ISS, the expected signal to noise ratio for polarization is greater than one only for reionized models with high reionization redshifts, and the ratio is less for anisotropy-polarization correlation. Correlation and covariance matrices for likelihood analyses of ground-based and satellite data are also given.

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