arXiv subjects

Kin-Wang Ng

Publications and source records attributed to Kin-Wang Ng.

At least 37 records · Page 2Linked to original sources

arXiv2021

Constraining the dark-energy equation of state with cosmological data

Recently, the observed equation of state for dark energy appears to favor values below $-1$. The tendency implies that the nature of dark energy may be quite different from that of the cosmological constant. In view of the adjustment on the equation of state keeps decreasing, the introduction of the phantom energy seems inevitable. By employing observational constraints from supernovae and from the acoustic scale in which the accuracy of the data has become extraordinary, we apply a phenomenological scenario to be acquainted with the evolution of our universe. The demonstration on the constrained unfolding of the phantom energy shows the model has high consistency with the current observation.

astro-ph.CO↗

arXiv2021

Observation of polarized stochastic gravitational-wave background in pulsar-timing-array experiments

We study the observation of polarized stochastic gravitational-wave background (SGWB) in pulsar-timing-array experiments. The time residual for an observed pulsar is formulated as a line-of-sight integral that incorporates the effects of the pulsar term, from which we construct the correlation function of the time residual between a pair of pulsars in terms of the overlap reduction functions (ORFs) for the SGWB intensity and polarization anisotropies. Our formulation provides a numerical scheme for computing the ORFs for high multipole moments and the lowest-moment ORFs for the SGWB linear polarization are worked out for the first time.

gr-qc↗

arXiv2021

Power spectrum of primordial perturbations during ultra-slow-roll inflation with back reaction effects

We develop a nonperturbative method through the Hartree factorization to examine the quantum fluctuation effects on the single-field inflationary models in a spatially flat FRW cosmological space-time. Apart from the background field equation as well as the Friedmann equation with the corrections of quantum field fluctuations, the modified Mukhanov-Sasaki equations for the mode functions of the quantum scalar field are also derived by introducing the nonzero $Δ_B$ term. We consider the Universe undergoing the slow roll (SR)-ultra slow roll (USR) -slow roll (SR) inflation where in particular the presence of the USR inflation triggers the huge growth of $Δ_B$ that in turn gives the boost effects to the curvature perturbations for the modes that leave horizon in the early times of the inflation. However, the cosmic friction term in the mode equation given by the Hubble parameter presumably prohibits the boost effects. Here we propose two representative models to illustrate these two competing terms.

astro-ph.CO↗

arXiv2021

Spherical harmonic analysis of anisotropies in polarized stochastic gravitational wave background with interferometry experiments

We study the interferometric observation of intensity and polarization anisotropies of a stochastic gravitational wave background (SGWB). We show that the observed correlated data is defined in the group manifold of the three-dimensional rotation. Explicit correlation between two detectors in the interferometry experiments such as LIGO-Virgo and KAGRA is constructed in terms of the Wigner D-functions. Our results may provide a tool for constructing data pipelines to estimate the power spectra of the SGWB anisotropies.

gr-qc↗

arXiv2020

Thermal Loop Effects on Large-Scale Curvature Perturbation in the Higgs Inflation

It is known that the Higgs potential in the Standard Model can drive a successful inflation as long as the Higgs field couples non-minimally to gravity. It is then inevitable to take into account the loop corrections of the Standard Model particles to the Higgs potential in the Higgs inflation. In this paper, we discuss the one-loop corrections at finite temperature to the curvature perturbation generated during the Higgs inflation. We find that the thermal loop effects can suppress the power of the curvature perturbation at large scales, thus resulting in a low quadrupole of the temperature anisotropy in the cosmic microwave background.

hep-ph↗

arXiv2020

Quantum loop effects to the power spectrum of primordial perturbations during ultra slow-roll inflation

We examine the quantum loop effects on the single-field inflationary models in a spatially flat Friedmann-Robertson-Walker (FRW) cosmological space-time with a general self-interacting scalar field potential, which is modeled in terms of the Hubble flow parameters in the effective field theory approach. In particular, we focus on the scenarios in both slow-roll to ultra-slow-roll (SR-USR) and SR-USR-SR inflation, in which it is shown that density perturbations originated from quantum vacuum fluctuations can be enhanced at small-scales, and then potentially collapse into primordial black holes (PBHs). Here our estimates indicate significant one-loop corrections around the peak of the density power spectrum in both scenarios. The induced large quantum loop effects should be confirmed by a more formal quantum field theory, and, if so, should be treated in a self-consistent manner that will be discussed.

gr-qc↗

arXiv2019

Dark Ultra-Light Scalars and Cosmic Parity Violation

If the dark sector of the Universe consists of ultra-light scalars, their coupling to photon via a Chern-Simons term would induce a rotation of the polarization plane of the cosmic microwave background (CMB). This rotation would convert E -mode polarization into B -mode polarization, resulting in new CMB BB correlation and parity-violating TB and EB cross correlations. We review the subject giving details about the derivation of the rotational effects and summarizing the possible signals in current and future CMB B-mode experiments.

astro-ph.CO↗

arXiv2019

Circularly Polarized Gamma Rays in Effective Dark Matter Theory

We study the loop-induced circularly polarized gamma rays from dark matter annihilation using the effective dark matter theory approach. Both neutral scalar and fermionic dark matter annihilating into monochromatic diphoton and $Z$-photon final states are considered. To generate the circular polarization asymmetry, $P$ and $CP$ symmetries must be violated in the couplings between dark matter and Standard Model fermions inside the loop with non-vanishing Cutkosky cut. The asymmetry can be sizable especially for $Z$-photon final state for which asymmetry of nearly $90\%$ can be reached. We discuss the prospect for detecting the circular polarization asymmetry of the gamma-ray flux from dark matter annihilation in the Galactic Center in future gamma-ray polarimetry experiments.

hep-ph↗

arXiv2019

Electromagnetic coupling effects in natural inflation

In this work we study the effects of the electromagnetic coupling in natural inflation in a systematic manner using the Schwinger-Keldysh formalism. The corresponding influence functional is evaluated to one-loop level. It can be interpreted as due to a single stochastic force. The equation of motion of the inflaton field is therefore given in the form of a Langevin equation. Lastly, the two-point and the three-point correlation functions of the inflaton field are worked out. They are related to the power spectrum and the nongaussianity of the inflaton field, respectively.

gr-qc↗

arXiv2019

Chaotic inflation on the brane and the Swampland Criteria

In this paper, we show that single-field chaotic inflation on the brane with the potential $V=a ϕ^p$ is compatible with the Swampland criteria. The spectral index and the running spectral index are within experimental bounds for $0<p \leq 2$. The tensor to scalar ratio is within observational bounds if $p \lesssim \mathcal{O}(1)$.

hep-ph↗

arXiv2018

Primordial black holes and associated gravitational waves in axion monodromy inflation

In the axion monodromy inflation, the inflation is driven by the axion with super-Planckian field values in a monomial potential with superimposed sinusoidal modulations. The coupling of the axion to massless gauge fields can induce copious particle production during inflation, resulting in large non-Gaussian curvature perturbation that leads to the formation of primordial black holes. In this paper, we explore the parameter space in the axion monodromy inflation model that favors the formation of primordial black holes with masses ranging from $10^8$ grams to $20$ solar masses. We also study the associated gravitational waves and their detection in pulsar timing arrays and interferometry experiments.

astro-ph.CO↗

arXiv2018

Superhorizon curvature perturbation in ultra-slow-roll inflation

We study the growth of superhorizon modes in the curvature perturbation during an ultra-slow-roll or a large-$η$ phase in single-field inflation. In a simple toy model, we derive the two-point correlation function of the curvature perturbation and show that the requirement for causality restricts the growth rate and hence puts a lower limit on the value of $η$. The toy model is then realized by considering an inflation potential with an inflection point. Our study is useful to assessing the growth of the curvature perturbation that seeds the formation of primordial black holes.

astro-ph.CO↗

arXiv2018

The impact of EDGES 21-cm data on dark matter interactions

The recently announced results on the 21-cm absorption spectrum by the EDGES experiment can place very stringent limits on dark matter annihilation cross-sections. We properly take into account the heating energy released from dark matter annihilation from the radiation epoch to the 21-cm observation redshifts in the radiative transfer to compute the evolution of the gas temperature. Our results show that the global 21-cm absorption profile is a powerful cosmological probe of the dark matter interactions. For dark matter annihilating into electron-positron pairs, the EDGES results give a more stringent upper limit than the PLANCK result on the annihilation cross section at the lower dark matter mass region.

astro-ph.CO↗

arXiv2018

Polarized gamma rays from dark matter annihilations

In this paper, we explore the possibility of a linearly polarized gamma-ray signal from dark matter annihilations in the Galactic center. Considering neutral weakly interacting massive particles, a polarized gamma-ray signal can be realized by a two-component dark matter model of Majorana fermions with an anapole moment. We discuss the spin alignment of such dark matter fermions in the Galactic center and then estimate the intensity and the polarizability of the final-state electromagnetic radiation in the dark matter annihilations. For low-mass dark matter, the photon flux at sub-GeV energies may be polarized at a level detectable in current X-ray polarimeters. Depending on the mass ratio between the final-state fermion and DM, the degree of polarization at the mass threshold can reach $70\%$ or even higher, providing us with a new tool for probing the nature of dark matter in future gamma-ray polarization experiments.

hep-ph↗

arXiv2017

Spherical Collapse Models with Clustered Dark Energy

We investigate the clustering effect of dark energy (DE) in the formation of galaxy clusters using the spherical collapse model. Assuming a fully clustered DE component, the spherical overdense region is treated as an isolated system which conserves the energy separately for both matter and DE inside the spherical region. Then, by introducing a parameter $r$ to characterize the degree of DE clustering, which is defined by the nonlinear density contrast ratio of matter to DE at turnaround in the recollapsing process, i.e. $r\equiv \nld_{\de,\ta}/\nld_{\m,\ta}$, we are able to uniquely determine the spherical collapsing process and hence obtain the virialized overdensity $\Dvir$ through a proper virialization scheme. Estimation of the virialized overdensities from current observation on galaxy clusters suggests that $0.5 < r < 0.8$ at $1σ$ level for the clustered DE with $w < -0.9$. Also, we compare our method to the linear perturbation theory that deals with the growth of DE perturbation at early times. While both results are consistent with each other, our method is practically simple and it shows that the collapse process is rather independent of initial DE perturbation and its evolution at early times.

astro-ph.CO↗

arXiv2017

Supernovae and Weinberg's Higgs Portal Dark Radiation and Dark Matter

The observed burst duration and energies of the neutrinos from Supernova 1987A strongly limit the possibility of any weakly-interacting light particle species being produced in the proto-neutron star (PNS) core and leading to efficient energy loss. We reexamine this constraint on Weinberg's Higgs portal model, in which the dark radiation particles (the Goldstone bosons) and the dark matter candidate (a Majorana fermion) interact with Standard Model (SM) fields solely through the mixing of the SM Higgs boson and a light Higgs boson. In order for the Goldstone bosons to freely stream out of the PNS core region, the Higgs portal coupling has to be about a factor of $4$--$9$ smaller than the current collider bound inferred from the SM Higgs invisible decay width. We find that in the energy loss rate calculations, results obtained by using the one-pion exchange (OPE) approximation and the SP07 global fits for the nucleon-nucleon total elastic cross section differ only by a factor $\lesssim 3$. The SN 1987A constraints surpass those set by laboratory experiments or by the energy loss arguments in other astrophysical objects such as the gamma-ray bursts, even with other nuclear uncertainties taken into account. Furthermore, the SN 1987A constraints are comparable to bounds from the latest dark matter direct search for low-mass WIMPs ($\lesssim 10~\Gev$.)

hep-ph↗

arXiv2017

Axion Dark Matter Induced Cosmic Microwave Background $B$-modes

It was known that isocurvature perturbation of a nearly massless cosmological axion field can lead to rotation of $E$-mode polarization into $B$-mode polarization in the cosmic microwave background (CMB) by the presence of a parity violating coupling of the field to the topological density of electromagnetism, resulting in a phenomenon known as anisotropic cosmic birefringence. In this {\em Letter}, we propose a new source of anisotropic cosmic birefringence induced by dark matter adiabatic density perturbation. If dark matter is ultralight axions that carry a coupling to photon, its adiabatic density fluctuations will induce anisotropic cosmic birefringence with a blue-tilted rotation power spectrum, thus generating CMB $B$-mode polarization on sub-degree angular scales. Using current POLARBEAR and SPTPol $B$-mode polarization data, we derive a constraint on the axion-photon coupling strength ($β$) and the axion mass ($m$), $β^2 (10^{-22}{\rm eV}/m)^2 < 8\times 10^{15}$. It is shown that the birefringence $B$ modes can dominate over CMB lensing $B$ modes at high $l$, manifesting as an excess power for $l>1500$ in future CMB lensing $B$-mode searches. In addition, we derive the lensing-rotation cross correlation that can be a potential test to the present model.

astro-ph.CO↗

arXiv2017

Production of high stellar-mass primordial black holes in trapped inflation

Trapped inflation has been proposed to provide a successful inflation with a steep potential. We discuss the formation of primordial black holes in the trapped inflationary scenario. We show that primordial black holes are naturally produced during inflation with a steep trapping potential. In particular, we have given a recipe for an inflaton potential with which particle production can induce large non-Gaussian curvature perturbation that leads to the formation of high stellar-mass primordial black holes. These primordial black holes could be dark matter observed by the LIGO detectors through a binary black-hole merger. At the end, we have given an attempt to realize the required inflaton potential in the axion monodromy inflation, and discussed the gravitational waves sourced by the particle production.

astro-ph.CO↗