arXiv subjects

Ming-Chung Chu

Publications and source records attributed to Ming-Chung Chu.

26 records · Page 2Linked to original sources

arXiv2010

Observational evidences for the existence of 17.4 keV decaying degenerate sterile neutrinos near the Galactic Center

We show that the existence of a degenerate halo of sterile neutrinos with rest mass of 17.4 keV near the Galactic Center can account for both the excess 8.7 keV emission observed by the $Suzaku$ mission and the power needed ($10^{40}$ erg s$^{-1}$) to maintain the high temperature of the hot gas (8 keV) near the Galactic Center. The required decay rate and mixing angle of the sterile neutrinos are $Γ\ge 5 \times 10^{-20}$ s$^{-1}$ and $\sin^22 θ\sim 10^{-3}-10^{-4}$ respectively. These values are consistent with a low reheating temperature in the inflation model, and suggest the exciting possibility that the sterile - active neutrino oscillation can be visible in near future experiments.

astro-ph.HE↗

arXiv2006

CMB and Matter Power Spectra of Early f(R) Cosmology in Palatini Formalism

We calculate in this article the CMB and matter power spectra of a class of early $f(R)$ cosmologies, which takes the form of $f(R) = R + λ_1 H_0^2\text{exp}[R/(λ_2 H_0^2)]$. Unlike the late-time $f(R)$ cosmologies such as $f(R) = R + α(-R)^β$ ($β<0$), the deviation from $Λ\text{CDM}$ of this model occurs at a higher redshift (thus the name \emph{Early $f(R)$ Cosmology}), and this important feature leads to rather different ISW effect and CMB spectrum. The matter power spectrum of this model is, at the same time, again very sensitive to the chosen parameters, and LSS observations such as SDSS should constrain the parameter space stringently. We expect that our results are applicable at least qualitatively to other models that produce $f(R)$ modification to GR at earlier times (\emph{e.g.}, redshifts $\mathcal{O}(10) \lesssim z \lesssim \mathcal{O}(1)$) than when dark energy begins to dominate -- such models are strongly disfavored by data on CMB and matter power spectra.

astro-ph↗

arXiv2005

Big Bang Nucleosynthesis with an Evolving Radion in the Brane World Scenario

We consider the big bang nucleosynthesis (BBN) in the Brane world scenario, where all matter fields are confined on our 3-brane and the radion of the Brane evolves cosmologically. In the Einstein frame fundamental fermion masses vary and the results of standard BBN (SBBN) are modified. We can thus use the observational primordial element abundances to impose constraints on the possible variations of the radion. The possibility of using the evolving radion to resolve the discrepancies between the Wilkinson Microwave Anisotropy Probe (WMAP) and SBBN values of the baryon-to-photon ratio ($η$) is also discussed. The results and constraints presented here are applicable to other models in which fundamental fermion masses vary.

astro-ph↗

arXiv2005

Big Bang Nucleosynthesis Constraints on Universal Extra Dimensions and Varying Fundamental Constants

The successful prediction of light element abundances from Big Bang Nucleosynthesis has been a pillar of the standard model of Cosmology. Because many of the relevant reaction rates are sensitive to the values of fundamental constants, such as the fine structure constant and the strong coupling constant, BBN is a useful tool to probe and to put constraints on possible cosmological variations of these constants, which arise naturally from many versions of extra-dimensional theories. In this paper, we study the dependences of fundamental constants on the radion field of the universal extra dimension model, and calculate the effects of such varying constants on BBN. We show that the discrepancy between BBN and the Wilkinson Microwave Anisotropy Probe data on the baryon-to-photon ratio may be reduced if the volume of the extra dimensions was slightly larger - by $\mathcal{O} (10^{-3})$ - at the BBN era compared to its present value, which would result in smaller gauge couplings at BBN by the same factor.

hep-ph↗

arXiv2005

Dark Energy as a Signature of Extra Dimensions

One of the most important and surprising discoveries in cosmology in recent years is the realization that our Universe is dominated by a mysterious dark energy, which leads to an accelerating expansion of space-time. A simple generalization of the standard Friedmann-Robertson-Walker equations based on General Relativity and the Cosmological Principles with the inclusion of a number of closed extra dimensions reproduces the currently observed data on dark energy, without the introduction of any cosmological constant or new particles. In particular, with a few extra dimensions, we obtain the redshift dependence of the deceleration parameter, the dark energy equation of state, as well as the age of the Universe in agreement with data, with essentially no free parameter. This model predicts that the extra dimensions have been compactified throughout the cosmic history, and it therefore suggests that signals from early Universe may give promising signatures of extra dimensions.

astro-ph↗

arXiv2003

Recombination Induced Softening and Reheating of the Cosmic Plasma

The atomic recombination process leads to a softening of the matter equation of state as reflected by a reduced generalized adiabatic index, with accompanying heat release. We study the effects of this recombination softening and reheating of the cosmic plasma on the ionization history, visibility function, Cold Dark Matter (CDM) transfer function, and the Cosmic Microwave Background (CMB) spectra. The resulting modifications of the CMB spectrm is 1/10 of WMAP's current error and is comparable to PLANCK's error. Therefore, this effect should be considered when data with higher accuracy are analysed.

astro-ph↗

arXiv2003

Gravitational Waves from Phase-Transition Induced Collapses of Neutron Stars

We study the gravitational wave signals emitted from phase-transition induced collapses of rapidly rotating neutron stars to strange stars by performing 3D numerical simulations. Our preliminary results suggest that the complete conversion of neutron stars to strange stars would occur within a fraction of millisecond. We also find that the gravitational waves generated from the collapse process may be detectable by the advanced LIGO for reasonable source distance. In addition, the study such gravitational wave signals would put useful constraint on the parameters of QCD.

astro-ph↗

arXiv1997

The Collapse of Neutron Stars in High-Mass Binaries as the Energy Source for the Gamma-Ray Bursts

The energy source has remained to be the great mystery in understanding of the gamma-ray bursts (GRBs) if the events are placed at cosmological distances as indicated by a number of recent observations. The currently popular models include (1)the merger of two neutron stars or a neutron star and a black hole binary and (2)the hypernova scenario of the collapse of a massive member in a close binary. Since a neutron star will inevitably collapse into a black hole if its mass exceeds the limit $M_{max}\approx3M_{\odot}$, releasing a total binding gravitational energy of $\sim10^{54}$ erg, we explore semi-empirically the possibility of attributing the energy source of GRB to the accretion- induced collapse of a neutron star (AICNS) in a massive X-ray binary system consisting of a neutron star and a type O/B companion. This happens because a significant mass flow of $\sim10^{-3}$--$10^{-4}M_{\odot}$ yr$^{-1}$ may be transferred onto the neutron star through the Roche-lobe overflow and primarily during the spiral-in phase when it plunges into the envelope of the companion, which may eventually lead to the AICNS before the neutron star merges with the core of the companion. In this scenario, a ``dirty'' fireball with a moderate amount of beaming is naturally expected because of the nonuniformity of the stellar matter surrounding the explosion inside the companion, and a small fraction ($\sim0.1%$) of the energy is sufficient to create the observed GRBs. In addition, the bulk of the ejecting matter of the companion star with a relatively slow expansion rate may act as the afterglow. Assuming a non-evolutionary model for galaxies, we estimate that the birthrate of the AICNS events is about 2 per day within a volume to redshift $z=1$ for an $Ω_0=1$ universe, consistent with the reported GRB rate.

astro-ph↗