M.-C. Chu

Is the Universe Rotating

Models of a rotating universe have been studied widely since the work of G?del, who showed an example that is consistent with general relativity. By now, the possibility of a rotating universe has been discussed comprehensively in the framework of some types of Bianchis models, such as Type V, VII, and IX and different approaches have been proposed to constrain the rotation. Recent discoveries of some non-Gaussian properties of the Cosmic Microwave Background Anisotropies (CMBA), such as the suppression of the quadrupole and the alignment of some multipoles draw attention to some Bianchi models with rotation. However, cosmological data, such as those of the CMBA, strongly prefer a homogeneous and isotropic model. Therefore, it is of interest to discuss the rotation of the universe as a perturbation of the Robertson-Walker metric, to constrain the rotating speed by cosmological data and to discuss whether it could be the origin of the non-Gaussian properties of the CMBA mentioned above. Here, we derive the general form of the metric (up to second-order perturbations) which is compatible with the rotation perturbation in a flat ?-CDM universe. By comparing the second-order Sachs-Wolfe effect due to rotation with the CMBA data, we constrain the angular speed of the rotation to be less than 10?9 rad?yr?1 at the last scattering surface. This provides the first constraint on the shear-free rotation of a ?CDM universe.

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, i.e., 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 Mmax ≈ 3 M☉, releasing a total binding gravitational energy of ~1054 ergs, we explore semiempirically the possibility of attributing the energy source of GRBs 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 ~10 -->−3-10 -->−4 M☉ 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 (~0.1%) 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 nonevolutionary model for galaxies, we estimate that the birthrate of the AICNS events is about two per day within a volume to redshift z=1 for an Ω -->0=1 universe, consistent with the reported GRB rate. It appears that the AICNS scenario, as a result of stellar evolution, may provide a natural explanation for the origin of GRBs and therefore deserves to be further investigated in the theoretical study of GRBs.

Heating of the intergalactic medium by the radiative decay of neutrinos

Assuming that there exists a species of heavy sterile neutrinos (mν > 1 keV) and that their decays can serve as a heating source for the hot gas in galaxy clusters, we study how the observational constraints on cooling flow limit the masses of these sterile neutrinos. We predict a relation among the luminosity, the total mass, and the redshift of a cluster, and we compare this relation with data from 12 clusters to obtain an estimate of the decay rate of sterile neutrinos.

The Quadrupole Oscillations of Strange-Quark Stars

We study the relativistic quadrupole oscillations (l=2) of strange-quark stars by calculating their quasi-normal modes, and we thus explore the possibility of using these oscillation frequencies and damping times to differentiate among relativistic polytropic stars, neutron stars, pure light-quark stars, and hybrid neutron stars with quark cores. We found that, whereas the general features of the w-mode spectra of quark stars and hybrid neutron stars are very similar to those of relativistic polytropes, they differ in the higher mode frequencies for both axial and polar oscillations. The profiles of f-modes for quark stars are different from those of realistic neutron stars, and from that we conclude that quark and strange-quark stars are more efficient gravitational-waves radiators than pure neutron stars. Also, the damping times of various modes are rather sensitive to the details of the quark model, such as the Bag constant, quark-gluon interactions, and quark mass corrections.

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 (GC) can account for both the excess 8.7 keV emission observed by the Suzaku mission and the power needed (10{sup 40} erg s{sup -1}) to maintain the high temperature of the hot gas (8 keV) near the GC. The required decay rate and mixing angle of the sterile neutrinos are {Gamma} {>=} 10{sup -19} s{sup -1} and sin{sup 2}2{theta} {approx} 10{sup -3}, respectively. These values are consistent with a low reheating temperature, which suppresses the production of sterile neutrinos, resulting in a small sterile neutrino density parameter ({Omega}{sub s} < 10{sup -8}). They are also allowed by X-ray background data and the isotope experiment. The large mixing angle leads to the exciting possibility that a sterile-active neutrino oscillation may be visible in near future experiments.

THE EXISTENCE OF STERILE NEUTRINO HALOS IN GALACTIC CENTERS AS AN EXPLANATION OF THE BLACK HOLE MASS-VELOCITY DISPERSION RELATION

If sterile neutrinos exist and form halos in galactic centers, they can give rise to observational consequences. In particular, the sterile neutrinos decay radiatively and heat up the gas in the protogalaxy to achieve hydrostatic equilibrium, and they provide the mass to form supermassive black holes (BHs). A natural correlation between the BH mass and velocity dispersion thus arises: log(M BH,f /M ☉) = αlog(σ/200 km s-1) + β with α ≈ 4 and β ≈ 8.

A predicted relation between the temperature and density profile of cluster hot gas

Based on the assumptions that a fraction of cluster dark matter is composed of degenerate neutrinos and they are in hydrostatic equilibrium with other matter, we predict a relation between the density profile and temperature of the cluster hot gas. The predicted relation agrees with observational data of 103 clusters.

The Importance of Quantum Pressure of Fuzzy Dark Matter on Lyα Forest

With recent Lyman-alpha forest data from BOSS and XQ-100, some studies suggested that the lower mass limit on the fuzzy dark matter (FDM) particles is lifted up to

Oscillations of Quark Stars

10^{-21}\,\mathrm{eV}

Ultralight Axion Dark Matter and Its Impact on Dark Halo Structure in N-body Simulations

. However, such a limit was obtained by