arXiv subjects

Xiangdong Shi

Publications and source records attributed to Xiangdong Shi.

27 records · Page 2Linked to original sources

arXiv1997

Expectations for the Difference Between Local and Global Measurements of the Hubble Constant

There are irreducible differences between the Hubble constant measured locally and the global value. They are due to density perturbations and finite sample volume (cosmic variance) and finite number of objects in the sample (sampling variance). We quantify these differences for a suite of COBE-normalized CDM models that are consistent with the observed large-scale structure. For small samples of objects that only extend out to 10,000 km/sec, the variance can approach 4%. For the largest samples of Type Ia supernovae (SNeIa), which include about 40 objects and extend out to almost 40,000 km/sec, the variance is 1-2% and is dominated by sampling variance. Sampling and cosmic variance may be an important consideration in comparing local determinations of the Hubble constant with precision determinations of the global value that will be made from high-resolution maps of CBR anisotropy.

astro-ph↗

arXiv1997

Neutrino Production of Deuterium in Supermassive Stars and Possible Implications for Deuterium Detections in Ly-Limit Systems

We describe how thermally produced anti-electron neutrinos ($\barν_e$) from the homologously collapsing core of a supermassive star ($M \ga 5\times10^4 M_\odot$) can lead to significant deuterium enrichment in the ejected envelope of such a star. Deuterium-enriched material at high redshift might then serve as a clue to the existence of pregalactic supermassive stars. Conceivably, the ejected deuterium-enriched material could intercept the line of sight to a distant QSO and mimic a Lyman limit absorber. In such a case, the deuterium abundance inferred from absorption lines might not reflect the true primordial abundance of deuterium. We discuss relevant theoretical uncertainties in supermassive star physics as well as potential observational signatures in Lyman $α$ absorber clouds for processing by stars of this kind.

astro-ph↗

arXiv1997

Measuring Cold Dark Matter Power Spectrum from Variations of Hubble Flows

When Cold Dark Matter (CDM) power spectrum normalized by COBE results, its amplitude at smaller scales can be parametrized by $Γ\simΩ_0h$. The expected variations of Hubble flows in two samples, the sample of 36 clusters in the Mark III catalogue, and the sample of 20 Type Ia supernovae (SNe), are calculated for the power spectrum in critical-density CDM models (including tilted CDM models and vacuum-dominated CDM models). The comparison between the expectations and the real variations in the data offers a bias-free way to constrain the power spectrum. The cluster sample yields $Γ\le 0.30-0.88(n_{ps}-1)+1.9 (n_{ps}-1)^2$ at 95% C.L., with best fits $Γ= 0.15-0.39(n_{ps}-1)+0.37(n_{ps}-1)^2$, where $n_{ps}$ is the spectral index of the power spectrum. The Type Ia SN sample yields $Γ\le 0.25-0.80(n_{ps}-1)+1.6 (n_{ps}-1)^2$ at 95% C.L., strongly favoring lower $Γ$'s. The results are inconsistent with a critical-density matter-dominated universe with $n_{ps}\ga 0.8$ and $H_0\ga 50$ km/sec/Mpc.

astro-ph↗

arXiv1996

Chaotic Amplification of Neutrino Chemical Potentials by Neutrino Oscillations in Big Bang Nucleosynthesis

We investigate in detail the parameter space of active-sterile neutrino oscillations that amplifies neutrino chemical potentials at the epoch of Big Bang Nucleosynthesis. We calculate the magnitude of the amplification and show evidences of chaos in the amplification process. We also discuss the implications of the neutrino chemical potential amplification in the Big Bang Nucleosynthesis. It is shown that with a $\sim 1$ eV $\nue$, the amplification of its chemical potential by active-sterile neutrino oscillations can lower the effective number of neutrino species at Big Bang Nucleosynthesis to significantly below 3.

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arXiv1995

The Proximity Effect of QSO Metal Line Absorption Systems on Lyman $α$ Forests

The proximity effect of metal line absorption systems in spectra of high redshift quasars and quasar pairs is investigated through the distribution of \Lyalpha forests. No strong proximity effect is found down to 500 km/sec away from a metal line absorption system for lyman $α$ clouds with $13.3\le\log\NHI <13.8$ at a redshift of $\sim 3$. Its implication on properties of high redshift galaxies and \Lyalpha clouds is discussed. More high resolution spectra of high redshift quasars and pairs of quasars (whose lines of sight are separated by $\sim 0.3h^{-1}$ to 1$h^{-1}$ Mpc) are needed so that the proximity effect can be reliably determined for \Lyalpha clouds with various $\NHI$ and down to a closer distance.

astro-ph↗

arXiv1995

Measuring Hubble's Constant in our Inhomogeneous Universe

Recent observations of Cepheids in the Virgo cluster have bolstered the evidence that supports a Hubble constant in 70-90 km/s/Mpc range. This evidence, by and large, probes the expansion of the Universe within 100 Mpc. We investigate the possibility that the expansion rate within this region is systematically higher than the true expansion rate due to the presence of a local, large underdense region or void. We begin by calculating the expected deviations between the locally measured Hubble constant and the true Hubble constant for a variety of models. We also discuss the expected correlations between these deviations and mass fluctuation for the sample volume. We find that the fluctuations are small for the standard cold dark matter as well as mixed dark matter models but can be substantial in a number of interesting and viable nonstandard scenarios. However, deviations in the Hubble flow for a region of radius 200 Mpc are small for virtually all reasonable models. Therefore, methods based on supernovae or the Sunyaev-Zel'dovich effect, which can probe 200 Mpc scales, will be essential in determining the true Hubble constant. We discuss, in detail, the fluctuations induced in the cosmic background radiation by voids at the last scattering surface. In addition, we discuss the dipole and quadrupole fluctuations one would expect if the void enclosing us is aspherical or if we lie off-center.

astro-ph↗

arXiv1995

On the Origin of the Clustered QSO Metal Absorption Lines

Observations show that there is significant clustering of QSO metal absorption lines within the range of velocity dispersion between 200km/sec and 600 km/sec. With a reasonable supernova rate, it is shown that high velocity gases driven by SNe and/or strong stellar winds could explain the clustered absorptions, provided that QSO metal-line absorbers are galactic halos or dwarf galaxies. Rich clusters of galaxies, on the other hand, cannot yield the observed clustering of QSO metal absorption lines.

astro-ph↗

arXiv1994

Can globular clusters be as young as 10 Gyr?--The Uncertainties in the Age of Globular Clusters from Their Helium Abundance and Mass Loss

The age of globular clusters inferred from observations depends sensitively on assumptions such as the initial helium abundance and the mass loss rate. A high helium abundance (e.g., $Y\approx$0.28), as well as an inclusion of helium diffusion and oxygen-enhancement in stellar models, can lower the current age estimate for metal-poor globular clusters from 14$\pm 1.5$ Gyr to about 11$\pm 1$ Gyr, significantly relaxing the constraints on the Hubble constant, allowing values as high as 60km/sec/Mpc for a universe with the critical density and 90km/sec/Mpc for a baryon-only universe. The uncertainties of a high helium abundance and an instability strip induced mass loss near the turn-off in globular clusters are discussed. Ages lower than 10 Gyr are not possible even with the operation of both of these mechanisms unless the initial helium abundance in globular clusters is $>0.28$, which can hardly be accomodated by indirect inferences of helium abundances in globular clusters.

astro-ph↗

arXiv1993

A Type II Supernovae Constraint on $ν_e$-$ν_s$

The role of a resonant $ν_e$-$ν_s$ oscillation is discussed in the event of a supernova explosion. It is concluded that a significant $ν_e$-$ν_s$ mixing may hinder the ability of the supernova to explode. It may also cool the proto-neutron star too quickly with respect to the observed cooling time of several seconds. The constraints on the $ν_e$-$ν_s$ mixing parameters based on the above arguments are calculated.

hep-ph↗