5,393 research outputs found
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.Comment: Revtex 15 pages, 2 ps figures available at
ftp://astro.queensu.ca/pub/shi/, or at http://astro.queensu.ca/~shi/, or by
request. Submitted to Ap. J
Higher dimensional Loop Quantum Cosmology
Loop quantum cosmology(LQC) is the symmetric model of loop quantum gravity.
In this paper, we generalize the structure of loop quantum cosmology to the
theories with arbitrary spacetime dimensions. The isotropic and homogenous
cosmological model in n+1 dimensions is quantized by the loop quantization
method. Interestingly, we find that the underlying quantum theories are divided
into two qualitatively different sectors according to spacetime dimensions. The
effective Hamiltonian and modified dynamical equations of n+1 dimensional LQC
are obtained. Moreover, our results indicate that the classical big bang
singularity is resolved in arbitrary spacetime dimensions by a quantum bounce.
We also briefly discuss the similarities and differences between the n+1
dimensional model and the 3+1 dimensional one. Our model serves as a first
example of higher dimensional loop quantum cosmology and offers possibility to
investigate quantum gravity effects in higher dimensional cosmology.Comment: 14 pages. Minor revision and references added, presentation improve
Testing Cold Dark Matter Models Using Hubble Flow Variations
COBE-normalized flat (matter plus cosmological constant) and open Cold Dark
Matter (CDM) models are tested by comparing their expected Hubble flow
variations and the observed variations in a Type Ia supernova sample and a
Tully Fisher cluster sample. The test provides a probe of the CDM power
spectrum on scales of Mpc^{-1}\la k\la 0.2h Mpc, free of the
bias factor . The results favor a low matter content universe, or a flat
matter-dominated universe with a very low Hubble constant and/or a very small
spectral index , with the best fits having to 0.4.
The test is found to be more discriminative to the open CDM models than to the
flat CDM models. For example, the test results are found to be compatible with
those from the X-ray cluster abundance measurements at smaller length scales,
and consistent with the galaxy and cluster correlation analysis of Peacock and
Dodds (1994) at similar length scales, if our universe is flat; but the results
are marginally incompatible with the X-ray cluster abundance measurements if
our universe is open. The open CDM results are consistent with that of Peacock
and Dodds only if the matter density of the universe is less than about 60% of
the critical density. The shortcoming of the test is discussed, so are ways to
minimize it.Comment: 8 pages, 10 figures, submitted to MNRA
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