153 research outputs found
Sharpening the predictions of big-bang nucleosynthesis
Motivated by the recent measurement of the primeval abundance of deuterium,
we re-examine the nuclear inputs to big-bang nucleosynthesis (BBN). Using
Monte-Carlo realization of the nuclear cross-section data to directly estimate
the theoretical uncertainties for the yields of D, 3-He and 7-Li, we show that
previous estimates were a factor of 2 too large. We sharpen the BBN
determination of the baryon density based upon deuterium, rho_B = (3.6 +/- 0.4)
* 10^{-31} g/cm^3 (Omega_B h^2 = 0.019 +/- 0.0024), which leads to a predicted
4-He abundance, Y_P = 0.246 +/- 0.0014 and a stringent limit to the equivalent
number of light neutrino species: N_nu < 3.20 (all at 95% cl). The predicted
7-Li abundance, 7-Li/H = (3.5 + 1.1 - 0.9) * 10^{-10}, is higher than that
observed in pop II stars, (1.7 +/- 0.3) * 10^{-10} (both, 95% cl). We identify
key reactions and the energies where further work is needed.Comment: 5 pages, 4 figures (epsfig), REVTeX; submitted to Phys. Rev. Let
A Catalog of Absorption Lines in Eight HST/STIS E230M 1.0 < z < 1.7 Quasar Spectra
We have produced a catalog of line identifications and equivalent width
measurements for all absorption features in eight ultraviolet echelle quasar
spectra. These spectra were selected as having the highest signal-to-noise
among the HST/STIS spectra obtained with the E230M grating. We identify 56
metal-line systems toward the eight quasars, and present plots of detected
transitions, aligned in velocity-space. We found that about 1/4 - 1/3 of the
features in the Lya forest region, redward of the incidence of the Lyb forest,
are metal lines. High ionization transitions are common. We see both O VI and C
IV in 88 - 90% of the metal-line systems for which the spectra cover the
expected wavelength. Si III is seen in 58%, while low ionization absorption in
C II, Si II, and/or Al II is detected in 50% of the systems for which they are
covered. This catalog will facilitate future studies of the Lya forest and of
metal-line systems of various types.Comment: 13 pages, 2 figures, submitted to Monthly Notices of the Royal
Astronomical Society, a complete version with the appendix and all figures is
available at http://www.astro.psu.edu/users/misawa/pub/Paper/qalcat.pdf.g
The Keck+Magellan Survey for Lyman Limit Absorption I: The Frequency Distribution of Super Lyman Limit Systems
We present the results of a survey for super Lyman limit systems (SLLS;
defined to be absorbers with 19.0 <= log(NHI) <= 20.3 cm^-2) from a large
sample of high resolution spectra acquired using the Keck and Magellan
telescopes. Specifically, we present 47 new SLLS from 113 QSO sightlines. We
focus on the neutral hydrogen frequency distribution f(N,X) of the SLLS and its
moments, and compare these results with the Lyman-alpha forest and the damped
Lyman alpha systems (DLA; absorbers with log(NHI) >= 20.3 cm^-2). We find that
that f(N,X) of the SLLS can be reasonably described with a power-law of index
alpha = -1.43^{+0.15}_{-0.16} or alpha = -1.19^{+0.20}_{-0.21} depending on
whether we set the lower N(HI) bound for the analysis at 10^{19.0} cm^-2 or
10^{19.3}$ cm^-2, respectively. The results indicate a flattening in the slope
of f(N,X) between the SLLS and DLA. We find little evidence for redshift
evolution in the shape of f(N,X) for the SLLS over the redshift range of the
sample 1.68 < z < 4.47 and only tentative evidence for evolution in the zeroth
moment of f(N,X), the line density l_lls(X). We introduce the observable
distribution function O(N,X) and its moment, which elucidates comparisons of HI
absorbers from the Lyman-alpha through to the DLA. We find that a simple three
parameter function can fit O(N,X) over the range 17.0 <= log(NHI) <=22.0. We
use these results to predict that f(N,X) must show two additional inflections
below the SLLS regime to match the observed f(N,X) distribution of the
Lyman-alpha forest. Finally, we demonstrate that SLLS contribute a minor
fraction (~15%) of the universe's hydrogen atoms and, therefore, an even small
fraction of the mass in predominantly neutral gas.Comment: 15 pages, 10 figures, accepted to the Astrophysical Journal. Revision
includes updated reference
Cosmic Concordance and Quintessence
We present a comprehensive study of the observational constraints on
spatially flat cosmological models containing a mixture of matter and
quintessence --- a time varying, spatially inhomogeneous component of the
energy density of the universe with negative pressure. Our study also includes
the limiting case of a cosmological constant. Low red shift constraints include
the Hubble parameter, baryon fraction, cluster abundance, age of the universe,
bulk velocity and shape of the mass power spectrum; intermediate red shift
constraints are due to type 1a supernovae, gravitational lensing, the Ly-a
forest, and the evolution of large scale structure; high red shift constraints
are based on cosmic microwave background temperature anisotropy. Mindful of
systematic errors, we adopt a conservative approach in applying these
constraints. We determine that quintessence models in which the matter density
parameter is 0.2 \ls \Omega_m \ls 0.5 and the effective, density-averaged
equation of state is -1 \le w \ls -0.2, are consistent with the most
reliable, current low red shift and CMB observations at the level.
Factoring in the constraint due to type 1a SNe, the range for the equation of
state is reduced to -1 \le w \ls -0.4, where this range represents models
consistent with each observational constraint at the 2 level or better
(concordance analysis). A combined maximum likelihood analysis suggests a
smaller range, -1 \le w \ls -0.6. We find that the best-fit and
best-motivated quintessence models lie near , , and spectral index , with an effective equation of state for ``tracker'' quintessence and for ``creeper''
quintessence. (abstract shortened)Comment: revised to match ApJ version; 33 pages; 20 figures, 4 in color; uses
emulateapj.st
The Increase in the Primordial He-4 Yield in the Two-Doublet Four-Neutrino Mixing Scheme
We assess the effects on Big Bang Nucleosynthesis (BBN) of lepton number
generation in the early universe resulting from the two-doublet four-neutrino
mass/mixing scheme. It has been argued that this neutrino mass/mixing
arrangement gives the most viable fit to the existing data. We study full 4 x 4
mixing matrices and show how possible symmetries in these can affect the BBN
He-4 abundance yields. Though there is as yet no consensus on the reliability
of BBN calculations with neutrino flavor mixing, we show that, in the case
where the sign of the lepton number asymmetry is unpredictable, BBN
considerations may pick out specific relationships between mixing angles. In
particular, reconciling the observed light element abundances with a
\bar\nu_\mu \bar\nu_e oscillation interpretation of LSND would allow unique
new constraints on the neutrino mixing angles in this model.Comment: 12 pages, including 4 figures, to appear in Phys. Rev.
Cosmological Constraints on Bulk Neutrinos
Recent models invoking extra spacelike dimensions inhabited by (bulk)
neutrinos are shown to have significant cosmological effects if the size of the
largest extra dimension is R > 1 fm. We consider effects on cosmic microwave
background anisotropies, big bang nucleosynthesis, deuterium and Li-6
photoproduction, diffuse photon backgrounds, and structure formation. The
resulting constraints can be stronger than either bulk graviton overproduction
constraints or laboratory constraints.Comment: matches published versio
Large extra dimensions, the galaxy power spectrum and the end of inflation
We consider the production of gravitational KK modes via cosmological
photon-photon and electron-positron annihilation in models with large
factorisable extra dimensions. We place constraints on this production using
recent results from a joint analysis of the power spectra of the 2dF Galaxy
Redshift Survey (2dFGS) and the cosmic microwave background (CMB) anisotropies.
We obtain a more accurate upper limit for the temperature corresponding to
matter-radiation equality and show that, even for the case of 6 extra
dimensions and a fundamental scale of 1 TeV, a period of inflation is required
that ends at a temperature much lower than that of the QCD phase transition.Comment: 12 pages, 2 figures, hadronic branching+typos corrected,accepted in
JHE
Constraining the Cosmological Density of Compact Objects with the Long-Term Variability of Quasars
By comparing the results from numerical microlensing simulations to the
observed long-term variability of quasars, strong upper limits on the
cosmological density of compact objects in the 0.0001-1 solar mass range may in
principle be imposed. Here, this method is generalized from the Einstein-de
Sitter universe to the currently favored Omega_M=0.3, Omega_Lambda=0.7
cosmology and applied to the latest observational samples. We show that the use
of high-redshift quasars from variability-selected samples has the potential to
substantially improve current constraints on compact objects in this mass
range. We also investigate to what extent the upper limits on such hypothetical
dark matter populations are affected by assumptions concerning the size of the
optical continuum-emitting region of quasars and the velocity dispersion of
compact objects. We find that mainly due to uncertainties in the typical value
of the source size, cosmologically significant populations of compact objects
cannot safely be ruled out with this method at the present time.Comment: 10 pages, 7 figures, accepted for publication in A&
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