612 research outputs found
Constraining The Universal Lepton Asymmetry
The relic cosmic background neutrinos accompanying the cosmic microwave
background (CMB) photons may hide a universal lepton asymmetry orders of
magnitude larger than the universal baryon asymmetry. At present, the only
direct way to probe such an asymmetry is through its effect on the abundances
of the light elements produced during primordial nucleosynthesis. The relic
light element abundances also depend on the baryon asymmetry, parameterized by
the baryon density parameter (eta_B = n_B/n_gamma = 10^(-10)*eta_10), and on
the early-universe expansion rate, parameterized by the expansion rate factor
(S = H'/H) or, equivalently by the effective number of neutrinos (N_nu = 3 +
43(S^2 - 1)/7). We use data from the CMB (and Large Scale Structure: LSS) along
with the observationally-inferred relic abundances of deuterium and helium-4 to
provide new bounds on the universal lepton asymmetry, finding for eta_L, the
analog of eta_B, 0.072 +/- 0.053 if it is assumed that N_nu = 3 and, 0.115 +/-
0.095 along with N_nu = 3.3^{+0.7}_{-0.6}, if N_nu is free to vary
Spontaneous baryogenesis in flat directions
We discuss a spontaneous baryogenesis mechanism in flat directions. After
identifying the Nambu-Goldstone mode which derivatively couples to the
associated (1) current and rotates due to the A-term, we show that
spontaneous baryogenesis can be naturally realized in the context of the flat
direction. As applications, we discuss two scenarios of baryogenesis in detail.
One is baryogenesis in a flat direction with a vanishing charge,
especially, with neither baryon nor lepton charge, which was recently proposed
by Chiba and the present authors. The other is a baryogenesis scenario
compatible with a large lepton asymmetry.Comment: 10 pages, no figure, the version accepted to Phys. Rev. D; a few
explanatory comments are adde
Gas Accretion via Lyman Limit Systems
In cosmological simulations, a large fraction of the partial Lyman limit
systems (pLLSs; 16<log N(HI)<17.2) and LLSs (17.2log N(HI)<19) probes
large-scale flows in and out of galaxies through their circumgalactic medium
(CGM). The overall low metallicity of the cold gaseous streams feeding galaxies
seen in these simulations is the key to differentiating them from metal rich
gas that is either outflowing or being recycled. In recent years, several
groups have empirically determined an entirely new wealth of information on the
pLLSs and LLSs over a wide range of redshifts. A major focus of the recent
research has been to empirically determine the metallicity distribution of the
gas probed by pLLSs and LLSs in sizable and representative samples at both low
(z2) redshifts. Here I discuss unambiguous evidence for
metal-poor gas at all z probed by the pLLSs and LLSs. At z<1, all the pLLSs and
LLSs so far studied are located in the CGM of galaxies with projected distances
<100-200 kpc. Regardless of the exact origin of the low-metallicity pLLSs/LLSs,
there is a significant mass of cool, dense, low-metallicity gas in the CGM that
may be available as fuel for continuing star formation in galaxies over cosmic
time. As such, the metal-poor pLLSs and LLSs are currently among the best
observational evidence of cold, metal-poor gas accretion onto galaxies.Comment: Invited review to appear in Gas Accretion onto Galaxies, Astrophysics
and Space Science Library, eds. A. J. Fox & R. Dav\'e, to be published by
Springe
Precision Primordial He Measurement with CMB Experiments
Big bang nucleosynthesis (BBN) and the cosmic microwave background (CMB) are
two major pillars of cosmology. Standard BBN accurately predicts the primordial
light element abundances (He, D, He and Li), depending on one
parameter, the baryon density. Light element observations are used as a
baryometers. The CMB anisotropies also contain information about the content of
the universe which allows an important consistency check on the Big Bang model.
In addition CMB observations now have sufficient accuracy to not only determine
the total baryon density, but also resolve its principal constituents, H and
He. We present a global analysis of all recent CMB data, with special
emphasis on the concordance with BBN theory and light element observations. We
find and
(fraction of baryon mass as He) using CMB data alone, in agreement with
He abundance observations. With this concordance established we show that
the inclusion of BBN theory priors significantly reduces the volume of
parameter space. In this case, we find
and . We also find that the inclusion of deuterium
abundance observations reduces the and ranges by a factor
of 2. Further light element observations and CMB anisotropy experiments
will refine this concordance and sharpen BBN and the CMB as tools for precision
cosmology.Comment: 7 pages, 3 color figures made minor changes to bring inline with
journal versio
Early-universe constraints on a time-varying fine structure constant
Higher-dimensional theories have the remarkable feature of predicting a time
(and hence redshift) dependence of the `fundamental' four dimensional constants
on cosmological timescales. In this paper we update the bounds on a possible
variation of the fine structure constant alpha at the time of BBN (z =10^10)
and CMB (z=10^3). Using the recently-released high-resolution CMB anisotropy
data and the latest estimates of primordial abundances of 4He and D, we do not
find evidence for a varying alpha at more than one-sigma level at either epoch.Comment: 5 pages, 1 figure, minor misprints corrected, references added. The
analysis has been updated using new BOOMERanG and DASI data on CMB anisotrop
Leaf reflectance spectra capture the evolutionary history of seed plants
Leaf reflection spectra have been increasingly used to assess plant diversity. However, we do not yet understand how spectra vary across the tree of life or how the evolution of leaf traits affects the differentiation of spectra among species and lineages. Here we describe a framework that integrates spectra with phylogenies and apply it to aglobal dataset of over 16 000 leaf-level spectra (400â2400 nm) for 544 seed plant species. We test for phylogenetic signal in spectra, evaluate their ability to classify lineages, and characterize their evolutionary dynamics. We show that phylogenetic signal is present in leaf spectra but that the spectral regions most strongly associated with the phylogeny vary among lineages. Despite among-lineage heterogeneity, broad plant groups, orders, and families can be identified from reflectance spectra. Evolutionary models also reveal that different spectral regions evolve at different rates and under different constraint levels, mirroring the evolution of their underlying traits. Leaf spectra capture the phylogenetic history of seed plants and the evolutionary dynamics of leaf chemistry and structure. Consequently, spectra have the potential to provide breakthrough assessments of leaf evolution and plant phylogenetic diversity at global scales
Constraining the dark energy with galaxy clusters X-ray data
The equation of state characterizing the dark energy component is constrained
by combining Chandra observations of the X-ray luminosity of galaxy clusters
with independent measurements of the baryonic matter density and the latest
measurements of the Hubble parameter as given by the HST key project. By
assuming a spatially flat scenario driven by a "quintessence" component with an
equation of state we place the following limits on the
cosmological parameters and : (i) and (1) if the
equation of state of the dark energy is restricted to the interval (\emph{usual} quintessence) and (ii) and
() if violates the null energy condition and assume values (\emph{extended} quintessence or ``phantom'' energy). These results are in
good agreement with independent studies based on supernovae observations,
large-scale structure and the anisotropies of the cosmic background radiation.Comment: 6 pages, 4 figures, LaTe
The Interstellar Medium of GRB Host Galaxies I. Echelle Spectra of Swift GRB Afterglows
We present optical echelle spectra of four gamma-ray burst (GRB) afterglows
(GRB 050730, GRB 050820, GRB 051111, and GRB 060418) discovered during the
first 1.5 years of operation of the Swift satellite and localized by either the
Swift telescope or follow-up ground-based imaging. We analyze the spectra to
derive accurate column density measurements for the transitions arising in the
interstellar medium (ISM) of the GRB host galaxies. These measurements can be
used to constrain the physical properties of the ISM including the metallicity,
dust-to-gas ratio, ionization state, and chemical abundances of the gas. We
also present measurements of the strong MgII systems in the GRB afterglow
spectra. With the publication of this paper, we provide the first data release
of echelle afterglow spectra by the GRAASP collaboration to the general
community.Comment: 52 pages, 18 figures. Accepted to ApJS. The spectra will be published
with the paper and is also available upon request. A complete version with
full-resolution figures is available at
http://lambda.uchicago.edu/public/tmp/echelle.pd
Update on neutrino mixing in the early Universe
From the current cosmological observations of CMB and nuclear abundances we
show, with an analytic procedure, that the total effective number of extra
neutrino species . We also describe the possible
signatures of non standard effects that could be revealed in future CMB
observations. This cosmological information is then applied to neutrino mixing
models. Taking into account the recent results from the SNO and SuperKamiokande
experiments, disfavouring pure active to sterile neutrino oscillations, we show
that all 4 neutrino mixing models, both of 2+2 and 3+1 type, lead to a full
thermalization of the sterile neutrino flavor. Moreover such a sterile neutrino
production excludes the possibility of an electron neutrino asymmetry
generation and we conclude that , in
disagreement with the cosmological bound. This result is valid under the
assumption that the initial neutrino asymmetries are small. We suggest the
existence of a second sterile neutrino flavor, with mixing properties such to
generate a large electron neutrino asymmetry, as a possible way out.Comment: 29 pages, 3 figures; to appear on Phys.Rev.D; added discussion (at
page 19) and references; typos correcte
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