24,936 research outputs found
Supernova Constraints on Models of Neutrino Dark Energy
In this paper we use the recently released Type Ia Supernova (SNIa) data to
constrain the interactions between the neutrinos and the dark energy scalar
fields. In the analysis we take the dark energy scalars to be either
Quintessence-like or Phantom-like. Our results show the data mildly favor a
model where the neutrinos couple to a phantom-like dark energy scalar, which
implies the equation of state of the coupled system behaves like Quintom
scenario in the sense of parameter degeneracy. We find future observations like
SNAP are potentially promising to measure the couplings between neutrino and
dark energy.Comment: Typos fixed and references updated. Version pressed in PR
Photometric properties and luminosity function of nearby massive early-type galaxies
We perform photometric analyses for a bright early-type galaxy (ETG) sample
with 2949 galaxies ( mag) in the redshift range of 0.05 to
0.15, drawn from the SDSS DR7 with morphological classification from Galaxy Zoo
1. We measure the Petrosian and isophotal magnitudes, as well as the
corresponding half-light radius for each galaxy. We find that for brightest
galaxies ( mag), our Petrosian magnitudes, and isophotal
magnitudes to 25 and 1\% of the sky brightness are on
average 0.16 mag, 0.20 mag, and 0.26 mag brighter than the SDSS Petrosian
values, respectively. In the first case the underestimations are caused by
overestimations in the sky background by the SDSS PHOTO algorithm, while the
latter two are also due to deeper photometry. Similarly, the typical half-light
radii () measured by the SDSS algorithm are smaller than our
measurements. As a result, the bright-end of the -band luminosity function
is found to decline more slowly than previous works. Our measured luminosity
densities at the bright end are more than one order of magnitude higher than
those of Blanton et al. (2003), and the stellar mass densities at and are a few tenths
and a factor of few higher than those of Bernardi et al. (2010). These results
may significantly alleviate the tension in the assembly of massive galaxies
between observations and predictions of the hierarchical structure formation
model.Comment: 43 pages, 14 figures, version accepted for publication in the
Astrophysical Journa
Dominant Eigenvalue-Eigenvector Pair Estimation via Graph Infection
We present a novel method to estimate the dominant eigenvalue and eigenvector
pair of any non-negative real matrix via graph infection. The key idea in our
technique lies in approximating the solution to the first-order matrix ordinary
differential equation (ODE) with the Euler method. Graphs, which can be
weighted, directed, and with loops, are first converted to its adjacency matrix
A. Then by a naive infection model for graphs, we establish the corresponding
first-order matrix ODE, through which A's dominant eigenvalue is revealed by
the fastest growing term. When there are multiple dominant eigenvalues of the
same magnitude, the classical power iteration method can fail. In contrast, our
method can converge to the dominant eigenvalue even when same-magnitude
counterparts exist, be it complex or opposite in sign. We conduct several
experiments comparing the convergence between our method and power iteration.
Our results show clear advantages over power iteration for tree graphs,
bipartite graphs, directed graphs with periods, and Markov chains with
spider-traps. To our knowledge, this is the first work that estimates dominant
eigenvalue and eigenvector pair from the perspective of a dynamical system and
matrix ODE. We believe our method can be adopted as an alternative to power
iteration, especially for graphs.Comment: 13 pages, 8 figures, 3 table
High-Redshift Cosmography
We constrain the parameters describing the kinematical state of the universe
using a cosmographic approach, which is fundamental in that it requires a very
minimal set of assumptions (namely to specify a metric) and does not rely on
the dynamical equations for gravity. On the data side, we consider the most
recent compilations of Supernovae and Gamma Ray Bursts catalogues. This allows
to further extend the cosmographic fit up to , i.e. up to redshift for
which one could start to resolve the low z degeneracy among competing
cosmological models. In order to reliably control the cosmographic approach at
high redshifts, we adopt the expansion in the improved parameter .
This series has the great advantage to hold also for and hence it is
the appropriate tool for handling data including non-nearby distance
indicators. We find that Gamma Ray Bursts, probing higher redshifts than
Supernovae, have constraining power and do require (and statistically allow) a
cosmographic expansion at higher order than Supernovae alone. Exploiting the
set of data from Union and GRBs catalogues, we show (for the first time in a
purely cosmographic approach parametrized by deceleration , jerk ,
snap ) a definitively negative deceleration parameter up to the
3 confidence level. We present also forecasts for realistic data sets
that are likely to be obtained in the next few years.Comment: 16 pages, 6 figures, 3 tables. Improved version matching the
published one, additional comments and reference
Simulation of the solute transport and microstructure evolution during the selective laser melting process
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