1,927 research outputs found
Nonparametric Methods in Astronomy: Think, Regress, Observe -- Pick Any Three
Telescopes are much more expensive than astronomers, so it is essential to
minimize required sample sizes by using the most data-efficient statistical
methods possible. However, the most commonly used model-independent techniques
for finding the relationship between two variables in astronomy are flawed. In
the worst case they can lead without warning to subtly yet catastrophically
wrong results, and even in the best case they require more data than necessary.
Unfortunately, there is no single best technique for nonparametric regression.
Instead, we provide a guide for how astronomers can choose the best method for
their specific problem and provide a python library with both wrappers for the
most useful existing algorithms and implementations of two new algorithms
developed here.Comment: 19 pages, PAS
Testing Inflation: A Bootstrap Approach
We note that the essential idea of inflation, that the universe underwent a
brief period of accelerated expansion followed by a long period of decelerated
expansion, can be encapsulated in a "closure condition" which relates the
amount of accelerated expansion during inflation to the amount of decelerated
expansion afterward. We present a protocol for systematically testing the
validity of this condition observationally.Comment: 4 pages, 2 figures, matches Phys. Rev. Lett. versio
The Cosmic Microwave Background and the Stellar Initial Mass Function
We argue that an increased temperature in star-forming clouds alters the
stellar initial mass function to be more bottom-light than in the Milky Way. At
redshifts , heating from the cosmic microwave background radiation
produces this effect in all galaxies, and it is also present at lower redshifts
in galaxies with very high star formation rates (SFRs). A failure to account
for it means that at present, photometric template fitting likely overestimates
stellar masses and star formation rates for the highest-redshift and
highest-SFR galaxies. In addition this may resolve several outstanding problems
in the chemical evolution of galactic halos.Comment: 9 pages, 5 figures. Published in MNRAS. Added further reference
Local growth of icosahedral quasicrystalline tilings
Icosahedral quasicrystals (IQCs) with extremely high degrees of translational
order have been produced in the laboratory and found in naturally occurring
minerals, yet questions remain about how IQCs form. In particular, the
fundamental question of how locally determined additions to a growing cluster
can lead to the intricate long-range correlations in IQCs remains open. In
answer to this question, we have developed an algorithm that is capable of
producing a perfectly ordered IQC, yet relies exclusively on local rules for
sequential, face-to-face addition of tiles to a cluster. When the algorithm is
seeded with a special type of cluster containing a defect, we find that growth
is forced to infinity with high probability and that the resultant IQC has a
vanishing density of defects. The geometric features underlying this algorithm
can inform analyses of experimental systems and numerical models that generate
highly ordered quasicrystals.Comment: 13 pages, 15 figures, 1 tabl
Interacting dark energy, holographic principle and coincidence problem
The interacting and holographic dark energy models involve two important
quantities. One is the characteristic size of the holographic bound and the
other is the coupling term of the interaction between dark energy and dark
matter. Rather than fixing either of them, we present a detailed study of
theoretical relationships among these quantities and cosmological parameters as
well as observational constraints in a very general formalism. In particular,
we argue that the ratio of dark matter to dark energy density depends on the
choice of these two quantities, thus providing a mechanism to change the
evolution history of the ratio from that in standard cosmology such that the
coincidence problem may be solved. We investigate this problem in detail and
construct explicit models to demonstrate that it may be alleviated provided
that the interacting term and the characteristic size of holographic bound are
appropriately specified. Furthermore, these models are well fitted with the
current observation at least in the low red-shift region.Comment: 20 pages, 3 figure
Gravitational Wave Spectrum Induced by Primordial Scalar Perturbations
We derive the complete spectrum of gravitational waves induced by primordial
scalar perturbations ranging over all observable wavelengths. This
scalar-induced contribution can be computed directly from the observed scalar
perturbations and general relativity and is, in this sense, independent of the
cosmological model for generating the perturbations. The spectrum is
scale-invariant on small scales, but has an interesting scale-dependence on
large and intermediate scales, where scalar-induced gravitational waves do not
redshift and are hence enhanced relative to the background density of the
Universe. This contribution to the tensor spectrum is significantly different
in form from the direct model-dependent primordial tensor spectrum and,
although small in magnitude, it dominates the primordial signal for some
cosmological models. We confirm our analytical results by direct numerical
integration of the equations of motion.Comment: 19 pages, 5 figure
A Highly Consistent Framework for the Evolution of the Star-Forming "Main Sequence" from z~0-6
Using a compilation of 25 studies from the literature, we investigate the
evolution of the star-forming galaxy (SFG) Main Sequence (MS) in stellar mass
and star formation rate (SFR) out to . After converting all
observations to a common set of calibrations, we find a remarkable consensus
among MS observations ( dex 1 interpublication scatter). By
fitting for time evolution of the MS in bins of constant mass, we deconvolve
the observed scatter about the MS within each observed redshift bins. After
accounting for observed scatter between different SFR indicators, we find the
width of the MS distribution is dex and remains constant over cosmic
time. Our best fits indicate the slope of the MS is likely time-dependent, with
our best fit , with the age of the Universe in Gyr. We use our fits to create
empirical evolutionary tracks in order to constrain MS galaxy star formation
histories (SFHs), finding that (1) the most accurate representations of MS SFHs
are given by delayed- models, (2) the decline in fractional stellar mass
growth for a "typical" MS galaxy today is approximately linear for most of its
lifetime, and (3) scatter about the MS can be generated by galaxies evolving
along identical evolutionary tracks assuming an initial spread in
formation times of Gyr.Comment: 59 pages, 10 tables, 12 figures, accepted to ApJS; v2, slight changes
to text, added new figure and fit
Hyperuniformity of Quasicrystals
Hyperuniform systems, which include crystals, quasicrystals and special
disordered systems, have attracted considerable recent attention, but rigorous
analyses of the hyperuniformity of quasicrystals have been lacking because the
support of the spectral intensity is dense and discontinuous. We employ the
integrated spectral intensity, , to quantitatively characterize the
hyperuniformity of quasicrystalline point sets generated by projection methods.
The scaling of as tends to zero is computed for one-dimensional
quasicrystals and shown to be consistent with independent calculations of the
variance, , in the number of points contained in an interval of
length . We find that one-dimensional quasicrystals produced by projection
from a two-dimensional lattice onto a line of slope fall into distinct
classes determined by the width of the projection window. For a countable dense
set of widths, ; for all others, . This
distinction suggests that measures of hyperuniformity define new classes of
quasicrystals in higher dimensions as well.Comment: 12 pages, 14 figure
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