51,152 research outputs found
Dark matter cores all the way down
We use high resolution simulations of isolated dwarf galaxies to study the
physics of dark matter cusp-core transformations at the edge of galaxy
formation: M200 = 10^7 - 10^9 Msun. We work at a resolution (~4 pc minimum cell
size; ~250 Msun per particle) at which the impact from individual supernovae
explosions can be resolved, becoming insensitive to even large changes in our
numerical 'sub-grid' parameters. We find that our dwarf galaxies give a
remarkable match to the stellar light profile; star formation history;
metallicity distribution function; and star/gas kinematics of isolated dwarf
irregular galaxies. Our key result is that dark matter cores of size comparable
to the stellar half mass radius (r_1/2) always form if star formation proceeds
for long enough. Cores fully form in less than 4 Gyrs for the M200 = 10^8 Msun
and 14 Gyrs for the 10^9 Msun dwarf. We provide a convenient two parameter
'coreNFW' fitting function that captures this dark matter core growth as a
function of star formation time and the projected stellar half mass radius.
Our results have several implications: (i) we make a strong prediction that
if LCDM is correct, then 'pristine' dark matter cusps will be found either in
systems that have truncated star formation and/or at radii r > r_1/2; (ii)
complete core formation lowers the projected velocity dispersion at r_1/2 by a
factor ~2, which is sufficient to fully explain the 'too big to fail problem';
and (iii) cored dwarfs will be much more susceptible to tides, leading to a
dramatic scouring of the subhalo mass function inside galaxies and groups.Comment: 20 pages; 9 figures; final version to appear in MNRAS including typos
corrected in proo
From urban to national heat island: The effect of anthropogenic heat output on climate change in high population industrial countries
The project presented here sought to determine whether changes in anthropogenic thermal emission can have a measurable effect on temperature at the national level, taking Japan and Great Britain as type examples. Using energy consumption as a proxy for thermal emission, strong correlations (mean r2 = 0.90 and 0.89, respectively) are found between national equivalent heat output (HO) and temperature above background levels Δt averaged over 5‐ to 8‐yr periods between 1965 and 2013, as opposed to weaker correlations for CMIP5 model temperatures above background levels Δmt (mean r2 = 0.52 and 0.10). It is clear that the fluctuations in Δt are better explained by energy consumption than by present climate models, and that energy consumption can contribute to climate change at the national level on these timescales
Momentum Flow Correlations from Event Shapes: Factorized Soft Gluons and Soft-Collinear Effective Theory
The distributions of two-jet event shapes contain information on
hadronization in QCD. Near the two-jet limit, these distributions can be
described by convolutions of nonperturbative event shape functions with the
same distributions calculated in resummed perturbation theory. The shape
functions, in turn, are determined by correlations of momentum flow operators
with each other and with light-like Wilson lines, which describe the coupling
of soft, wide-angle radiation to jets. We observe that leading power
corrections to the mean values of event shapes are determined by the
correlation of a single momentum flow operator with the relevant Wilson lines.
This generalizes arguments for the universality of leading power corrections
based on the low-scale behavior of the running coupling or resummation. We also
show how a study of the angularity event shapes can provide information on
correlations involving multiple momentum flow operators, giving a window to the
system of QCD dynamics that underlies the variety of event shape functions. In
deriving these results, we review, develop and compare factorization techniques
in conventional perturbative QCD and soft-collinear effective theory (SCET). We
give special emphasis to the elimination of double counting of momentum regions
in these two formalisms.Comment: 25 pages revtex
Statistical comparison of ensemble implementations of Grover's search algorithm to classical sequential searches
We compare pseudopure state ensemble implementations, quantified by their
initial polarization and ensemble size, of Grover's search algorithm to
probabilistic classical sequential search algorithms in terms of their success
and failure probabilities. We propose a criterion for quantifying the resources
used by the ensemble implementation via the aggregate number of oracle
invocations across the entire ensemble and use this as a basis for comparison
with classical search algorithms. We determine bounds for a critical
polarization such that the ensemble algorithm succeeds with a greater
probability than the probabilistic classical sequential search. Our results
indicate that the critical polarization scales as N^(-1/4) where N is the
database size and that for typical room temperature solution state NMR, the
polarization is such that the ensemble implementation of Grover's algorithm
would be advantageous for N > 10^2
Squeezed: Why Rising Exposure to Health Care Costs Threatens the Health and Financial Well-Being of American Families
Examines U.S. healthcare costs compared with other industrialized countries, individual health insurance coverage, individual market regulations, and the impact of high deductible plans on the health of individuals with chronic disease
Gaps in Health Insurance: An All-American Problem
Presents findings from a survey that examines health insurance coverage, rising healthcare costs, and the health and financial consequences to families that experience breaks in insurance
Failure to Protect: Why the Individual Insurance Market Is Not a Viable Option for Most U.S. Families
Based on the Commonwealth Fund 2007 Biennial Health Insurance Survey, examines access to and affordability of individual insurance. Reviews obstacles to obtaining coverage, such as health issues and costs, and out-of-pocket costs of those who obtain it
Out of Options: Why So Many Workers in Small Businesses Lack Affordable Health Insurance, and How Health Care Reform Can Help
Based on the 2007 Commonwealth Fund Biennial Health Insurance Survey, examines small business employees' limited access to health insurance and contributing factors. Explores how small businesses and employees could benefit from proposed reforms
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