9 research outputs found
\u3cem\u3eN\u3c/em\u3e-Body Simulations of Alternative Gravity Models
Theories in which gravity is weaker on cosmological scales have been proposed to explain the observed acceleration of the universe. The nonlinear regime in such theories is not well studied, though it is likely that observational tests of structure formation will lie in this regime. A class of alternative gravity theories may be approximated by modifying Poisson’s equation. We have run N-body simulations of a set of such models to study the nonlinear clustering of matter on 1–100 Mpc scales. We find that nonlinear gravity enhances the deviations of the power spectrum of these models from standard gravity. This occurs due to mode coupling, so that models with an excess or deficit of large-scale power (at k \u3c 0.2 Mpc-1) lead to deviations in the power spectrum at smaller scales as well (up to k ~ 1 Mpc-1), even though the linear spectra match very closely on the smaller scales. This makes it easier to distinguish such models from general relativity using the three-dimensional power spectrum probed by galaxy surveys and the weak lensing power spectrum. If the potential for light deflection is modified in the same way as the potential that affects the dark matter, then weak lensing constrains deviations from gravity even more strongly. Our simulations show that, even with a modified potential, gravitational evolution is approximately universal. Based on this, the Peacock-Dodds approach can be adapted to get an analytical fit for the nonlinear power spectra of alternative gravity models, though the recent Smith et al. formula is less successful. Our conclusions extend to models with modifications of gravity on scales of 1–20 Mpc. We also use a way of measuring projected power spectra from simulations that lowers the sample variance, so that fewer realizations are needed to reach a desired level of accuracy
N-Body Simulations of Alternate Gravity Models
Theories in which gravity is weaker on cosmological scales have been proposed
to explain the observed acceleration of the universe. The nonlinear regime in
such theories is not well studied, though it is likely that observational tests
of structure formation will lie in this regime. A class of alternate gravity
theories may be approximated by modifying Poisson's equation. We have run
N-body simulations of a set of such models to study the nonlinear clustering of
matter on 1-100 Mpc scales. We find that nonlinear gravity enhances the
deviations of the power spectrum of these models from standard gravity. This
occurs due to mode-coupling, so that models with an excess or deficit of
large-scale power (at k < 0.2/Mpc) lead to deviations in the power spectrum at
smaller scales as well (up to k ~ 1/Mpc), even though the linear spectra match
very closely on the smaller scales. This makes it easier to distinguish such
models from general relativity using the three-dimensional power spectrum
probed by galaxy surveys and the weak lensing power spectrum. If the potential
for light deflection is modified in the same way as the potential that affects
the dark matter, then weak lensing constrains deviations from gravity even more
strongly. Our simulations show that even with a modified potential,
gravitational evolution is approximately universal. Based on this, the
Peacock-Dodds approach can be adapted to get an analytical fit for the
nonlinear power spectra of alternate gravity models, though the recent Smith et
al formula is less successful. Our conclusions extend to models with
modifications of gravity on scales of 1-20 Mpc. We also use a way of measuring
projected power spectra from simulations that lowers the sample variance, so
that fewer realizations are needed to reach a desired level of accuracy.Comment: 26 pages, 10 figures, matches published versio
Spherical Collapse and Cluster Counts in Modified Gravity Models
Modifications to the gravitational potential affect the nonlinear
gravitational evolution of large scale structures in the Universe. To
illustrate some generic features of such changes, we study the evolution of
spherically symmetric perturbations when the modification is of Yukawa type;
this is non-trivial, because we should not and do not assume that Birkhoff's
theorem applies. We then show how to estimate the abundance of virialized
objects in such models. Comparison with numerical simulations shows reasonable
agreement: When normalized to have the same fluctuations at early times, weaker
large scale gravity produces fewer massive halos. However, the opposite can be
true for models that are normalized to have the same linear theory power
spectrum today, so the abundance of rich clusters potentially places
interesting constraints on such models. Our analysis also indicates that the
formation histories and abundances of sufficiently low mass objects are
unchanged from standard gravity. This explains why simulations have found that
the nonlinear power-spectrum at large k is unaffected by such modifications to
the gravitational potential. In addition, the most massive objects in
CMB-normalized models with weaker gravity are expected to be similar to the
high-redshift progenitors of the most massive objects in models with stronger
gravity. Thus, the difference between the cluster and field galaxy populations
is expected to be larger in models with stronger large-scale gravity.Comment: 9 pages, 8 figures Accepted by Phys. Rev.
Photometric Redshifts with Surface Brightness Priors
We use galaxy surface brightness as prior information to improve photometric
redshift (photo-z) estimation. We apply our template-based photo-z method to
imaging data from the ground-based VVDS survey and the space-based GOODS field
from HST, and use spectroscopic redshifts to test our photometric redshifts for
different galaxy types and redshifts. We find that the surface brightness prior
eliminates a large fraction of outliers by lifting the degeneracy between the
Lyman and 4000 Angstrom breaks. Bias and scatter are improved by about a factor
of 2 with the prior for both the ground and space data. Ongoing and planned
surveys from the ground and space will benefit, provided that care is taken in
measurements of galaxy sizes and in the application of the prior. We discuss
the image quality and signal-to-noise requirements that enable the surface
brightness prior to be successfully applied.Comment: 15 pages, 13 figures, matches published versio
Spitzer MIPS 24 and 70 micron Imaging near the South Ecliptic Pole: Maps and Source Catalogs
We have imaged an 11.5 sq. deg. region of sky towards the South Ecliptic Pole
(RA = 04h43m, Dec = -53d40m, J2000) at 24 and 70 microns with MIPS, the
Multiband Imaging Photometer for Spitzer. This region is coincident with a
field mapped at longer wavelengths by AKARI and the Balloon-borne Large
Aperture Submillimeter Telescope. We discuss our data reduction and source
extraction procedures. The median depths of the maps are 47 microJy/beam at 24
micron and 4.3 mJy/beam at 70 micron. At 24 micron, we identify 93098 point
sources with signal-to-noise ratio (SNR) >5, and an additional 63 resolved
galaxies; at 70 micron, we identify 891 point sources with SNR >6. From
simulations, we determine a false detection rate of 1.8% (1.1%) for the 24
micron (70 micron) catalog. The 24 and 70 micron point-source catalogs are 80%
complete at 230 microJy and 11 mJy, respectively. These mosaic images and
source catalogs will be available to the public through the NASA/IPAC Infrared
Science Archive.Comment: 30 pages, 9 figures, 4 tables. Submitted to ApJS. Maps and catalogs
can be downloaded from
http://blastexperiment.info/release/SEP_MIPS/sep-mips.php, and will be soon
be available through IRS
Finishing the euchromatic sequence of the human genome
The sequence of the human genome encodes the genetic instructions for human physiology, as well as rich information about human evolution. In 2001, the International Human Genome Sequencing Consortium reported a draft sequence of the euchromatic portion of the human genome. Since then, the international collaboration has worked to convert this draft into a genome sequence with high accuracy and nearly complete coverage. Here, we report the result of this finishing process. The current genome sequence (Build 35) contains 2.85 billion nucleotides interrupted by only 341 gaps. It covers ∼99% of the euchromatic genome and is accurate to an error rate of ∼1 event per 100,000 bases. Many of the remaining euchromatic gaps are associated with segmental duplications and will require focused work with new methods. The near-complete sequence, the first for a vertebrate, greatly improves the precision of biological analyses of the human genome including studies of gene number, birth and death. Notably, the human enome seems to encode only 20,000-25,000 protein-coding genes. The genome sequence reported here should serve as a firm foundation for biomedical research in the decades ahead
A Focus on the Right Atrium
We report a case of a 70-year-old woman who presented for a cavotricuspid isthmus atrial flutter ablation that was aborted prematurely. On subsequent imaging, she was discovered to have a right atrial diverticulum, which was present on prior imaging but not reported, likely due to unfamiliarity with the entity. (Level of Difficulty: Intermediate.