780 research outputs found
Probing Dark Energy with Baryonic Acoustic Oscillations from Future Large Galaxy Redshift Surveys
We show that the measurement of the baryonic acoustic oscillations in large
high redshift galaxy surveys offers a precision route to the measurement of
dark energy. The cosmic microwave background provides the scale of the
oscillations as a standard ruler that can be measured in the clustering of
galaxies, thereby yielding the Hubble parameter and angular diameter distance
as a function of redshift. This, in turn, enables one to probe dark energy. We
use a Fisher matrix formalism to study the statistical errors for redshift
surveys up to z=3 and report errors on cosmography while marginalizing over a
large number of cosmological parameters including a time-dependent equation of
state. With redshifts surveys combined with cosmic microwave background
satellite data, we achieve errors of 0.037 on Omega_x, 0.10 on w(z=0.8), and
0.28 on dw(z)/dz for cosmological constant model. Models with less negative
w(z) permit tighter constraints. We test and discuss the dependence of
performance on redshift, survey conditions, and fiducial model. We find results
that are competitive with the performance of future supernovae Ia surveys. We
conclude that redshift surveys offer a promising independent route to the
measurement of dark energy.Comment: submitted to ApJ, 24 pages, LaTe
Dark energy and curvature from a future baryonic acoustic oscillation survey using the Lyman-alpha forest
We explore the requirements for a Lyman-alpha forest (LyaF) survey designed
to measure the angular diameter distance and Hubble parameter at 2~<z~<4 using
the standard ruler provided by baryonic acoustic oscillations (BAO). The goal
would be to obtain a high enough density of sources to probe the
three-dimensional density field on the scale of the BAO feature. A
percent-level measurement in this redshift range can almost double the Dark
Energy Task Force Figure of Merit, relative to the case with only a similar
precision measurement at z~1, if the Universe is not assumed to be flat. This
improvement is greater than the one obtained by doubling the size of the z~1
survey, with Planck and a weak SDSS-like z=0.3 BAO measurement assumed in each
case. Galaxy BAO surveys at z~1 may be able to make an effective LyaF
measurement simultaneously at minimal added cost, because the required number
density of quasars is relatively small. We discuss the constraining power as a
function of area, magnitude limit (density of quasars), resolution, and
signal-to-noise of the spectra. For example, a survey covering 2000 sq. deg.
and achieving S/N=1.8 per Ang. at g=23 (~40 quasars per sq. deg.) with an
R~>250 spectrograph is sufficient to measure both the radial and transverse
oscillation scales to 1.4% from the LyaF (or better, if fainter magnitudes and
possibly Lyman-break galaxies can be used). At fixed integration time and in
the sky-noise-dominated limit, a wider, noisier survey is generally more
efficient; the only fundamental upper limit on noise being the need to identify
a quasar and find a redshift. Because the LyaF is much closer to linear and
generally better understood than galaxies, systematic errors are even less
likely to be a problem.Comment: 18 pages including 6 figures, submitted to PR
Supervoid Origin of the Cold Spot in the Cosmic Microwave Background
We use a WISE-2MASS-Pan-STARRS1 galaxy catalog to search for a supervoid in
the direction of the Cosmic Microwave Background Cold Spot. We obtain
photometric redshifts using our multicolor data set to create a tomographic map
of the galaxy distribution. The radial density profile centred on the Cold Spot
shows a large low density region, extending over 10's of degrees. Motivated by
previous Cosmic Microwave Background results, we test for underdensities within
two angular radii, , and . Our data, combined with an
earlier measurement by Granett et al 2010, are consistent with a large supervoid with centered at . Such a supervoid, constituting a
fluctuation in the model, is a plausible cause
for the Cold Spot.Comment: 4 pages, 2 figures, Proceedings of IAU 306 Symposium: Statistical
Challenges in 21st Century Cosmolog
The Dynamics of the Neuropeptide Y Receptor Type 1 Investigated by Solid-State NMR and Molecular Dynamics Simulation
We report data on the structural dynamics of the neuropeptide Y (NPY) G-protein-coupled receptor (GPCR) type 1 (Y1R), a typical representative of class A peptide ligand GPCRs, using a combination of solid-state NMR and molecular dynamics (MD) simulation. First, the equilibrium dynamics of Y1R were studied using 15N-NMR and quantitative determination of 1H-13C order parameters through the measurement of dipolar couplings in separated-local-field NMR experiments. Order parameters reporting the amplitudes of the molecular motions of the C-H bond vectors of Y1R in DMPC membranes are 0.57 for the Cα sites and lower in the side chains (0.37 for the CH2 and 0.18 for the CH3 groups). Different NMR excitation schemes identify relatively rigid and also dynamic segments of the molecule. In monounsaturated membranes composed of longer lipid chains, Y1R is more rigid, attributed to a higher hydrophobic thickness of the lipid membrane. The presence of an antagonist or NPY has little influence on the amplitude of motions, whereas the addition of agonist and arrestin led to a pronounced rigidization. To investigate Y1R dynamics with site resolution, we conducted extensive all-atom MD simulations of the apo and antagonist-bound state. In each state, three replicas with a length of 20 μs (with one exception, where the trajectory length was 10 μs) were conducted. In these simulations, order parameters of each residue were determined and showed high values in the transmembrane helices, whereas the loops and termini exhibit much lower order. The extracellular helix segments undergo larger amplitude motions than their intracellular counterparts, whereas the opposite is observed for the loops, Helix 8, and termini. Only minor differences in order were observed between the apo and antagonist-bound state, whereas the time scale of the motions is shorter for the apo state. Although these relatively fast motions occurring with correlation times of ns up to a few µs have no direct relevance for receptor activation, it is believed that they represent the prerequisite for larger conformational transitions in proteins
Large-Scale Anisotropic Correlation Function of SDSS Luminous Red Galaxies
We study the large-scale anisotropic two-point correlation function using
46,760 luminous red galaxies at redshifts 0.16 -- 0.47 from the Sloan Digital
Sky Survey. We measure the correlation function as a function of separations
parallel and perpendicular to the line-of-sight in order to take account of
anisotropy of the large-scale structure in redshift space. We find a slight
signal of baryonic features in the anisotropic correlation function, i.e., a
``baryon ridge'' which corresponds to a baryon acoustic peak in the spherically
averaged correlation function which has already been reported using the same
sample. The baryon ridge has primarily a spherical structure with a known
radius in comoving coordinates. It enables us to divide the redshift distortion
effects into dynamical and geometrical components and provides further
constraints on cosmological parameters, including the dark energy
equation-of-state. With an assumption of a flat cosmology, we find
the best-fit values of and
(68% C.L.) when we use the overall
shape of the anisotropic correlation function of 40 including a
scale of baryon acoustic oscillations. When an additional assumption
is adopted, we obtain and . These constraints
are estimated only from our data of the anisotropic correlation function, and
they agree quite well with values both from the cosmic microwave background
(CMB) anisotropies and from other complementary statistics using the LRG
sample. With the CMB prior from the 3 year WMAP results, we give stronger
constraints on those parameters.Comment: 11 pages, 9 figures, 1 table, typo corrected, references added with
respect to published versio
2006 SQ372: A Likely Long-Period Comet from the Inner Oort Cloud
We report the discovery of a minor planet (2006 SQ372) on an orbit with a
perihelion of 24 AU and a semimajor axis of 796 AU. Dynamical simulations show
that this is a transient orbit and is unstable on a timescale of 200 Myrs.
Falling near the upper semimajor axis range of the scattered disk and the lower
semimajor axis range of the Oort Cloud, previous membership in either class is
possible. By modeling the production of similar orbits from the Oort Cloud as
well as from the scattered disk, we find that the Oort Cloud produces 16 times
as many objects on SQ372-like orbits as the scattered disk. Given this result,
we believe this to be the most distant long-period comet ever discovered.
Furthermore, our simulation results also indicate that 2000 OO67 has had a
similar dynamical history. Unaffected by the "Jupiter-Saturn Barrier," these
two objects are most likely long-period comets from the inner Oort Cloud
Continuously-variable survival exponent for random walks with movable partial reflectors
We study a one-dimensional lattice random walk with an absorbing boundary at
the origin and a movable partial reflector. On encountering the reflector, at
site x, the walker is reflected (with probability r) to x-1 and the reflector
is simultaneously pushed to x+1. Iteration of the transition matrix, and
asymptotic analysis of the probability generating function show that the
critical exponent delta governing the survival probability varies continuously
between 1/2 and 1 as r varies between 0 and 1. Our study suggests a mechanism
for nonuniversal kinetic critical behavior, observed in models with an infinite
number of absorbing configurations.Comment: 5 pages, 3 figure
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