152 research outputs found
To Bin or Not To Bin: Decorrelating the Cosmic Equation of State
The physics behind the acceleration of the cosmic expansion can be elucidated
through comparison of the predictions of dark energy equations of state to
observational data. In seeking to optimize this, we investigate the advantages
and disadvantages of using principal component analysis, uncorrelated
bandpowers, and the equation of state within redshift bins. We demonstrate that
no one technique is a panacea, with tension between clear physical
interpretation from localization and from decorrelated errors, as well as model
dependence and form dependence. Specific lessons include the critical role of
proper treatment of the high redshift expansion history and the lack of a
unique, well defined signal-to-noise or figure of merit.Comment: 26 pages, 28 figure
Calibrating Dark Energy
Exploring the diversity of dark energy dynamics, we discover a calibration
relation, a uniform stretching of the amplitude of the equation of state time
variation with scale factor. This defines homogeneous families of dark energy
physics. The calibration factor has a close relation to the standard time
variation parameter w_a, and we show that the new, calibrated w_a describes
observables, i.e. distance and Hubble parameter as a function of redshift,
typically to an accuracy level of 10^{-3}. We discuss implications for figures
of merit for dark energy science programs.Comment: 9 pages, 10 figure
CMB Lensing Constraints on Neutrinos and Dark Energy
Signatures of lensing of the cosmic microwave background radiation by
gravitational potentials along the line of sight carry with them information on
the matter distribution, neutrino masses, and dark energy properties. We
examine the constraints that Planck, PolarBear, and CMBpol future data,
including from the B-mode polarization or the lensing potential, will be able
to place on these quantities. We simultaneously fit for neutrino mass and dark
energy equation of state including time variation and early dark energy
density, and compare the use of polarization power spectra with an optimal
quadratic estimator of the lensing. Results are given as a function of
systematics level from residual foreground contamination. A realistic CMBpol
experiment can effectively constrain the sum of neutrino masses to within 0.05
eV and the fraction of early dark energy to 0.002. We also present a
surprisingly simple prescription for calculating dark energy equation of state
constraints in combination with supernova distances from JDEM.Comment: 18 pages, 14 figures. Small changes made to match version to be
published in Phys. Rev.
Signature of odd-frequency pairing correlations induced by a magnetic interface
We investigate the mutual proximity effect in a normal metal contacted to a
superconductor through a magnetic interface. Analytical and self-consistent
numerical results are presented, and we consider both the diffusive and
ballistic regimes. We focus on the density of states in both the normal and
superconducting region, and find that the presence of spin-dependent
phase-shifts occurring at the interface qualitatively modifies the density of
states. In particular, we find that the proximity-induced pairing amplitudes in
the normal metal region undergo a conversion at the Fermi level from pure
even-frequency to odd-frequency. Above a critical value of the interface
spin-polarization (or, equivalently, for fixed interface spin-polarization,
above a critical interface resistance), only odd frequency correlations remain.
This is accompanied by the replacement of the familiar proximity minigap or
pseudogap in the normal layer by an enhancement of the density of states above
its normal state value for energies near the chemical potential. The robustness
of this effect towards inelastic scattering, impurity scattering, and the
depletion of the superconducting order parameter close to the interface is
investigated. We also study the inverse proximity effect in the diffusive
limit. We find that the above-mentioned conversion persists also for thin
superconducting layers comparable in size to the superconducting coherence
length , as long as the inverse proximity effect is relatively
weak. Concomitantly, we find a shift in the critical interface resistance where
the pairing conversion occurs. Our findings suggest a robust and simple method
for producing purely odd-frequency superconducting correlations, that can be
tested experimentally.Comment: 14 pages, 12 figures. Submitted to Physical Review. Chosen as
Editors' Suggestio
Future CMB Constraints on Early, Cold, or Stressed Dark Energy
We investigate future constraints on early dark energy (EDE) achievable by
the Planck and CMBPol experiments, including cosmic microwave background (CMB)
lensing. For the dark energy, we include the possibility of clustering through
a sound speed c_s^2 <1 (cold dark energy) and anisotropic stresses
parameterized with a viscosity parameter c_vis^2. We discuss the degeneracies
between cosmological parameters and EDE parameters. In particular we show that
the presence of anisotropic stresses in EDE models can substantially undermine
the determination of the EDE sound speed parameter c_s^2. The constraints on
EDE primordial energy density are however unaffected. We also calculate the
future CMB constraints on neutrino masses and find that they are weakened by a
factor of 2 when allowing for the presence of EDE, and highly biased if it is
incorrectly ignored.Comment: 12 pages, 19 figure
Inflationary Freedom and Cosmological Neutrino Constraints
The most stringent bounds on the absolute neutrino mass scale come from cosmological data. These bounds are made possible because massive relic neutrinos affect the expansion history of the universe and lead to a suppression of matter clustering on scales smaller than the associated free streaming length. However, the resulting effect on cosmological perturbations is relative to the primordial power spectrum of density perturbations from inflation, so freedom in the primordial power spectrum affects neutrino mass constraints. Using measurements of the cosmic microwave background (CMB), the galaxy power spectrum and the Hubble constant, we constrain neutrino mass and number of species for a model-independent primordial power spectrum. Describing the primordial power spectrum by a 20-node spline, we find that the neutrino mass upper limit is a factor 3 weaker than when a power law form is imposed, if only CMB data are used. The primordial power spectrum itself is constrained to better than 10% in the wave vector range k≈0.01−0.25  Mpc^(−1) . Galaxy clustering data and a determination of the Hubble constant play a key role in reining in the effects of inflationary freedom on neutrino constraints. The inclusion of both eliminates the inflationary freedom degradation of the neutrino mass bound, giving for the sum of neutrino masses Σm_ν<0.18  eV (at 95% confidence level, Planck+BOSS+H_0), approximately independent of the assumed primordial power spectrum model. When allowing for a free effective number of species, N_(eff) , the joint constraints on Σm_ν and N_(eff) are loosened by a factor 1.7 when the power law form of the primordial power spectrum is abandoned in favor of the spline parametrization
Galaxies as Fluctuations in the Ionizing Background Radiation at Low Redshift
Some Lyman continuum photons are likely to escape from most galaxies, and
these can play an important role in ionizing gas around and between galaxies,
including gas that gives rise to Lyman alpha absorption. Thus the gas
surrounding galaxies and in the intergalactic medium will be exposed to varying
amounts of ionizing radiation depending upon the distances, orientations, and
luminosities of any nearby galaxies. The ionizing background can be
recalculated at any point within a simulation by adding the flux from the
galaxies to a uniform quasar contribution. Normal galaxies are found to almost
always make some contribution to the ionizing background radiation at redshift
zero, as seen by absorbers and at random points in space. Assuming that about 2
percent of ionizing photons escape from a galaxy like the Milky Way, we find
that normal galaxies make a contribution of at least 30 to 40 percent of the
assumed quasar background. Lyman alpha absorbers with a wide range of neutral
column densities are found to be exposed to a wide range of ionization rates,
although the distribution of photoionization rates for absorbers is found to be
strongly peaked. On average, less highly ionized absorbers are found to arise
farther from luminous galaxies, while local fluctuations in the ionization rate
are seen around galaxies having a wide range of properties.Comment: 10 pages, 8 figures, references added, clarified explanation of first
two equation
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