6,391 research outputs found
Exploring the Expansion History of the Universe
Exploring the recent expansion history of the universe promises insights into
the cosmological model, the nature of dark energy, and potentially clues to
high energy physics theories and gravitation. We examine the extent to which
precision distance-redshift observations can map out the history, including the
acceleration-deceleration transition, and the components and equations of state
of the energy density. We consider the ability to distinguish between various
dynamical scalar field models for the dark energy, as well as higher dimension
and alternate gravity theories. Finally, we present a new, advantageous
parametrization for the study of dark energy.Comment: 4 pages, 3 figures, submitted to Phys. Rev. Letter
Physics of SNeIa and Cosmology
We give an overview of the current understanding of Type Ia supernovae
relevant for their use as cosmological distance indicators. We present the
physical basis to understand their homogeneity of the observed light curves and
spectra and the observed correlations. This provides a robust method to
determine the Hubble constant, 67 +- 8 (2 sigma) km/Mpc/sec, independently from
primary distance indicators.
We discuss the uncertainties and tests which include SNe Ia based distance
determinations prior to delta-Ceph. measurements for the host galaxies. Based
on detailed models, we study the small variations from homogeneities and their
observable consequences. In combination with future data, this underlines the
suitability and promises the refinements needed to determine accurate relative
distances within 2 to 3 % and to use SNe Ia for high precision cosmology.Comment: to be published in "Stellar Candles", eds. Gieren et al. Lecture
Notes in Physics (http://link.springer.de/series/lnpp
Net ecosystem productivity and its uncertainty in a diverse boreal peatland
Net ecosystem exchange (NEE) of CO2 was measured in four peatlands along plant community, hydrologic, and water chemistry gradients from bog to rich fen in a diverse peatland complex near Thompson, Manitoba, as part of the Boreal Ecosystem-Atmosphere Study (BOREAS). A simple model for estimating growing season net ecosystem productivity (NEP) using continuous measurements of photosynthetically active radiation (PAR), and peat temperature was constructed with weekly chamber measurements of NEE from May to October 1996. The model explained 79–83% of the variation in NEE across the four sites. Model estimation and parameter uncertainty calculations were performed using nonlinear regression analyses with a maximum likelihood objective function. The model underestimated maximum NEE and respiration during the midseason when the standard errors for each parameter were greatest. On a daily basis, uncertainty in the midday NEE simulation was higher than at night. Although the magnitude of both photosynthesis and respiration rates followed the trophic gradient bog less than poor fen less than intermediate fen less than rich fen, NEP did not follow the same pattern. NEP in the bog and rich fen was close to zero, while the poor and intermediate fens had higher NEP due to a greater imbalance between uptake and release of CO2. Although all sites had a positive growing season NEP, upper and lower 95% confidence limits showed that the bog and rich fen were either a source or sink of CO2 to the atmosphere, while the sedge-dominated poor and intermediate fens were accumulating approximately 20–100 g CO2 C m−2over the 5 month period in 1996. Peatland ecosystems may switch from a net sink to a source of carbon on short timescales with small changes in soil temperature or water table position. Since the difference between production and decomposition is small, it is important to understand and quantify the magnitude of uncertainty in these measurements in order to predict the effect of climatic change on peatland carbon exchange
Phase-controlled proximity-effect in ferromagnetic Josephson junctions: calculation of DOS and electronic specific heat
We study the thermodynamic properties of a dirty ferromagnetic
SFS Josephson junction with s-wave superconducting leads in the
low-temperature regime. We employ a full numerical solution with a set of
realistic parameters and boundary conditions, considering both a uniform and
non-uniform exchange field in the form of a Bloch domain wall ferromagnetic
layer. The influence of spin-active interfaces is incorporated via a
microscopic approach. We mainly focus on how the electronic specific heat and
density of states (DOS) of such a system is affected by the \textit{proximity
effect}, which may be tuned via the superconducting phase difference. Our main
result is that it is possible to \textit{strongly modify the electronic
specific heat} of the system by changing the phase difference between the two
superconducting leads from 0 up to nearly at low temperatures. An
enhancement of the specific heat will occur for small values of
the exchange field, while for large values of the specific heat is
suppressed by increasing the phase difference between the superconducting
leads. These results are all explained in terms of the proximity-altered DOS in
the ferromagnetic region, and we discuss possible methods for experimental
detection of the predicted effect.Comment: 7 pages, 5 figure
Vegetative morphology and interfire survival strategies in the Cape Fynbos grasses
It is shown that there is a wide range of structural variation in the habit of the Arundineae and Ehrharteae of the fynbos of the Cape Floristic Region (Cape Province, South Africa). Structural differences in the bases of the fynbos grasses have been classified into four groups: swollen, knotty tillering, weak and annual. Variation in the position of the innovation buds occurs with one group having basal perennating buds, implying that all the culm material is annual, while the second group has cauline innovation buds, leading to the development of a divaricate perennial herb. The recognition of caducous, mesic (orthophyllous) and sclerophyllous leaf blades is also possible, based on leaf morphology and anatomy. These variations in growth forms allow the classification of the Cape grasses into five guilds adapted for survival in the dense fynbos vegetation that develops between the well-spaced fires in these heathlands. The following guilds have been recognized: competition avoiders that grow on rock ledges and outcrops where competition from shrubby vegetation is reduced; reseeders, that survive the protracted interfire period as seed; geophytes, that survive this period as underground organs; coppicers, that survive as small plants; and competitors, that grow tall by means of cauline innovation buds, and so are able to compete with the shrubby heath vegetation
Strongly spin-polarized current generated in Zeeman-split unconventional superconductors
We consider a thin-film normal metal/superconductor junction in the presence
of an externally applied in-plane magnetic field for several symmetries of the
superconducting order parameter. For p-wave superconductors, a strongly
spin-polarized current emerges due to an interplay between the nodal structure
of the superconducting order parameter, the existence or non-existence of
zero-energy surface states, and the Zeeman-splitting of the bands which form
superconductivity. Thus, the polarization depends strongly on the orbital
symmetry of the superconducting state. Our findings suggest a mechanism for
obtaining fully spin-polarized currents crucially involving zero-energy surface
states, not present in s-wave superconductors.Comment: 5 pages, 5 figures. Accepted for publication in Phys. Rev.
Dark energy records in lensed cosmic microwave background
We consider the weak lensing effect induced by linear cosmological
perturbations on the cosmic microwave background (CMB) polarization
anisotropies. We find that the amplitude of the lensing peak in the BB mode
power spectrum is a faithful tracer of the dark energy dynamics at the onset of
cosmic acceleration. This is due to two reasons. First, the lensing power is
non-zero only at intermediate redshifts between the observer and the source,
keeping record of the linear perturbation growth rate at the corresponding
epoch. Second, the BB lensing signal is expected to dominate over the other
sources. The lensing distortion on the TT and EE spectra do exhibit a similar
dependence on the dark energy dynamics, although those are dominated by primary
anisotropies. We investigate and quantify the effect by means of exact tracking
quintessence models, as well as parameterizing the dark energy equation of
state in terms of the present value () and its asymptotic value in the
past (); in the interval allowed by the present constraints on dark
energy, the variation of induces a significant change in the BB
mode lensing amplitude. A Fisher matrix analysis, under conservative
assumptions concerning the increase of the sample variance due to the lensing
non-Gaussian statistics, shows that a precision of order 10% on both
and is achievable by the future experiments probing a large sky
area with angular resolution and sensitivity appropriate to detect the lensing
effect on the CMB angular power spectrum. These results show that the CMB can
probe the differential redshift behavior of the dark energy equation of state,
beyond its average.Comment: New version including substantial text change, three more figures and
two more table
Gravitational Wave Sirens as a Triple Probe of Dark Energy
Gravitational wave standard sirens have been considered as precision distance
indicators to high redshift; however, at high redshift standard sirens or
standard candles such as supernovae suffer from lensing noise. We investigate
lensing noise as a signal instead and show how measurements of the maximum
demagnification (minimum convergence) probe cosmology in a highly complementary
manner to the distance itself. Revisiting the original form for minimum
convergence we quantify the bias arising from the commonly used approximation.
Furthermore, after presenting a new lensing probability function we discuss how
the width of the lensed standard siren amplitude distribution also probes
growth of structure. Thus standard sirens and candles can serve as triple
probes of dark energy, measuring both the cosmic expansion history and growth
history.Comment: 7 pages, 5 figures; v2 minor changes matching published versio
The Limits of Quintessence
We present evidence that the simplest particle-physics scalar-field models of
dynamical dark energy can be separated into distinct behaviors based on the
acceleration or deceleration of the field as it evolves down its potential
towards a zero minimum. We show that these models occupy narrow regions in the
phase-plane of w and w', the dark energy equation-of-state and its
time-derivative in units of the Hubble time. Restricting an energy scale of the
dark energy microphysics limits how closely a scalar field can resemble a
cosmological constant. These results, indicating a desired measurement
resolution of order \sigma(w')\approx (1+w), define firm targets for
observational tests of the physics of dark energy.Comment: 4 pages, 2 figure
Contiguous redshift parameterizations of the growth index
The growth rate of matter perturbations can be used to distinguish between
different gravity theories and to distinguish between dark energy and modified
gravity at cosmological scales as an explanation to the observed cosmic
acceleration. We suggest here parameterizations of the growth index as
functions of the redshift. The first one is given by that
interpolates between a low/intermediate redshift parameterization
and a high
redshift constant value. For example, our interpolated form
can be used when including the CMB to the rest of the data while
the form can be used otherwise. It is found that the
parameterizations proposed achieve a fit that is better than 0.004% for the
growth rate in a CDM model, better than 0.014% for
Quintessence-Cold-Dark-Matter (QCDM) models, and better than 0.04% for the flat
Dvali-Gabadadze-Porrati (DGP) model (with ) for the entire
redshift range up to . We find that the growth index parameters
take distinctive values for dark energy models and
modified gravity models, e.g. for the CDM model
and for the flat DGP model. This provides a means for future
observational data to distinguish between the models.Comment: 7 pages, 6 figures, matches PRD accepted versio
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