4,409 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.
Observational constraints on thawing quintessence models
We use a dynamical systems approach to study thawing quintessence models,
using a multi-parameter extension of the exponential potential which can
approximate the form of typical thawing potentials. We impose observational
constraints using a compilation of current data, and forecast the tightening of
constraints expected from future dark energy surveys, as well as discussing the
relation of our results to analytical constraints already in the literature.Comment: 6 pages MNRAS style with 8 figures included. Minor updates to match
MNRAS accepted versio
The Struve-Sahade effect in the optical spectra of O-type binaries I. Main-sequence systems
We present a spectroscopic analysis of four massive binary systems that are
known or are good candidates to display the Struve-Sahade effect (defined as
the apparent strengthening of the secondary spectrum of the binary when the
star is approaching, and the corresponding weakening of the lines when it is
receding).
We use high resolution optical spectra to determine new orbital solutions and
spectral types of HD 165052, HD 100213, HD 159176 and DH Cep. As good knowledge
of the fundamental parameters of the considered systems is necessary to examine
the Struve-Sahade effect. We then study equivalent width variations in the
lines of both components of these binaries during their orbital cycle.
In the case of these four systems, variations appear in the equivalent widths
of some lines during the orbital cycle, but the definition given above can any
longer be valid, since it is now clear that the effect modifies the primary
spectrum as much as the secondary spectrum. Furthermore, the lines affected,
and the way in which they are affected, depend on the considered system. For at
least two of them (HD 100213 and HD 159176) these variations probably reflect
the ellipsoidal variable nature of the system.Comment: 12 pages, 20 figures, in press A&
Shifting the Universe: Early Dark Energy and Standard Rulers
The presence of dark energy at high redshift influences both the cosmic sound
horizon and the distance to last scattering of the cosmic microwave background.
We demonstrate that through the degeneracy in their ratio, early dark energy
can lie hidden in the CMB temperature and polarization spectra, leading to an
unrecognized shift in the sound horizon. If the sound horizon is then used as a
standard ruler, as in baryon acoustic oscillations, then the derived
cosmological parameters can be nontrivially biased. Fitting for the absolute
ruler scale (just as supernovae must be fit for the absolute candle magnitude)
removes the bias but decreases the leverage of the BAO technique by a factor 2.Comment: 6 pages, 3 figure
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