16976292
Self-calibration of cluster dark energy studies: Observable-mass distribution
The exponential sensitivity of cluster number counts to the properties of the dark energy implies a comparable sensitivity to not only the mean but also the actual distribution of an observable-mass proxy given the true cluster mass. For example a 25% scatter in mass can provide a $\ensuremath{\sim}50%$ change in the number counts at $z\ensuremath{\sim}2$ for the upcoming SPT survey. Uncertainty in the scatter of this amount would degrade dark energy constraints to uninteresting levels. Given the shape of the actual mass function, the properties of the distribution may be internally monitored by the shape of the observable mass function. As a proof of principle, for a simple mass-independent Gaussian distribution the scatter may be self-calibrated to allow a measurement of the dark energy equation of state of $\ensuremath{\sigma}(w)\ensuremath{\sim}0.1$. External constraints on the mass variance of the distribution that are more accurate than $\ensuremath{\Delta}{\ensuremath{\sigma}}_{\mathrm{ln}﻿M}^{2}l0.01$ at $z\ensuremath{\sim}1$ can further improve constraints by up to a factor of 2. More generally, cluster counts and their sample variance measured as a function of the observable provide internal consistency checks on the assumed form of the observable-mass distribution that will protect against misinterpretation of the dark energy constraints.
17593627
PHOTOMETRIC REDSHIFT ERROR ESTIMATORS
Photometric redshift (photo-z) estimates are playing an increasingly important role in extragalactic astronomy and cosmology. Crucial to many photo-z applications is the accurate quantification of photometric redshift errors and their distributions, including identification of likely catastrophic failures in photo-z estimates. We consider several methods of estimating photo-z errors, and propose new training-set based error estimators based on spectroscopic training set data. Using data from the Sloan Digital Sky Survey and simulations of the Dark Energy Survey as examples, we show that this method provides a robust, relatively unbiased estimate of photo-z errors. We show that culling objects with large, accurately estimated photo-z errors from a sample can reduce the incidence of catastrophic photo-z failures.
15689441
Cross-correlation Weak Lensing of SDSS Galaxy Clusters. I. Measurements
This is the first in a series of papers on the weak lensing effect caused by clusters of galaxies in Sloan Digital Sky Survey. The photometrically selected cluster sample, known as MaxBCG, includes ~130,000 objects between redshift 0.1 and 0.3, ranging in size from small groups to massive clusters. We split the clusters into bins of richness and luminosity and stack the surface density contrast to produce mean radial profiles. The mean profiles are detected over a range of scales, from the inner halo (25 kpc h^(–1)) well into the surrounding large-scale structure (30 Mpc h^(–1)), with a significance of 15 to 20 in each bin. The signal over this large range of scales is best interpreted in terms of the cluster-mass cross-correlation function. We pay careful attention to sources of systematic error, correcting for them where possible. The resulting signals are calibrated to the ~10% level, with the dominant remaining uncertainty being the redshift distribution of the background sources. We find that the profiles scale strongly with richness and luminosity. We find that the signal within a given richness bin depends upon luminosity, suggesting that luminosity is more closely correlated with mass than galaxy counts. We split the samples by redshift but detect no significant evolution. The profiles are not well described by power laws. In a subsequent series of papers, we invert the profiles to three-dimensional mass profiles, show that they are well fit by a halo model description, measure mass-to-light ratios, and provide a cosmological interpretation.
119201134
DARK ENERGY EQUATION OF STATE AND COSMIC TOPOLOGY
The immediate observational consequence of a non-trivial spatial topology of the Universe is that an observer could potentially detect multiple images of radiating sources. In particular, a non-trivial topology will generate pairs of correlated circles of temperature fluctuations in the anisotropies maps of the cosmic microwave background (CMB), the so-called circles-in-the-sky. In this way, a detectable non-trivial spatial topology may be seen as an observable attribute, which can be probed through the circles-in-the-sky for all locally homogeneous and isotropic universes with no assumptions on the cosmological dark energy (DE) equation of state (EOS) parameters. We show that the knowledge of the spatial topology through the circles-in-the-sky offers an effective way of reducing the degeneracies in the DE EOS parameters. We concretely illustrate the topological role by assuming, as an exanple, a Poincar\'{e} dodecahedral space topology and reanalyzing the constraints on the parameters of a specific EOS which arise from the supernovae type Ia, baryon acoustic oscillations and the CMB plus the statistical topological contribution.
118387259
Energy conditions bounds and their confrontation with supernovae data
The energy conditions play an important role in the understanding of several properties of the Universe, including the current accelerating expansion phase and the possible existence of the so-called phantom fields. We show that the integrated bounds provided by the energy conditions on cosmological observables such as the distance modulus {mu}(z) and the lookback time t{sub L}(z) are not sufficient (or necessary) to ensure the local fulfillment of the energy conditions, making explicit the limitation of these bounds in the confrontation with observational data. We recast the energy conditions as bounds on the deceleration and normalized Hubble parameters, obtaining new bounds which are necessary and sufficient for the local fulfillment of the energy conditions. A statistical confrontation, with 1{sigma}-3{sigma} confidence levels, between our bounds and supernovae data from the gold and combined samples is made for the recent past. Our analyses indicate, with 3{sigma} confidence levels, the fulfillment of both the weak energy condition (WEC) and dominant energy condition (DEC) for z{<=}1 and z < or approx. 0.8, respectively. In addition, they suggest a possible recent violation of the null energy condition (NEC) with 3{sigma}, i.e. a very recent phase of superacceleration. Our analyses also show the possibility of violationmore » of the strong energy condition (SEC) with 3{sigma} in the recent past (z{<=}1), but interestingly the q(z)-best-fit curve crosses the SEC--fulfillment divider at z{approx_equal}0.67, which is a value very close to the beginning of the epoch of cosmic acceleration predicted by the standard concordance flat {lambda}CDM scenario.« less