Munch: Monday, May 14, 2007

                               


 

 WHERE: 6TH FLOOR CONFERENCE ROOM
WHEN  : 12:30, MONDAY

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 


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       Munch Archive

 arXiv:0705.0898 [ps, pdf, other] : Title: Testing anthropic predictions for Lambda and the CMB temperature
Authors: J.A. Peacock
Comments: 9 pages. To appear in MNRAS
It has been claimed that the observed magnitude of the vacuum energy density is consistent with the distribution predicted in anthropic models, in which an ensemble of universes is assumed. This calculation is revisited, without making the assumption that the CMB temperature is known, and considering in detail the possibility of a recollapsing universe. New accurate approximations for the growth of perturbations and the mass function of dark haloes are presented. Structure forms readily in the recollapsing phase of a model with negative Lambda, so collapse fraction alone cannot forbid Lambda from being large and negative. A negative Lambda is disfavoured only if we assume that formation of observers can be neglected once the recollapsing universe has heated to T > 8 K. For the case of positive Lambda, however, the current universe does occupy a extremely typical position compared to the predicted distribution on the Lambda-T plane. Contrasting conclusions can be reached if anthropic arguments are applied to the curvature of the universe, and we discuss the falsifiability of this mode of anthropic reasoning.

arXiv:0705.0979 [ps, pdf, other] : Title: Precision measurements of large scale structure with future type Ia supernova surveys
Authors: Steen Hannestad, Troels Haugboelle, Bjarne Thomsen
Comments: 18 pages, 9 figures, submitted to JCAP
Type Ia supernovae are currently the best known standard candles at cosmological distances. In addition to providing a powerful probe of dark energy they are an ideal source of information about the peculiar velocity field of the local universe. Even with the very small number of supernovae presently available it has been possible to measure the dipole and quadrupole of the local velocity field out to z~0.025. With future continuous all-sky surveys like the LSST project the luminosity distances of tens of thousands of nearby supernovae will be measured accurately. This will allow for a determination of the local velocity structure of the universe as a function of redshift with unprecedented accuracy, provided the redshifts of the host galaxies are known. Using catalogues of mock surveys we estimate that future low redshift supernova surveys will be able to probe sigma-8 to a precision of roughly 5% at 95% C.L. This is comparable to the precision in future galaxy and weak lensing surveys and with a relatively modest observational effort it will provide a crucial cross-check on future measurements of the matter power spectrum.

arXiv:0705.1158 [ps, pdf, other] : Title: Models of f(R) Cosmic Acceleration that Evade Solar-System Tests
Authors: Wayne Hu, Ignacy Sawicki (KICP, U. Chicago)
Comments: 13 pages, 10 figures. Submitted to Phys. Rev. D
We study a class of metric-variation f(R) models that accelerates the expansion without a cosmological constant and satisfies both cosmological and solar-system tests in the small-field limit of the parameter space. Solar-system tests alone place only weak bounds on these models, since the additional scalar degree of freedom is locked to the high-curvature general-relativistic prediction across more than 25 orders of magnitude in density, out through the solar corona. This agreement requires that the galactic halo be of sufficient extent to maintain the galaxy at high curvature in the presence of the low-curvature cosmological background. If the galactic halo and local environment in f(R) models do not have substantially deeper potentials than expected in LCDM, then cosmological field amplitudes |f_R| > 10^{-6} will cause the galactic interior to evolve to low curvature during the acceleration epoch. Viability of large-deviation models therefore rests on the structure and evolution of the galactic halo, requiring cosmological simulations of f(R) models, and not directly on solar-system tests. Even small deviations that conservatively satisfy both galactic and solar-system constraints can still be tested by future, percent-level measurements of the linear power spectrum, while they remain undetectable to cosmological-distance measures. Although we illustrate these effects in a specific class of models, the requirements on f(R) are phrased in a nearly model-independent manner.

arXiv:0705.0358 [ps, pdf, other] : Title: Comparison of weak lensing masses and X-ray properties of galaxy clusters
Authors: Henk Hoekstra
Comments: 16 pages, 8 figures. Revised version submitted to MNRAS (after minor corrections suggested by referee)
We present measurements of the masses of 20 X-ray luminous clusters of galaxies at intermediate redshifts, determined from a weak lensing analysis of deep archival R-band data obtained using the Canada-France-Hawaii-Telescope. Compared to previous work, our analysis accounts for a number of effects that are typically ignored, but can lead to small biases, or incorrect error estimates. We derive masses that are essentially model independent and find that they agree well with measurements of the velocity dispersion of cluster galaxies and with the results of X-ray studies. Assuming a power law between the lensing mass and the X-ray temperature, M_2500 T^alpha, we find a best fit slope of alpha=1.34^{+0.30}_{-0.28}. This slope agrees with self-similar cluster models and studies based on X-ray data alone. For a cluster with a temperature of kT=5keV we obtain a mass M_{2500}=(1.4+-0.2)\times 10^{14}h^{-1}Msun in fair agreement with recent Chandra and XMM studies.

arXiv:0705.0163 [ps, pdf, other] : Title: Probing dark energy with cluster counts and cosmic shear power spectra: including the full covariance (suggested by Josh)
Authors: Masahiro Takada (Tohoku Univ., Japan), Sarah Bridle (University College London)
Comments: 30 pages, 13 figures, invited original contribution to gravitational lensing focus issue, New Journal of Physics
(Abridged) Combining cosmic shear power spectra and cluster counts is powerful to improve cosmological parameter constraints and/or test inherent systematics. However they probe the same cosmic mass density field, if the two are drawn from the same survey region, and therefore the combination may be less powerful than first thought. We investigate their cross-covariance based on the halo model approach, where the cross-covariance arises from the three-point correlations of the underlying mass density field as a result of non-linear gravitational clustering in structure formation. Taking into account the cross-covariance as well as non-Gaussian errors on the lensing power spectrum covariance, we find a significant cross-correlation between the lensing power spectrum signals at multipoles l~1000 and the cluster counts containing halos with masses M > 10^14 M_sun. Including the cross-covariance for the combined measurement degrades and in some cases improves the total signal-to-noise by up to plus or minus 20% relative to when the two are independent. For cosmological parameter determination, the cross-covariance has a smaller effect as a result of working in a multi-dimensional parameter space, implying that the two observables can be considered independent to a good approximation. We also found that cluster counts using lensing selected mass peaks are more complementary to lensing tomography than mass-selected cluster counts. Using lensing selected clusters with a realistic usable detection threshold (S/N~6 for a ground-based survey) the uncertainty on each dark energy parameter is roughly halved by combining cluster counts and lensing power spectra, relative to using power spectra alone.

arXiv:0705.0354 [ps, pdf, other] : Title: Constraining dark energy via baryon acoustic oscillations in the (an)isotropic light-cone power spectrum
Authors: Christian Wagner, Volker Müller, Matthias Steinmetz
Comments: submitted to A&A, 11 pages, 12 figures
The measurement of the scale of the baryon acoustic oscillations (BAO) in the galaxy power spectrum as a function of redshift is a promising method to constrain the equation-of-state parameter of the dark energy w. In order to measure precisely the scale of the BAO a huge volume has to be surveyed. We test whether light-cone effects become important and whether the scaling relations used to compensate for a wrong reference cosmology are accurate enough in this case. We compare two different fitting methods to extract the scale of the BAO. Further, we analyze the advantage of using the two-dimensional anisotropic power spectrum. Finally, we estimate the uncertainty with which an effectively constant w can be measured with proposed surveys around redshifts of z=3 and z=1, respectively. We find that light-cone effects for the simulated survey are negligible and that the simple scaling relations used to correct for the cosmological distortions work well even for such large survey volumes. The two different fitting approaches deliver consistent results and both should be considered further. The analysis of the two-dimensional anisotropic power spectra allows independent determination of the apparent scale of BAO perpendicular and parallel to the line of sight but it does not significantly lower the uncertainty of an effectively constant w. Nevertheless, for less constrained models of w independent measurements of the apparent scale of BAO perpendicular and parallel to the line of sight are essential. We estimate that with planned surveys around z=3 and z=1 one will be able to measure an effectively constant w with sigma_w ~ 4% in both cases.