750 research outputs found
Modelling the relative velocities of isolated pairs of galaxies
We study the comoving relative velocities, v12, of model isolated galaxy
pairs at z=0.5. For this purpose, we use the predictions from the GALFORM
semi-analytical model of galaxy formation and evolution based on a Lambda cold
dark matter cosmology consistent with the results from WMAP7. In real space, we
find that isolated pairs of galaxies are predicted to form an angle t with the
line-of-sight that is uniformily distributed as expected if the Universe is
homogeneous and isotropic. We also find that isolated pairs of galaxies
separated by a comoving distance between 1 and 3 Mpc/h are predicted to have
=0. For galaxies in this regime, the distribution of the angle t is
predicted to change minimally from real to redshift space, with a change
smaller than 5% in . However, the distances defining the comoving
regime strongly depends on the applied isolation criteria.Comment: 4 pages, 4 figures, SF2A 2013 Proceedin
Sensitivity and figures of merit for dark energy supernovae surveys
Tracking the origin of the accelerating expansion of the Universe remains one of the most challenging research activities today. The final answer will depend on the precision and on the consistency of future data. The sensitivity of future surveys and the control of the errors are crucial. We focus on futur supernovae surveys in the light of the figure of merit defined by the Dark Energy Task Force. We compare different optimisation and emphasize the importance of the understanding of the systematic error level in this approach and their impact on the conclusions. We discuss different representations of the results to distinguish CDM from other theoretical models. We conclude that all representations should be controlled through combined analyses and consistency checks to avoid some bias
Prospects for Dark Energy Evolution: a Frequentist Multi-Probe Approach
A major quest in cosmology is the understanding of the nature of dark energy.
It is now well known that a combination of cosmological probes is required to
break the underlying degeneracies on cosmological parameters. In this paper, we
present a method, based on a frequentist approach, to combine probes without
any prior constraints, taking full account of the correlations in the
parameters. As an application, a combination of current SNIa and CMB data with
an evolving dark energy component is first compared to other analyses. We
emphasise the consequences of the implementation of the dark energy
perturbations on the result for a time varying equation of state. The impact of
future weak lensing surveys on the measurement of dark energy evolution is then
studied in combination with future measurements of the cosmic microwave
background and type Ia supernovae. We present the combined results for future
mid-term and long-term surveys and confirm that the combination with weak
lensing is very powerful in breaking parameter degeneracies. A second
generation of experiment is however required to achieve a 0.1 error on the
parameters describing the evolution of dark energy.Comment: Submitted to Astronomy & Astrophysics 14 pages, 8 figure
Image Slicer Performances from a Demonstrator for the SNAP/JDEM Mission - Part I: Wavelength Accuracy
A well-adapted visible and infrared spectrograph has been developed for the
SNAP (SuperNova/Acceleration Probe) experiment proposed for JDEM. The
instrument should have a high sensitivity to see faint supernovae but also a
good redshift determination better than 0.003(1+z) and a precise
spectrophotometry (2%). An instrument based on an integral field method with
the powerful concept of imager slicing has been designed. A large prototyping
effort has been performed in France which validates the concept. In particular
a demonstrator reproducing the full optical configuration has been built and
tested to prove the optical performances both in the visible and in the near
infrared range. This paper is the first of two papers. The present paper focus
on the wavelength measurement while the second one will present the
spectrophotometric performances. We adress here the spectral accuracy expected
both in the visible and in the near infrared range in such configuration and we
demonstrate, in particular, that the image slicer enhances the instrumental
performances in the spectral measurement precision by removing the slit effect.
This work is supported in France by CNRS/INSU/IN2P3 and by the French spatial
agency (CNES) and in US by the University of California.Comment: Submitted to PAS
Jackknife resampling technique on mocks: an alternative method for covariance matrix estimation
We present a fast and robust alternative method to compute covariance matrix
in case of cosmology studies. Our method is based on the jackknife resampling
applied on simulation mock catalogues. Using a set of 600 BOSS DR11 mock
catalogues as a reference, we find that the jackknife technique gives a similar
galaxy clustering covariance matrix estimate by requiring a smaller number of
mocks. A comparison of convergence rates show that 7 times fewer
simulations are needed to get a similar accuracy on variance. We expect this
technique to be applied in any analysis where the number of available N-body
simulations is low.Comment: 11 pages, 11 figures, 2 table
On the determination of the deceleration parameter from Supernovae data
Supernovae searches have shown that a simple matter-dominated and
decelerating universe should be ruled out. However a determination of the
present deceleration parameter through a simple kinematical description
is not exempt of possible drawbacks. We show that, with a time dependent
equation of state for the dark energy, a bias is present for : models
which are very far from the so-called Concordance Model can be accommodated by
the data and a simple kinematical analysis can lead to wrong conclusions. We
present a quantitative treatment of this bias and we present our conclusions
when a possible dynamical dark energy is taken into account.Comment: 4 pages, 3 figures, submitte
SNAP: an integral field spectrograph for supernova identification
A well-adapted spectrograph concept has been developed for the SNAP (SuperNova/Acceleration Probe) experiment. The goal is to ensure proper identification of Type Ia supernovae and to standardize the magnitude of each candidate by determining explosion parameters. An instrument based on an integral field method with the powerful concept of imager slicing has been designed and is presented in this paper. The spectrograph concept is optimized to have very high efficiency and low spectral resolution (R~100), constant through the wavelength range (0.35-1.7μm), adapted to the scientific goals of the mission
Probing Dark Energy with Supernovae : a concordant or a convergent model?
We present a revised interpretation of recent analysis of supernovae data. We
evaluate the effect of the priors on the extraction of the dark energy equation
of state. We find that the conclusions depend strongly on the prior
value and on its uncertainty, and show that a biased fitting procedure applied
on non concordant simulated data can converge to the "concordance model".
Relaxing the prior on points to other sets of solutions, which are
not excluded by observational data.Comment: 1+4 pages, 6 figures, submitted to Phys. Re
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