Proposed Work:

Amongst the different probe, WL has emerged as one of the most effective cosmological probes (DETF report, and the more recent JDEM Figure of Merit (FoM) working group results by Albrecht09), since it is sensitive to both the geometry (through its dependence on angular-diameter distance ratio) and the growth of structure. Indeed, the observed shape of a distant galaxy depends on the amount of mass distributed along the line of sight. In order to obtain the best cosmological constraints, it is critical to have accurate redshift measurements of all the galaxies for which one can measure their shape (Massey et al 2007). In other words, any future WL imaging survey must address the question of the complementary redshift survey. Measuring the redshifts of all galaxies used in future WL surveys by direct spectroscopy is practically impossible with current technology. Currently, the only route is to use photometric redshift. Although photometric redshift have now been used for many years, the technique has foremost been developed using data available at various telescopes. However, very rarely has an instrument or a survey been designed in order to optimize the photometric redshift measurement needed to reach a specific goal.

Previous work aimed particularly at optimizing photometric redshifts for future Dark Energy surveys looked at the filter properties, their number and the photometry efficiency (Benitez et al 2009, Dahlen et al 2008). More recently, Bordoloi et al (2009), Schulz et al (2009), Quadri et al (2009), and Sheth et al (2010) explored the possible improvement of the photometric redshift techniques using respectively, work on likelihood functions, cross-correlation methods, close galaxy pairs, convolution and de-convolution methods from a sub-sample of spectroscopic redshifts. However, only the recent paper by Jouvel et al (2010) is exploring photometric redshift in the global context of the DE mission optimization. Indeed, as we are preparing the future cosmological surveys, it is important to understand how to best design the observational strategy and the photometric data of a WL survey to maximize its cosmological return which is often cast in a single FoM number.

Baryonic Acoustic Oscillations (BAO) are the imprint, on the distribution of (almost) present-day galaxies, of the large scale acoustic oscillations that occurred right after the Big Bang. They can be measured by mapping the galaxy distribution over the whole sky in 3D, i.e., using the crucial redshift information. The position of the BAO peaks in the power spectrum of the distribution of galaxies is a sensitive tool to measure the mass density of the Universe and the baryon fraction (Percival et al 2010). Wang et al (2009) explored the dependence on the BAO FoM as a function of the redshift distribution, size of the survey. However, we need to extend such an analysis, to include the full design of a space mission, taking into account the potential problems in the redshift measurements. There might also be some instrumental design, which may improve the redshift determination, and those ones needs to be explored in the context of the strategy of observation. Furthermore, BAO is only part of the cosmological information imprinted in a spectroscopic survey, and a full account of the information buried in the power spectrum of the galaxy distribution must be included in a global analysis.

Galaxy clusters are the most massive gravitationally bound structures in the Universe, forming at the highest mass-density peaks of the primordial density perturbations. Clusters are usually used to constrain the amplitude of the mass density fluctuation and the mass density of the Universe, but they also convey information on the cosmological parameters beyond these 2 parameters. When including lensing information such as strong and weak lensing, they can also be used to constrain distances in particular when combined with X-ray or SZ surveys.

Empirically, the peak luminosity of supernovae of Type Ia (SNe Ia) can be used as an efficient distance indicator (e.g., Leibundgut et al 2001) and was successfully used to first probe the acceleration of the Universe expansion (Riess et al 1998, Perlmutter et al 1999). The favorite theoretical explanation for SNe Ia is the thermonuclear disruption of carbon-oxygen white dwarfs. Although not perfect ‘standard candles,’ it has been demonstrated that SNe Ia can be “standardize” in particular if multi-band light curves or spectroscopic information can be obtained. The SNe Ia probe is now likely limited by systematic effects and only a tight control of these systematics that could be achieved by a dedicated space mission would make such a probe useful in the future. Including a SNe Ia component in a space mission is challenging because of the implications on the observation survey strategy, but it has the advantage to only probe the geometrical part of the world model, similarly as the BAO measurement.

To summarize, we believe that it is now time to confront the different research work conducted by the different groups, to agree and confort our understanding of what is the optimal mission that will at no risk gives the best constraints on the cosmological world model. As we have seen in some recent analysis, such optimization can only be achieved if a global approach of the problem is taken (making a joint analysis of the different probes), which includes both the observational survey strategy and the instrument parameters. We also believe that confronting the experienced gained on EUCLID and JDEM would help in making the right decision in preparing these future space missions.