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Sci TR-32

Updates: 11.05.2012 (Aliş)

3.2 Observational Cosmology

Observational studies in cosmology mainly aim one thing: What is the implied density parameter (Ωm) of the universe?

This can be obtained from cosmic background radiation, type Ia supernovae and galaxy clusters. In the standard structure formation scenario (i.e. CDM) clusters are formed at the nodes of the cosmic web (see Millennium Simulation, Springel et al., 2005). Their formation process is still ongoing, is mainly driven by gravitational processes making the abundance of clusters as function of mass and redshift a sensitive probe of mass density parameter and amplitude of mass fluctuations (σ8). Comparison of values obtained from different methods can give new insights and are important for consistency checks. In the near future EUCLID satellite will investigate galaxy clusters to be able to constrain those parameters.

Fig.3.2-1. From the EUCLID Red Book

The Dark Energy implied by a group of astronomers in 1998 by studying type Ia supernovae led to a consiredable interest in using galaxy clusters. It has been implied that the universe is expanding with an accelerating speed. There are new surveys and efforts are being done to characterize better this acceleration. Observations for the Dark Energy Survey (DES) has just started. The Blanco telescope in CTIO (Chile) is used to observe around 1000 type Ia SNe. Dark Energy became the hottest topic in observational cosmology studies. On the other side, Dark Matter is known since 1930s and became almost a standard approach to explain structure formation in the universe. It is still needs to be explained. The nature of dark matter remains unknown but particle physicists and astrophysicists are gathered their power to solve this puzzle. Examples like the Bullet Cluster give very important clues to understand its properties. Therefore investigating merging clusters is another hot topic in observational cosmology. 4m class telescopes are very good opportunuties to focus on individual clusters detected from large surveys.

It has been a while that the third method to obtain cosmological parameters (i.e. CMB) is used extensively. Firstly, COBE (in 1999) and then WMAP (2003) satellites changed our knowledge significantly. Now it is known that the fluctuations in the CMB around orders of 10-5 result structure formation in the universe just after the Radiation Era.

Galaxy clusters amongs these methods always attracts people more than others. Key issues in the determination of cosmological parameters are control and decrease the systematic uncertainties due to our imperfect knowledge of the physics that govern cluster formation and evolution, handle properly the selection function of cluster catalogues. Efforts were made building up large samples of clusters especially since 2000 with the SDSS and before the 2dF survey. These samples were in the low redshift universe (z=0.3-0.4) that is known as local universe in cosmology. To be able to investigate evolution of universe and evolution of clusters itself it is needed to go deeper in terms of redshift. Recently completed CFHTLS, ongoing DES and future LSST are the main surveys to study evolutionary aspects.

As the case is reach large redshifts infrared observations play an important role. Current surveys are designed in multibands but it is not possible to reach beyond z=1.3 with filter set in use (i.e. ugriz). There are efforts to complete current surveys in the IR domain such WIRCAM Deep Survey for the CFHTLS. The DAG project, very likely, may play an important role in this aspect especially when EUCLID considered.


Springel et al., 2005, Nature, 435, 629.

What can be done with DAG
  • Masses of galaxy clusters can be detected via velocity dispersions and these masses can be used to constrain growth rate of structures in the universe. In this sense, spectroscopic follow-ups of clusters which will be detected by DES project and EUCLID satellite can be done with the DAG telescope.