One of the most astonishing results in physics at the end of the twentieth century was the realization that the universe is expanding at an increasing speed. The agent of this accelerated expansion has been termed “dark energy”. In fact, in the decade since then the combined analysis of observations of the Large Scale Structure of the universe, of measurements of Cosmic Microwave Background temperature anisotropies and of distance-scale measurements provided by the observation of type Ia Supernova, have led to a remarkably consistent cosmological model, in which the universe is described by Einstein’s theory of general relativity with flat geometry, containing the known baryonic matter, radiation and light neutrinos, an unknown form of collisionless (weakly interacting) matter known as “cold dark matter”, and dark energy. The observations show that at present the matter part (baryonic and dark) accounts only for about 25% of the total matter-energy density, the radiation and neutrino part is almost negligible and the other 75% is in the form of "dark energy". The dark energy exerts negative pressure and its fraction of the total matter-energy of the universe is variable with time, having become dominant only recently. A group at the IFAE with interest in Observational and Theoretical Cosmology, together with other six groups in Spain, promoted a project submitted to the Consolider Ingenio 2010 Program of the Ministry of Science and Innovation of Spain, consistent with the goals of that program, namely to lead, from Spain, an internationally competitive project in dark energy studies. Dark energy is detected indirectly, by its effects in observable quantities of the universe. In particular it influences the expansion history, the rate at which the universe is expanding as a function of time. This in turn can be inferred by measuring known scales as a function of time, or of red-shift, which is the quantity measured experimentally. One such scale is provided by the so called baryon acoustic oscillation peak (BAO), a correlation in the large-scale distribution of mass, which owes its name to the fact that it originates from the pressure (sound) waves present in the radiation-matter plasma before recombination. The most important goal of the project is to construct an instrument consisting of a very wide-field CCD camera (~ 6 deg2) for galaxy photometric red-shift measurements based on a large number of narrow-band filters, and to prepare a large galaxy photometric survey, of about 8000 deg2 and up to z=0.9 in red-shift, with such an instrument installed in a 2 meter class telescope. Simulations show that with the data of the survey it would be possible to measure the BAO scale in both the radial and angular directions, constraining the equation of state parameter, and thus the nature of dark energy. The same survey will also contain a wealth of other astronomical data for cosmological and astrophysical studies. The telescope would be an existing telescope, suitably modified, or, preferably, an entirely new telescope, fully dedicated to the survey in an initial phase. The project was approved as part of the Consolider Program in 2007. IFAE is the Coordinating Institution of the project and the Coordinator is also a member of IFAE.
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