The Universe Tonight
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By Stephane Arnouts
A Journey Through the Universe: the Bright and Dark Sides of the Universe
As far as we can explore the sky, the universe is filled with galaxies (vast aggregates of gas and stars, similar to our own galaxy, the milky way). Those entities are priceless for the astronomers, not only because they look pretty but because they provide a unique window to explore the universe from our neighborhood up to the time when the universe was very young (~500 million years). Therefore they play a major role in our understanding of the universe. Although they represent the bright side of the universe, they are used by cosmologists to reveal its dark side: the dark matter and the dark energy that we think the universe is essentially made of. In this talk, I will first give a brief overview about the world of galaxies with a special focus on recent observations collected by the satellites GALEX and SPITZER, which explore different parts of the elecromagnetic spectrum than our optical telescopes. They give us a detailed picture of their contents and how stars are continually created and destroyed. This is just the start of our journey, we have to go one step further if we want to understand why the universe is full of galaxies, how do they form and why are they distributed into groups, clusters and large scale structures called the cosmic web? Astronomers do not have yet a clear answer to this question. One of the reasons is that galaxy formation is bound up to the cosmology (study of the universe as a whole). In contrast to stars who evolve independently of the universe, galaxies may have emerged from seeds that appear when the universe was young and still hot and dense. Their subsequent evolutions is a subtle balance between the expansion of the universe and the gravitational attraction and will depend on the global content of the universe (visible and dark matter + dark energy). Thanks to supercomputers, and using the main force in action (gravity), theorists can predict the formation and distribution of galaxies and confront their results with state of the art observations obtained at Keck and VLT telescopes where dedicated programs are currently mapping their distribution. As prelimary to this part, I will first review how the theory of a hot Big-Bang has been able to supersede other theories and what is the observational evidence supporting this. In this framework, I will describe how, by using galaxies as a laboratory, astronomers have detected the presence of a mysterious matter surrounding galaxies that we call, by ignorance, dark matter. More surprising, was the recent discovery of a mysterious energy, again called dark energy, that causes the universe to expand at an ever-increasing rate. I will discuss the recent results of a large campaign of observations undertaken at CFHT and which made a major contribution to its discovery. This dark energy is a new and mysterious component of our Universe that will determine its ultimate fate. Understanding its nature remains a major challenge for the future.
Stephane Arnouts obtained his PhD in Astronomy in 1996 in Paris. During is PhD he was working on the large scale structure of the universe. He went to Italy (Padova) for a couple of years where he worked on the Hubble Deep Fields. He developed a code to estimate the distance of galaxies based on multi-color images of the HDFs. Thanks to this technique he investigated how galaxies cluster with look back time in the hope to provide constraints on the cosmological content of the Universe. He then moved to ESO (European Southern Observatory) where he was involved in the first public imaging survey (EIS) based on the Wide Field Imager of the 2.2m telescope at La Silla. He moved back to France in Marseille where he obtained a position offered by the CNRS (Scientific National Research Center). He became members of large projects that perform a cartography of the distant universe (VVDS) and collect multi-wavelength data in UV (with GALEX satellite) and Infrared (with SPITZER satellite). Three years ago, he moved to CFHT, as resident astronomer, where he was enthusiastic to contribute to the ongoing CFHTLS survey.