Cosmology

The initial conditions of our Universe can be summarized on a single sheet of paper. Yet the Universe is full of complex structures today, such as stars, galaxies and groups of galaxies. I will describe how complexity emerged in the form of the first stars out of the simple initial state of the Universe at early cosmic times. The future of the Universe is even more surprising. Over the past decade it was realized that the cosmic expansion has been accelerating. If this accelerated expansion will continue into the future, then within a hundred billion years there will be no galaxies left for us to observe within the cosmic horizon except one: the merger product between our own Milky Way galaxy and its nearest neighbor, the Andromeda galaxy.

How do we weigh the Universe? Where is the Dark Matter? I will discuss these questions and show that several independent methods, including the observed present-day abundance of rich clusters , the evolution of cluster abundance with redshift, the baryon-fraction in clusters, the observed Mass-to-Light function from galaxies to superclusters, and other large-scale structure observations, all reveal a universe with a low mass density parameter of ~20% of the critical density. The data suggest that the mass in the Universe, including the dark-matter, approximately follows light on large scales and that most of the mass resides in huge dark halos around galaxies. I will review the combined observational evidence for dark-matter and for dark-energy in the universe and their cosmological implications.