CfA Colloquium Schedule Spring 2013
 7 March 2013

7 March 2013

Speaker: Neta Bahcall (Princeton)

Title:Cecilia Payne-Gaposhkin Lecture: Where is the dark matter?

Abstract:Where is the Dark Matter? How is it distributed in the Universe? What is the connection between the dark and light side of the Universe? I will discuss these questions utilizing recent gravitational lensing observations that trace the mass distribution from small to large scales and help reveal the connection between the dark and bright side of the Universe. We show that while the observed mass distribution is considerably more extended than light on galactic scales -- representing the huge dark matter halos around galaxies -- the mass follows light remarkably well on larger scales. On scales larger than a few hundred Kpc, there is no significant separation between dark and luminous matter; the dark and light sides trace each other with a nearly constant mass-to-light ratio. This universal ratio indicates the end of the dark matter excess over light and reflects the total mass-density of the universe. The results further suggest that most of the dark matter in the Universe is located in very large halos (~300 Kpc) around galaxies and inside clusters of galaxies, but with no significant increase in the dark matter component on larger scales; groups, clusters, and superclusters appear to be made up mostly by the total mass of their individual galaxy members (including their extended halos, which may be stripped inside the dense clusters but still remain within the cluster potential). The 'end' of the dark matter distribution is reached on these relatively small scales of a few hundred Kpc. The implications for cosmology and the mass-density of the universe, the baryonic component, as well as for galaxy and structure formation will be discussed.

Video of the Presentation (Talks can be viewed with RealPlayer. Free download is available from )


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