Since 2002 Perimeter Institute has been recording seminars, conference talks, and public outreach events using video cameras installed in our lecture theatres. Perimeter now has 7 formal presentation spaces for its many scientific conferences, seminars, workshops and educational outreach activities, all with advanced audio-visual technical capabilities. Recordings of events in these areas are all available On-Demand from this Video Library and on Perimeter Institute Recorded Seminar Archive (PIRSA). PIRSA is a permanent, free, searchable, and citable archive of recorded seminars from relevant bodies in physics. This resource has been partially modelled after Cornell University's arXiv.org.
Diffeomorphism symmetry is the underlying symmetry of general
relativity and deeply intertwined with its dynamics. The notion of
diffeomorphism symmetry is however obscured in discrete gravity, which
underlies most of the current quantum gravity models. We will propose
a notion of diffeomorphism symmetry in discrete models and find that
such a symmetry is weakly broken in many models. This is connected to
the problem of finding a consistent canonical dynamics for discrete
gravity. Finally we will discuss methods to construct models with
The hot, gaseous atmospheres of galaxies and clusters of galaxies are
repositories for the energy output from accreting, supermassive black holes located in the nuclei of galaxies.
X-ray observations show that star formation fueled by gas condensing out of hot atmospheres is strongly suppressed by feedback from active galactic nuclei (AGN). This mechanism
may solve several outstanding problems in astrophysics, including the
numbers of luminous galaxies and their colors, and the excess number of