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.
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Modified gravity theories under consideration typically reduce to a scalar-tensor form in the appropriate limits.
I will discuss in what sense a universal scalar coupling is stable against quantum corrections, when the scalar equivalence principle is violated, how to look for such violations, and the connection with cosmic acceleration.
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Instead of adding another dark component to the energy budget of the Universe, one can ask whether the observed accelerated expansion might in fact be due to the behavior of gravity itself on the largest scales.
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I will discuss an alternative to inflation based on a Galileon field. The model starts in a (contracting or expanding) quasi Minkowski phase and all the energy of the Universe in generated suddenly in a sort of Genesis associated with a strong violation of the Null Energy Condition. The symmetries of the model force any additional scalar field to acquire a scale invariant spectrum of perturbations.
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How can we rule out whole classes of dark energy models? And what quantities, at what redshift, and with what accuracy, should be measured in order to rule out these classes of models? I present answers to these questions by discussing an approach that utilizes the principal component parametrization of dark energy. I show results based on current data, and future forecasted data from SNAP and Planck.
We show that the existence of the bullet cluster, 1E0657-56,
is incompatible with the prediction of the standard Lambda CDM
cosmology. The probability of finding the large infall velocity (3000 km/s)
necessary for explaining the X-ray and weak lensing data of 1E0657-56 is
between 3.3x10^{-11} and 3.6x10^{-9}. The existence of the bullet cluster
poses a serious challenge to LCDM cosmology, unless a lower infall
velocity solution for 1E0657-56 with <1800 km/s is found.