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# Video Library

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.

## Why there is something rather than nothing

Dimanche mai 20, 2007
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We show that the recently suggested Euclidean quantum gravity density matrix of the Universe which generates the set of inflationary universes is, in fact, the density matrix of the microcanonical ensemble in {\em Lorentzian} quantum cosmology.

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## Random Observations in the Landscape

Dimanche mai 20, 2007
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## Dark Energy, Lorentz Violation and Ghosts

Dimanche mai 20, 2007
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I describe how vacuum energy can be controlled by a symmetry principle that necessitates a ghost sector. I argue that the implied instability of Minkowski spacetime is natural and consistent with experience if gravity is fundamentally Lorentz-violating, and describe attempts to construct such exotic dynamics. I briefly discuss the more robust experimental/observational signatures of such a scenario.

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## Consistent Lorentz Violation in Flat and Curved Space

Dimanche mai 20, 2007
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Motivated by the severity of the bounds on Lorentz violation in the presence of ordinary gravity, we study frameworks in which Lorentz violation does not affect the spacetime geometry. We show that there are at least two inequivalent classes of spontaneous Lorentz breaking that even in the presence of gravity result in Minkowski space. The first one generically corresponds to the condensation of tensor fields with tachyonic mass, which in turn is related to ghost-condensation.

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## The DGP Braneworld

Dimanche mai 20, 2007
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In this talk, I summarize a current status of the DGP braneworld emphasizing the theoretical consistency of the model.First I review the behaviour of the linearized gravity and show the existence of the ghost. Then I discuss the issue of the non-linearity of gravity in this model.

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## Gravity Probes and Graviton Hair

Dimanche mai 20, 2007
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## Why the Vacuum Energy is Enormous (Just Not Here and Now)

Samedi mai 19, 2007
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## Nonlinear Structure Formation and Apparent Acceleration: An Investigation

Samedi mai 19, 2007
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We present analytically solvable nonlinear models of structure formation in a Universe with only dust, using LTB solutions. We show that the luminosity distance-redshift relation has significant corrections at low redshift (Doppler effect). We discuss different possibilities that could further enhance this effect and mimick Dark Energy. We find negligible integrated effect, suppressed by $(L/R_{H})^3$ (where $L$ is the size of the structure, and $R_{H}$ is the Hubble radius) and we make contact with cosmological perturbation theory.

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## Non-Anthropic Approaches to the String Landscape

Samedi mai 19, 2007
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We consider the effect that dynamical selection principles could have on the string landscape and for determining the value of the cosmological constant. The underlying symmetries of string theory, along with the dynamics of moduli in the low energy effective field theory, seem to suggest that not all vacua are created equal. However, in some simple models many vacua are alike and this degeneracy may suggest a *non-anthropic* approach to understanding the observed value of the cosmological constant.

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## Large Extra Dimensions and Darkness From String Theory

Samedi mai 19, 2007
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## Next Public Lecture

### MARCIA BARBOSA, UNIVERSIDADE FEDERAL DO RIO GRANDE DO SUL

Mercredi mar 04, 2015

## RECENT PUBLIC LECTURE

### Kendrick Smith, Perimeter Institute

Speaker: Kendrick Smith