# 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.

## Efficient quantum algorithms for an additive approximation of the Tutte polynomial and the q-state Potts model.

Monday May 21, 2007
Speaker(s):

I will present an efficient quantum algorithm for an additive
approximation of the famous Tutte polynomial of any planar graph at
any point. The Tutte polynomial captures an extremely wide range of
interesting combinatorial properties of graphs, including the
partition function of the q-state Potts model. This provides a new
class of quantum complete problems.

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Scientific Areas:

## Modified Gravity and its Consequences for the Solar System, Astrophysics and Cosmology

Sunday May 20, 2007
Speaker(s):

A relativistic modified gravity (MOG) leads to a self-consistent, stable gravity theory that can describe the solar system, galaxy and clusters of galaxies data and cosmology without dark matter. A review is given of fits to galaxy rotation curves, mass profiles of X-ray clusters and weak and strong lensing of galaxy clusters including the bullet cluster E10657-56. MOG can explain the CMB power spectrum and the observed acceleration of the expansion of the universe.

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## Why there is something rather than nothing

Sunday May 20, 2007
Speaker(s):

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

Sunday May 20, 2007
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## Dark Energy, Lorentz Violation and Ghosts

Sunday May 20, 2007
Speaker(s):

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

Sunday May 20, 2007
Speaker(s):

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

Sunday May 20, 2007
Speaker(s):

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

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

Saturday May 19, 2007
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## Nonlinear Structure Formation and Apparent Acceleration: An Investigation

Saturday May 19, 2007
Speaker(s):

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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## Next Public Lecture

Check back for details on the next lecture in Perimeter's Public Lectures Series

## RECENT PUBLIC LECTURE

### Kendrick Smith, Perimeter Institute

Speaker: Kendrick Smith