C07008 - Excursions in the Dark Workshop - 2007Excursions in the Dark Workshop
http://pirsa.org/podcast/C07008
Science2020http://blogs.law.harvard.edu/tech/rssen-caWed, 25 Nov 2020 23:59:55 -0500Wed, 25 Nov 2020 23:59:55 -0500G180help@perimeterinstitute.capirsa.org<![CDATA[Dark Energy from variation of the fundamental scale]]>We explore the hypothesis that a dynamical dark energy is related to a time-variation of the fundamental mass scale. A dilatation anomaly induced by quantum fluctuations could explain the small value of the present dark energy. Reformulated as a scalar field theory this would predict a quintessence potential which asymptotically relaxes to zero rather than to a nonvanishing constant. An observable consequence of such a scenario results in "early dark energy " contributing a few percent to the energy density of the Universe even at high redshift. Quintessence would be related to a new "fundamental" macroscopic force and induce a small time variation of fundamental constants.]]>Christof Wetterich
http://streamer2.perimeterinstitute.ca/mp3/07050028.mp3
Sciencehttp://streamer2.perimeterinstitute.ca/mp3/07050028.mp3Fri, 18 May 2007 09:00:00 -0400<![CDATA[Embedding Quintessence in Spontaneously Broken N=1 Supergravity]]>I will present the embedding of runaway quintessence models in supergravity coupled to observable matter and hidden supersymmetry breaking. Serious obstructions appear either in the gravitational sector of the theory or cosmologically. Alternatives will be discussed.]]>Philippe Brax
http://streamer2.perimeterinstitute.ca/mp3/07050029.mp3
Sciencehttp://streamer2.perimeterinstitute.ca/mp3/07050029.mp3Fri, 18 May 2007 11:00:00 -0400<![CDATA[Dynamics of Linear Perturbations in Modified Gravity Theories]]>A modification of Gravity in the low-curvature regime may account for the late time acceleration of our universe, and is therefore an interesting alternative to Dark Energy.In such models, the modified Einstein equations admit self-accelerated solutions in the presence of negligible matter. At the level of perturbation theory,the modified equations give rise to new dynamics for the perturbations of the metric and matter. I will consider scalar perturbations, presenting in some details the dynamics of linear perturbations for two specific models, f(R) Gravity and Modified Source Gravity. I will conclude by demonstrating how some characteristic features of these models are likely to be common to general models of Modified Gravity and how Large Scale Structure formation and the Integrated Sachs Wolfe effect might be useful probes to distinguish between Dark Energy and Modified Gravity.]]>Alessandra Silvestri
http://streamer2.perimeterinstitute.ca/mp3/07050031.mp3
Sciencehttp://streamer2.perimeterinstitute.ca/mp3/07050031.mp3Fri, 18 May 2007 11:30:00 -0400<![CDATA[Two Topics on the Accelerating Universe]]>Gregory Gabadadze
http://streamer2.perimeterinstitute.ca/mp3/07050030.mp3
Sciencehttp://streamer2.perimeterinstitute.ca/mp3/07050030.mp3Fri, 18 May 2007 16:00:00 -0400<![CDATA[Dark Energy and Neutrino Mass]]>One of the possible explanations for the current acceleration of the universe comes from a coupling between the Dark Energy and the Neutrino sectors. This coupling causes the neutrino mass to vary with cosmic time, what opens a new window to constrain this dark energy candidate. In this work, we analyze the mass-varying neutrino scenario in a model independent way, focusing on its effects for the Cosmic Microwave Background and Large Scale Structure.]]>Urbano Franca
http://streamer2.perimeterinstitute.ca/mp3/07050032.mp3
Sciencehttp://streamer2.perimeterinstitute.ca/mp3/07050032.mp3Fri, 18 May 2007 17:00:00 -0400<![CDATA[Explorations in Supersymmetric Large Extra Dimensions]]>Brane worlds may provide insight into the cosmological constant problem because a large vacuum energy on the brane can curve the extra dimensions rather than the local 4D spacetime. Moreover, such models with supersymmetric large extra dimensions reveal a tantalizing numerology, in which the size of the two extra dimensions can lead not only to the electroweak hierarchy but also to the observed dark energy scale. I will review this proposal, its promises and problems, and then describe some of the novel physics that can arise in 6D brane worlds. The dynamical stability of models in 6D supergravity constrains the matter content of the theory, but surprisingly this can be relaxed with negative tension branes. Meanwhile, the Kaluza-Klein mass gap can remain finite even in the infinite volume limit.]]>Susha Parameswaran
http://streamer2.perimeterinstitute.ca/mp3/07050033.mp3
Sciencehttp://streamer2.perimeterinstitute.ca/mp3/07050033.mp3Fri, 18 May 2007 17:30:00 -0400<![CDATA[Large Extra Dimensions and Darkness From String Theory]]>Fernando Quevedo
http://streamer2.perimeterinstitute.ca/mp3/07050035.mp3
Sciencehttp://streamer2.perimeterinstitute.ca/mp3/07050035.mp3Sat, 19 May 2007 11:00:00 -0400<![CDATA[Non-Anthropic Approaches to the String Landscape ]]>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. The approach may also lead to a viable model of inflation, without the need of fine-tuned potentials.]]>G. Watson
http://streamer2.perimeterinstitute.ca/mp3/07050036.mp3
Sciencehttp://streamer2.perimeterinstitute.ca/mp3/07050036.mp3Sat, 19 May 2007 16:00:00 -0400<![CDATA[Nonlinear Structure Formation and Apparent Acceleration: An Investigation]]>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.]]>Alessio Notari
http://streamer2.perimeterinstitute.ca/mp3/07050037.mp3
Sciencehttp://streamer2.perimeterinstitute.ca/mp3/07050037.mp3Sat, 19 May 2007 16:30:00 -0400<![CDATA[Why the Vacuum Energy is Enormous (Just Not Here and Now)]]>Anthony Aguirre
http://streamer2.perimeterinstitute.ca/mp3/07050038.mp3
Sciencehttp://streamer2.perimeterinstitute.ca/mp3/07050038.mp3Sat, 19 May 2007 17:00:00 -0400<![CDATA[Gravity Probes and Graviton Hair]]>Georgi Dvali
http://streamer2.perimeterinstitute.ca/mp3/07050039.mp3
Sciencehttp://streamer2.perimeterinstitute.ca/mp3/07050039.mp3Sun, 20 May 2007 09:00:00 -0400<![CDATA[The DGP Braneworld]]>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.]]>Kazuya Koyama
http://streamer2.perimeterinstitute.ca/mp3/07050040.mp3
Sciencehttp://streamer2.perimeterinstitute.ca/mp3/07050040.mp3Sun, 20 May 2007 11:00:00 -0400<![CDATA[Consistent Lorentz Violation in Flat and Curved Space]]>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. In the second class, realized in the DGP model or in theories of massive gravity, spontaneous Lorentz breaking is induced by the expectation value of sources. The generalization to de-Sitter space is also discussed.]]>Oriol Pujolas
http://streamer2.perimeterinstitute.ca/mp3/07050041.mp3
Sciencehttp://streamer2.perimeterinstitute.ca/mp3/07050041.mp3Sun, 20 May 2007 11:30:00 -0400<![CDATA[Dark Energy, Lorentz Violation and Ghosts]]>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.]]>Raman Sundrum
http://streamer2.perimeterinstitute.ca/mp3/07050042.mp3
Sciencehttp://streamer2.perimeterinstitute.ca/mp3/07050042.mp3Sun, 20 May 2007 14:00:00 -0400<![CDATA[Random Observations in the Landscape]]>Vitaly Vanchurin
http://streamer2.perimeterinstitute.ca/mp3/07050043.mp3
Sciencehttp://streamer2.perimeterinstitute.ca/mp3/07050043.mp3Sun, 20 May 2007 15:00:00 -0400<![CDATA[Why there is something rather than nothing]]>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. This ensemble corresponds to the uniform (weight one) distribution over an entire phase space of {em true physical variables}, but in terms of the observable spacetime geometry it is peaked about complex saddle-points of the Lorentzian path integral, given by the recently obtained Euclidean gravitational instantons belonging to the bounded range of the cosmological constant. It is shown that the conformal rotation, designed to solve the problem of unboundedness of the Euclidean gravitational action, is, in fact, the manifestation of correct integration range over the lapse and shift functions in Lorentzian quantum gravity, enforcing the Hamiltonian and momentum constraints. Inflationary cosmologies generated by the gravitational instantons at late stages of expansion incorporate cosmological acceleration phenomenon whose low-energy scale can be attained by invoking the ideas of dynamically evolving compactified extra dimensions. Thus, together with the bounded range of the early cosmological constant, this initial state of the Universe simultaneously suggests the dynamical restriction mechanism for the landscape of string vacua and the solution of the dark energy problem.]]>Andrei Barvinski
http://streamer2.perimeterinstitute.ca/mp3/07050044.mp3
Sciencehttp://streamer2.perimeterinstitute.ca/mp3/07050044.mp3Sun, 20 May 2007 16:00:00 -0400<![CDATA[Modified Gravity and its Consequences for the Solar System, Astrophysics and Cosmology]]>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.]]>John Moffat
http://streamer2.perimeterinstitute.ca/mp3/07050045.mp3
Sciencehttp://streamer2.perimeterinstitute.ca/mp3/07050045.mp3Sun, 20 May 2007 16:30:00 -0400