Talks

15229 - 15240 of 16392 Results

Dark Energy, Lorentz Violation and Ghosts

Raman Sundrum
University of Maryland, College Park

May 20, 2007

PIRSA:07050042

Cosmology
Consistent Lorentz Violation in Flat and Curved Space

Oriol Pujolas
New York University (NYU)

May 20, 2007

PIRSA:07050041

Cosmology
The DGP Braneworld

Kazuya Koyama
University of Portsmouth

May 20, 2007

PIRSA:07050040

Cosmology
Gravity Probes and Graviton Hair

Georgi Dvali
New York University (NYU)

May 20, 2007

PIRSA:07050039

Cosmology
Why the Vacuum Energy is Enormous (Just Not Here and Now)

Anthony Aguirre
University of California, Santa Cruz

May 19, 2007

PIRSA:07050038

Cosmology
Nonlinear Structure Formation and Apparent Acceleration: An Investigation

Alessio Notari

May 19, 2007

PIRSA:07050037

Cosmology
Non-Anthropic Approaches to the String Landscape

G. Watson
University of Michigan–Ann Arbor

May 19, 2007

PIRSA:07050036

Cosmology
Large Extra Dimensions and Darkness From String Theory

Fernando Quevedo
University of Cambridge

May 19, 2007

PIRSA:07050035

Cosmology
Explorations in Supersymmetric Large Extra Dimensions

Susha Parameswaran
University of Liverpool

May 18, 2007

PIRSA:07050033

Cosmology
Dark Energy and Neutrino Mass

Urbano Franca
Harvard University

May 18, 2007

PIRSA:07050032

Cosmology
Two Topics on the Accelerating Universe

Gregory Gabadadze
New York University (NYU)

May 18, 2007

PIRSA:07050030

Cosmology
Dynamics of Linear Perturbations in Modified Gravity Theories

Alessandra Silvestri
Leiden University

May 18, 2007

PIRSA:07050031

Cosmology

Dark Energy, Lorentz Violation and Ghosts

Raman Sundrum
University of Maryland, College Park

May 20, 2007

PIRSA:07050042

Cosmology

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.
Consistent Lorentz Violation in Flat and Curved Space

Oriol Pujolas
New York University (NYU)

May 20, 2007

PIRSA:07050041

Cosmology

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.
The DGP Braneworld

Kazuya Koyama
University of Portsmouth

May 20, 2007

PIRSA:07050040

Cosmology

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.
Gravity Probes and Graviton Hair

Georgi Dvali
New York University (NYU)

May 20, 2007

PIRSA:07050039

Cosmology
Why the Vacuum Energy is Enormous (Just Not Here and Now)

Anthony Aguirre
University of California, Santa Cruz

May 19, 2007

PIRSA:07050038

Cosmology
Nonlinear Structure Formation and Apparent Acceleration: An Investigation

Alessio Notari

May 19, 2007

PIRSA:07050037

Cosmology

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.
Non-Anthropic Approaches to the String Landscape

G. Watson
University of Michigan–Ann Arbor

May 19, 2007

PIRSA:07050036

Cosmology

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.
Large Extra Dimensions and Darkness From String Theory

Fernando Quevedo
University of Cambridge

May 19, 2007

PIRSA:07050035

Cosmology
Explorations in Supersymmetric Large Extra Dimensions

Susha Parameswaran
University of Liverpool

May 18, 2007

PIRSA:07050033

Cosmology

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.
Dark Energy and Neutrino Mass

Urbano Franca
Harvard University

May 18, 2007

PIRSA:07050032

Cosmology

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.
Two Topics on the Accelerating Universe

Gregory Gabadadze
New York University (NYU)

May 18, 2007

PIRSA:07050030

Cosmology
Dynamics of Linear Perturbations in Modified Gravity Theories

Alessandra Silvestri
Leiden University

May 18, 2007

PIRSA:07050031

Cosmology

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.

Title	Date	Collection	Type	Info
Dark Energy, Lorentz Violation and Ghosts	2007‑05‑20	Excursions in the Dark Workshop - 2007	Conference	View details
Consistent Lorentz Violation in Flat and Curved Space	2007‑05‑20	Excursions in the Dark Workshop - 2007	Conference	View details
The DGP Braneworld	2007‑05‑20	Excursions in the Dark Workshop - 2007	Conference	View details
Gravity Probes and Graviton Hair	2007‑05‑20	Excursions in the Dark Workshop - 2007	Conference	View details
Why the Vacuum Energy is Enormous (Just Not Here and Now)	2007‑05‑19	Excursions in the Dark Workshop - 2007	Conference	View details
Nonlinear Structure Formation and Apparent Acceleration: An Investigation	2007‑05‑19	Excursions in the Dark Workshop - 2007	Conference	View details
Non-Anthropic Approaches to the String Landscape	2007‑05‑19	Excursions in the Dark Workshop - 2007	Conference	View details
Large Extra Dimensions and Darkness From String Theory	2007‑05‑19	Excursions in the Dark Workshop - 2007	Conference	View details
Explorations in Supersymmetric Large Extra Dimensions	2007‑05‑18	Excursions in the Dark Workshop - 2007	Conference	View details
Dark Energy and Neutrino Mass	2007‑05‑18	Excursions in the Dark Workshop - 2007	Conference	View details
Two Topics on the Accelerating Universe	2007‑05‑18	Excursions in the Dark Workshop - 2007	Conference	View details
Dynamics of Linear Perturbations in Modified Gravity Theories	2007‑05‑18	Excursions in the Dark Workshop - 2007	Conference	View details

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