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Search results in Astrophysics from PIRSA

2005 - 2016 of 2134 Results

Dark Energy, Lorentz Violation and Ghosts

Raman Sundrum University of Maryland, College Park
May 20, 2007

PIRSA:07050042
Astrophysics
Consistent Lorentz Violation in Flat and Curved Space

Oriol Pujolas New York University (NYU)
May 20, 2007

PIRSA:07050041
Astrophysics
The DGP Braneworld

Kazuya Koyama University of Portsmouth
May 20, 2007

PIRSA:07050040
Astrophysics
Gravity Probes and Graviton Hair

Georgi Dvali New York University (NYU)
May 20, 2007

PIRSA:07050039
Astrophysics
Why the Vacuum Energy is Enormous (Just Not Here and Now)

Anthony Aguirre University of California, Santa Cruz
May 19, 2007

PIRSA:07050038
Astrophysics
Nonlinear Structure Formation and Apparent Acceleration: An Investigation

Alessio Notari
May 19, 2007

PIRSA:07050037
Astrophysics
Non-Anthropic Approaches to the String Landscape

G. Watson University of Michigan–Ann Arbor
May 19, 2007

PIRSA:07050036
Astrophysics
Large Extra Dimensions and Darkness From String Theory

Fernando Quevedo University of Cambridge
May 19, 2007

PIRSA:07050035
Astrophysics
Explorations in Supersymmetric Large Extra Dimensions

Susha Parameswaran University of Liverpool
May 18, 2007

PIRSA:07050033
Astrophysics
Dark Energy and Neutrino Mass

Urbano Franca Harvard University
May 18, 2007

PIRSA:07050032
Astrophysics
Two Topics on the Accelerating Universe

Gregory Gabadadze New York University (NYU)
May 18, 2007

PIRSA:07050030
Astrophysics
Dynamics of Linear Perturbations in Modified Gravity Theories

Alessandra Silvestri Leiden University
May 18, 2007

PIRSA:07050031
Astrophysics

Dark Energy, Lorentz Violation and Ghosts

Raman Sundrum University of Maryland, College Park
May 20, 2007

PIRSA:07050042
Astrophysics
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
Astrophysics
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
Astrophysics
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
Astrophysics
Why the Vacuum Energy is Enormous (Just Not Here and Now)

Anthony Aguirre University of California, Santa Cruz
May 19, 2007

PIRSA:07050038
Astrophysics
Nonlinear Structure Formation and Apparent Acceleration: An Investigation

Alessio Notari
May 19, 2007

PIRSA:07050037
Astrophysics
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
Astrophysics
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
Astrophysics
Explorations in Supersymmetric Large Extra Dimensions

Susha Parameswaran University of Liverpool
May 18, 2007

PIRSA:07050033
Astrophysics
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
Astrophysics
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
Astrophysics
Dynamics of Linear Perturbations in Modified Gravity Theories

Alessandra Silvestri Leiden University
May 18, 2007

PIRSA:07050031
Astrophysics
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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Format results

Dark Energy, Lorentz Violation and Ghosts

Consistent Lorentz Violation in Flat and Curved Space

The DGP Braneworld

Gravity Probes and Graviton Hair

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

Nonlinear Structure Formation and Apparent Acceleration: An Investigation

Non-Anthropic Approaches to the String Landscape

Large Extra Dimensions and Darkness From String Theory

Explorations in Supersymmetric Large Extra Dimensions

Dark Energy and Neutrino Mass

Two Topics on the Accelerating Universe

Dynamics of Linear Perturbations in Modified Gravity Theories

Dark Energy, Lorentz Violation and Ghosts

Consistent Lorentz Violation in Flat and Curved Space

The DGP Braneworld

Gravity Probes and Graviton Hair

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

Nonlinear Structure Formation and Apparent Acceleration: An Investigation

Non-Anthropic Approaches to the String Landscape

Large Extra Dimensions and Darkness From String Theory

Explorations in Supersymmetric Large Extra Dimensions

Dark Energy and Neutrino Mass

Two Topics on the Accelerating Universe

Dynamics of Linear Perturbations in Modified Gravity Theories