**Niayesh Afshordi, Perimeter Institute**

Gravitational aether as a solution to the CC problem and more!

**Cliff Burgess, PI/McMaster University **

The Cosmological Constant Problem and Extra Dimensions

**Daniel Chung, University of Wisconsin-Madison**

Phenomenological Connections between Particle Physics and Dark Energy

I explore possible connections between the Higgs sector and dark energy.

**Aharon Davidson, ****Ben-Guiron University**

**Zero cosmological Constant from Normalized General Relativity**

**Ghazal Geshnizjani, Perimeter Instiutute**

**Observational Evidence for Cosmological-Scale Extra Dimensions**

**Lam Hui, Columbia University**

Modified gravity, degravitation and equivalence principle

**Alberto Iglesias, UC Davis**

**Dilution of the Cosmological Constant: Higher Codimension Branes and Higher Curvature Terms**

**Nissan Itzhaki, Tel-Aviv University**

Dynamics and the Cosmological Constant Problem

**Nemanja Kaloper, UC Davis,**

Where in the String Landscape is Quintessence

I will make the case that the Bousso-Polchinski landscape may contain quintessential corners.

**Justin Khoury, University of Pennsylvania**

Cascading Gravity and the Cosmological Constant

**Hyun-Min Lee, Carnegie Mello University**

Inflation on a codimension-two brane

We consider a codimension-2 brane inflation scenario in a warped flux compactification of 6D gauged supergravity. The volume modulus of the model is stabilized by means of potentials localized on the regularized background branes. We discuss the cosmological evolution of the world-volume of a probe codimension-2 brane when it moves along the radial direction of the internal space. In order to have slow-roll inflation, we find that the warping of the internal space is required to be weak, in contrast to the string inflation constructions with strong warping. We discuss the parameter range that the inflation is in agreement with the observationally inferred parameters and which furthermore is consistent with the probe brane approximation. We argue that from a multi-brane solution, the backreaction of the probe brane on the weak warp factor is ignorable.

**John Moffat, Perimeter Institute**

Positive and Negative Energy Symmetry and the Cosmological Constant Problem

The Hamiltonian for the quantized gravitational and matter fields contains both positive and negative energy particle contributions, which leads through a positive and negative energy symmetry of the vacuum to a cancellation of the zero-point vacuum energy and a vanishing cosmological constant in the presence of a gravitational field. The positive and negative energy particles interact only weakly through gravity. As in the case of antimatter, the negative energy matter is not found naturally on Earth or in galaxies in the universe. We introduce a graviton momentum cutoff ΛG ≤ 2 x10-3 eV that leads to a gravitational stability of the Minkowski spacetime vacuum with a lifetime greater than the age of the universe. A positive energy spectrum and a consistent unitary field theory for a pseudo- Hermitian Hamiltonian is obtained by demanding that the pseudo-Hermitian Hamiltonian is PT symmetric. The quadratic divergences in the two-point vacuum fluctuations and the self- energy of a scalar field are removed. By adopting a Higgsless model of electroweak theory, we remove the fine-tuning associated with a spontaneous symmetry breaking vacuum density. We also postulate that there are no phase transitions associated with QCD and at higher particle physics energy scales, removing all theorized quark-gluon vacuum density condensates and their fine-tuned vacuum densities and cosmological constant contributions.

**Susha Parameswaran, DESY**

6D Brane Models and their Perturbations

Models with two supersymmetric large extra dimensions (SLED) may provide a way to approach both the cosmological constant problem and dark energy. After reviewing these ideas, I shall discuss warped brane world solutions in 6D supergravity, as a laboratory in which to explore SLED and codimension two branes in general. Solving the linearized perturbations for all the bosonic fields, and some of the fermions, we can observe how the corresponding physics compares with 5D models and standard Kaluza-Klein compactifications. These results should help to better our understanding of the cancellations in the 4D effective vacuum energy with SLED

**Federico Piazza, Perimeter Institute**

Modifying Gravity in the Infra-Red by imposing an Ultra-Strong equivalence principle.

I will give account of a work in progress in which I attempt to modify the metric-manifold structure of GR in the infra-red. The proposed modification does not contain any massive parameter as it is effective at length scales comparable with the inverse (extrinsic) curvature. The guiding line for this modification is an "ultra-strong" equivalence principle, according to which even semi-classical gravitational effects (i.e. particle production) are definitely banned from a sufficiently small free-falling elevator. Some cosmological consequences of this modification will be discussed.

**Seifallah Randjbar-Daemi, ICTP**

Infrared Modification of Gravity with Dynamical Torsion

Theories of gravity having connection as an independent dynamical variable can have a massive spin -2 particle together with the massless graviton in their spectrum. We study the potentiality of such models as a consistent infrared modification of gravity. It will be shown that these models are free from ghost like Boulware-Deser mode in the background of arbitrary torsionless Einstein manifolds. At least for weak for weak enough curvature the dangerous spin zero mode has a healthy kinetic energy in our backgrounds. vDVZ singularity seems to be a generic feature which may be cured by Vainshtein mechanism as in other infrared modified theories of gravity.

**Yanwen Shang, University of Toronto**

Massive gravity in 3-D and the Chern-Simons-Proca theory

**Lee Smolin, Perimeter Institute**

The quantization of unimodular gravity and the cosmological constant problem

**Rafael Sorkin, Perimiter Institute**

Is the cosmological constant a nonlocal quantum residue of discreteness of the causal set type?

The currently accelerating Hubble expansion is in accord with the old heuristic prediction, from causal set theory, of a fluctuating and ever-present cosmological "constant''. More recently, a phenomenological model based on certain of the ideas behind the prediction has been devised, but it remains incomplete. I will review these developments and also mention a possible consequence for the dimensionality of spacetime.

**Henry Tye, Cornell University **

Is there Eternal Inflation in the Cosmic Landscape?