### Conveners

#### Dark Energy: DE

- Dragan Huterer

#### Dark Energy: DE

- Dragan Huterer

#### Dark Energy: DE

- Marek Biesiada

#### Dark Energy: DE

- Ruth Gregory

#### Dark Energy: DE

- David Polarski

Matteo Fasiello
(Stanford University, United States)

9/8/15, 2:00 PM

The quest for a stable, compelling, cosmology of massive gravity. I will introduce some of the everyday tools of massive (bi)gravity and then illustrate the latest developements. Particular attention will be devoted to the so-called generalized massive gravity model, its observational viability as well as its stability as a qft.

Benedict Broy
(Theory Group, DESY, Germany)

9/8/15, 2:20 PM

Motivated by UV realisations of Starobinsky-like inflation models, we study generic exponential plateau-like potentials to understand whether an exact $f(R)$-formulation may still be obtained when the asymptotic shift-symmetry of the potential is broken for larger field values. Potentials which break the shift symmetry with rising exponentials at large field values only allow for corresponding...

Ryo Saito
(Laboratoire APC, France)

9/8/15, 2:40 PM

Many theories of modified gravity, including the well studied Horndeski models, are characterized by a screening mechanism that ensures that standard gravity is recovered near astrophysical bodies. In a recently introduced class of gravitational theories that goes beyond Horndeski, it has been found that new derivative interactions lead to a partial breaking of the Vainshtein screening...

Sandrine Schloegel
(UNamur, Belgium)

9/8/15, 3:00 PM

Current acceleration of the cosmic expansion leads to co%u010Fncidence as well as fine-tuning issues in the framework of general relativity. Dynamical scalar field
have been introduced in response of these problems, some of them invoking screening mechanisms for passing local tests of gravity. Recent lab experiments based on atom interferometry in a vacuum chamber have been proposed for...

Kazufumi Takahashi
(RESCEU The University of Tokyo, Japan)

9/8/15, 3:20 PM

f(R) gravity is one of the simplest generalizations of general relativity, which may explain the accelerated cosmic expansion without introducing a cosmological constant. Transformed into the Einstein frame, a new scalar degree of freedom appears and it couples with matter fields. In order for f(R) theories to pass the local tests of general relativity, it has been known that the chameleon...

Frank Koennig
(Institut fuer Theoretische Physik -Uni Heidelberg, Germany)

9/8/15, 4:10 PM

Bimetric theory describes gravitational interactions in the presence of an extra
spin-2 field. It allows for many different types of cosmological
solutions but not all of them are theoretically allowed, most of them are
generically plagued by instabilities. We discuss the
conditions of the absence of these ghost- and gradient instabilities and present
a ghost-free model in which the gradient...

Alexandra Terrana
(York University, Canada)

9/8/15, 4:30 PM

Massive bigravity, a theoretically consistent modification of general relativity with an additional dynamical rank two tensor, successfully describes the observed accelerated expansion of the Universe without a cosmological constant. Previous analyses of perturbations around a cosmological background have revealed power law instabilities in both the scalar and tensor sectors, leading to...

Henrik Nersisyan
(Heidelberg University ITP, Germany)

9/8/15, 4:50 PM

Massive bigravity models are interesting alternatives to standard cosmology. In most cases however these models have been studied for a simplified scenario in which both metrics take homogeneous and isotropic forms (Friedmann-Lemaitre-Robertson-Walker; FLRW) with the same spatial curvatures.The interest to consider more general geometries arises in particular in view of the difficulty so far...

Ana Caterina Leite
(Centro de Astrofísica da Universidade do Porto, Portugal)

9/8/15, 5:10 PM

We recently extended Principal Component Analysis based methods to constrain the dark energy equation of state (originally developed for Type Ia supernovae and other low redshift probes) to spectroscopic tests of the stability of fundamental couplings, which can probe higher redshifts. In this talk I will use these methods to quantify the gains in sensitivity obtained by combining...

Daisuke Yoshida
(Tokyo Institute of Technology, Japan)

9/8/15, 5:50 PM

We investigate perturbations of a class of time-dependent spherically symmetric solutions in bi-gravity, which includes both FLRW space time and Schwarzschild de Sitter space time.
We consider the solutions which can be obtained only when the parameters of bi-gravity satisfy a special relation. In this case, It is known that the equation of motion for spherically symmetric space time reduces...

Alexander Leithes
(Queen Mary University of London, United Kingdom)

9/9/15, 9:00 AM

In this talk I will outline our research into the evolution of perturbations in coupled quintessence with multiple fluids and scalar fields. I will explain what coupled quintessence is and why it is of interest and will introduce a new Python code developed by myself in conjunction with my collaborators. There are a vast number of different coupled quintessence models and potentials and we...

Aleksandra Piorkowska
(University of Silesia, Poland)

9/9/15, 9:20 AM

In the era of massive galactic surveys, strong gravitational lensing systems have entered a stage in which we observe their growing significance in astrophysics and cosmology.
In our work we present a catalog of 118 strong gravitational lensing systems built on SLACS, BELLS, LSD and SL2S samples and we use them to demonstrates that strong lensing data provide a good quality tool for...

Elina Palmgren
(Helsinki institute of physics, Finland)

9/9/15, 9:40 AM

This presentation will introduce the research [1] in which we employed the Planck CMB temperature anisotropy and lensing data, and baryon acoustic oscillation (BAO) data to constrain a phenomenological $w$CDM model, where dark matter and dark energy interact. We assumed time-dependent equation of state parameter for dark energy, and treated dark matter and dark energy as fluids whose...

Douglas Spolyar
(Stockholm University OKC, Sweden)

9/9/15, 10:00 AM

Massive gravity can dynamically and transparently provide of the acceleration of the of the universe,
where the mass of the graviton sets the energy scale of dark enregy. We find the parameter space of massive graivty which easily accomodates the observed acceleration of the Universe by SNa, and BAO with the most recent Planck data. The same parameter space also makes unique predictions for...

Martina Donnari
(Sapienza University of Rome, Italy)

9/9/15, 10:20 AM

The role of Dark Energy (DE) in the long term evolution of galaxy clusters is the main topic of this talk.
Recently, observational data of the outflow of galaxies in the Virgo cluster, suggest that DE can also act on a small cosmic scales, like a single galaxy cluster.
By means of direct N-body simulations and taking advantage of the latest generation of computational hardware and software,...

Francesco Pace
(University of Manchester, United Kingdom)

9/10/15, 2:00 PM

Dark matter and dark energy are usually considered to be two separate components in the energy budget of the Universe. A simple extension to this hypothesis is to consider that they could be different aspects of a single component. This class of models is hence dubbed Unified Dark Matter (UDM) models.
UDM models could cluster and hence give origin to cosmic structures if the adiabatic sound...

Juan Magana
(Institute of Physics and Astronomy Universidad de Valparaiso, Chile)

9/10/15, 2:20 PM

In this work we constrain four alternative models to explain the late cosmic acceleration in the Universe: Chevallier-Polarski-Linder, interacting dark energy, Ricci holographic dark energy, and modified polytropic Cardassian. To test these models, we use mainly several strong gravitational lensing images of background galaxies produced by the galaxy cluster Abell 1689. We compare the value...

Pablo Gonzalez
(Universidad de Chile, Chile)

9/10/15, 2:40 PM

We present a model of the gravitational field based on two symmetric tensors, $g_{\mu \nu}$ and $\tilde{g}_{\mu \nu}$. Besides, we have a new matter field given by $\tilde{\phi}_I = \tilde{\delta} \phi_I$, where $\phi_I$ are the original matter fields. We call them $\tilde{\delta}$ matter fields. This model, called $\tilde{\delta}$ Gravity, has excellent properties at the quantum level. It...

Vanessa Smer Barreto
(University of Edinburgh, United Kingdom)

9/10/15, 3:00 PM

Two models of the hybrid metric-Palatini theory of gravitation are introduced. Their background evolution is explored, showing explicitly that one recovers standard General Relativity with an effective Cosmological Constant at late times. This happens because the Palatini Ricci scalar evolves towards and asymptotically settles at the minimum of its effective potential during cosmological...

Mike Hewitt
(Canterbury Christ Church University, United Kingdom)

9/10/15, 3:20 PM

The successful $\Lambda CDM$ cosmological model requires a small but nonzero $\Lambda$ which appears to have an unnaturally small value compared to the supersymmetry breaking scale, typically $O(10^{-60}) m_{3/2}^4$ for $m_{3/2} \sim 10 TeV$. We explore the possibility of solving this naturalness problem in a special class of no-scale supergravity models which arise from a supersymmetric...

Ilia Musco
(LUTH - Observatoire de Paris, France)

9/10/15, 4:10 PM

It has been suggested that a scalar field φ non-minimally coupled to matter could be responsible for the observed accelerated expansion of the Universe. However, the fact that we are able to measure its effect only on cosmological scales but not on local ones, such as that of our solar system, might be the consequence of a screening mechanism. This is the essence of the Chameleon model....

Ali Nayeri
(Chapman University, United States)

9/10/15, 4:30 PM

Some years ago in the context of a mechanism that thermally generates the primordial perturbations through a Hagedorn phase of string cosmology (Nayeri-Brandenberger-Vafa (NBV) setup) a blue tensor tilt with an induced and complimentary red tilt to the scalar spectrum, with a naturally large tensor to scalar ratio that relates to both, was found. One of the shortcomings of this setup, however,...

Purnendu Karmakar
(Universita degli Studi di Padova, Italy)

9/10/15, 4:50 PM

P. Karmakar, in collaboration with: F. Arroja, N. Bartolo and S. Matarrese
(arXiv:1506.08575).
We show that very general scalar-tensor theories of gravity (including, e.g., Horndeski models) are generically invariant under disformal transformations. However there is a special subset, when the transformation is not invertible, that yields new equations of motion which are a generalization...

Thorsten Battefeld
(University of Goettingen, Germany)

9/10/15, 5:10 PM

Moduli spaces in string theory, often dubbed Landscapes, are usually of high dimensionality and feature a complicated potential. Is multi-field inflation on such landscapes consistent with current observations? Modeling such landscapes by random potentials offers the opportunity to asses generic features of inflation. Random matrix theory provides a tool (complementing numerical experiments)...

Wessel Valkenburg
(Leiden University, Netherlands)

9/10/15, 5:30 PM

I will present a novel description for setting initial particle displacements and field values under arbitrary metric theories of gravity, for perfect and imperfect fluids with arbitrary characteristics. We extend the Zel'dovich Approximation to nontrivial theories of gravity, and show how scale dependence implies curved particle paths, even in the entirely linear regime of perturbations....

Mathew Hull
(University of Portsmouth, United Kingdom)

9/10/15, 5:50 PM

Vectors with ghost-free derivative self-interactions have been shown to have interesting cosmological applications. They form a ghost-free extension of the Proca theory that has a Galileonic longitudinal mode and can be generated naturally via a Higgs mechanism. In this talk I will discuss recent results on the covariantisation of these models and their relation to general ghost-free...