### Conveners

#### 03 - Modifications of gravity

- Anne-Christine Davis (Cambridge University)

#### 03 - Modifications of gravity

- Anne-Christine Davis (Cambridge University)

#### 03 - Modifications of gravity

- Anne-Christine Davis (Cambridge University)

Prof.
Jounghun Lee
(Seoul National University)

12/15/15, 4:15 PM

Talk

The radial velocities of the galaxies in the vicinity of a massive cluster shows deviation from the pure Hubble flow due to their gravitational interaction with the cluster. According to a recent study of Falco et al. with a high-resolution N-body simulation based on General Relativity (GR), the radial velocity profile of the galaxies located at distances larger than three times the virial...

Mr
Reginaldo Durazo
(Instituto de Astronomía, Universidad Nacional Autónoma de México)

12/15/15, 4:35 PM

Talk

For any MONDian extended theory of gravity where the rotation curves of spiral galaxies are explained through a change in physics rather than the hypothesis of dark matter, a generic dynamical behaviour is expected for pressure supported systems: an outer flattening of the velocity dispersion profile occurring at a characteristic radius, where both the amplitude of this flat velocity...

David Moore
(Stanford University)

12/15/15, 4:55 PM

Talk

We are developing a novel technique to search for non-Newtonian gravitational forces at micron length scales using optically levitated dielectric microspheres. At high vacuum, dissipation of the microsphere's motion due to residual gas collisions becomes small, allowing sub-attonewton force sensitivity. As a first demonstration of the ability to perform sensitive force measurements with...

Mr
Jurgen Mifsud
(Consortium for Fundamental Physics, School of Mathematics and Statistics, University of Sheffield)

12/15/15, 5:25 PM

Talk

We study a theory in which the electromagnetic field is disformally coupled to a scalar field, in addition to a usual non--minimal electromagnetic coupling. We show that disformal couplings modify the expression for the fine--structure constant, $\alpha$. As a result, the theory we consider can explain the non--zero reported variation in the evolution of $\alpha$ by purely considering...

Diego Blas Temino
(CERN)

12/15/15, 5:45 PM

Talk

After briefly explaining why Lorentz violating theories of gravity are interesting for quantum gravity, I will discuss how they can be tested with current astrophysical and cosmological observations.

Dr
Lubos Neslusan
(Astronomical Institute, Slovak Academy of Sciences, 05960 Tatranska Lomnica, Slovakia)

12/15/15, 6:10 PM

Poster

In 2011, Jun Ni published solution of the equations in the classical Tolman-Oppenheimer-Volkoff (TOV) modeling of spherically symmetric neutron star. The Ni's solution implies no upper-mass limit and the outer surface of modeled object always appears to be above the event horizon. In fact, Ni found an infinite variety of sets of the TOV-problem solutions. The original Oppenheimer-Volkoff...

Dr
Sergio Mendoza
(Instituto de Astronomia, Universidad Nacional Autonoma de Mexico (UNAM))

12/15/15, 6:13 PM

Poster

A metric extension of gravity based on the Tully-Fisher law is
presented. It will be shown that the Tully-Fisher law extends from the
dynamics of globular clusters up to the dynamics of groups of galaxies
and how it can be consider as a modified version of Kepler's third law.
With it, it will be shown how at second perturbation order lensing can
be fully understood and that the...

Hussain Gohar
(University of Szczecin, Poland)

12/15/15, 6:16 PM

Poster

We formulate the basic framework of thermodynamical entropic force cosmology which allows
variation of the gravitational constant G and the speed of light c. Three different approaches to the
formulation of the field equations are presented. Some cosmological solutions for each framework
are given and one of them is tested against combined observational data (supernovae, BAO, and
CMB)....

Prof.
Pradyumn Kumar Sahoo
(Birla Institute of Technology and Science-Pilani, Hyderabad Campus)

12/15/15, 6:19 PM

Poster

A class of Kaluza-Klein cosmological models in $f(R,T)$ theory of
gravity have been investigated. In the work, we have considered the
functional $f(R,T)$ to be in the form $f(R,T)=f(R)+f(T)$ with
$f(R)=\lambda R$ and $f(T)=\lambda T$. Such a choice of the
functional $f(R,T)$ leads to an evolving effective cosmological
constant $\Lambda$ which depends on the stress energy tensor....

Ernesto Barrientos Rodríguez
(Universidad Nacional Autónoma de México)

12/15/15, 6:22 PM

Poster

In this work we construct a relativistic extension of the MODified
Newtonian Dynamics (MOND) in the metric formalism $f(\chi)$ using the
Palatini approach. We show that a simple power law: $f(\chi)=\chi^b$,
with $b = 3/2$ corresponds to the non-relativistic form of MOND.
Amongst the many approaches proposed to extend MOND to a relativistic
regime, the Palatini metric formalism discussed...

Dr
Hossein Ghaffarnejad
(Semnan University of IRAN)

12/15/15, 6:25 PM

Poster

Combinations of Lovelock polynomials $R^2,R_{\mu\nu}R^{\mu\nu}$ and
$R_{\mu\nu\eta\delta}R^{\mu\nu\eta\delta}$ is added with
Einstein-Hilbert action to obtain interior metric of an anisotropic
spherically symmetric collapsing (ASSC) stellar cloud. We assume
that time dependent interior metric of the ASSC cloud is flat
Minkowski at beginning of the collapse. We solved linearized...

Ms
Giulia Cusin
(University of Geneva)

12/16/15, 2:00 PM

Talk

We introduce some recently proposed non-local infrared modifications of general relativity. We discuss which are the motivations to introduce non-localities in a theory of gravity. We then present a particular class of models which has been recently shown to be cosmologically viable, with an extremely good compatibility with cosmological data. We present the general features of such a class...

Jorge Ovalle
(Simon Bolivar University)

12/16/15, 2:21 PM

Talk

In the context of extra-dimensional gravity, as the Randall-Sundrum brane-world, a consistent extension of the minimal geometric deformation approach (MGD) is used to study the exterior spacetime
around spherically symmetric self-gravitating system. A modified Schwarzschild geometry is obtained and new black hole solutions are shown. A possible extension of this approach in $F(R)$ theories is...

Dr
Lucas Lombriser
(University of Edinburgh)

12/16/15, 2:43 PM

Talk

Modifications of gravity arising in the presence of a nonminimally coupled scalar field and capable of accelerating the expansion of our Universe can be suppressed at the linear level of cosmological perturbations, only introducing deviations from concordance cosmology at the largest observable scales. I will classify the theory space capable of this mechanism in the effective field theory of...

Wessel Valkenburg
(Leiden University)

12/16/15, 3:04 PM

Talk

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....

Laura BERNARD
(Université de Paris 6 - Pierre et Marie Curie)

12/16/15, 4:15 PM

Talk

The only consistent linear theory for a massive spin-2 field on a flat space-time has been known for a long time as being the Fierz-Pauli theory. Its promotion to a non-linear theory, although essential, has long been thought impossible because of the appearance of the Boulware-Deser (BD) ghost. Recently, de Rham, Gabadadze and Tolley (dRGT) proposed a family of massive gravity theories, free...

Mariele Motta
(University of Geneva)

12/16/15, 4:35 PM

Talk

In this talk I will discuss linear perturbations of dRGT massive bi-gravity with a single metric coupled to matter. First, I will introduce the formal derivation of the second order action for generic metrics. I will then use this result to identify stability bounds. Finally, I will discuss the linear perturbations on a FRW background, the cosmology of different branches, the number of degrees...

Pietro Guarato
(Université de Genève)

12/16/15, 4:55 PM

Talk

In this talk, a detailed analysis of the evolution of tensor perturbations in a cosmological background for a model of Hassan-Rosen theory of bigravity is presented. It is shown that gravitational waves are unstable in this setting, but also that in practice the amplitude of tensor perturbations generated during inflation is sufficiently suppressed to avoid this instability from showing up...

Johannes Noller
(University of Oxford)

12/16/15, 5:15 PM

Talk

Galileons appear in the low-energy limit of several cosmologically motivated theories, e.g. Massive Gravity, Bigravity and DGP. Yet we are only just beginning to understand some of their features. I will discuss newly discovered dualities and enhanced symmetries for (subsets of) Galileons and how they are related to scalar-theories of gravity and Massive gravity/Bigravity in particular.

Massimiliano Rinaldi
(University of Trento)

12/16/15, 5:35 PM

Talk

Recently Kallosh, Linde, and collaborators have provided a unified description of single-field inflation in terms of just one parameter α. These so-called α-attractors predict a spectral index n_s and a tensor-to-scalar ratio r, which are fully compatible with the latest Planck data. The only common feature of all α-attractors is a non-canonical kinetic term with a pole, and a potential...

Dr
Ilia Musco
(Laboratoire Univers et Théories (LUTH) - Observatoire de Paris)

12/16/15, 5:55 PM

Talk

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....