Have primordial gravitational waves been detected?

• Sabino Matarrese (Padova University)

Room: Pontecorvo room Abstract available at: http://agenda.infn.it/event/matarrese

Sequestering the Standard Model Vacuum Energy

• Antonio Padilla (University of Nottingham)

Room: Pontecorvo room We propose a very simple reformulation of General Relativity, which completely sequesters from gravity {\it all} of the vacuum energy from a matter sector, including all loop corrections and renders all contributions from phase transitions automatically small. The idea is to make the dimensional parameters in the matter sector functionals of the 4-volume element of the universe. For them to be nonzero, the universe should be finite in spacetime. If this matter is the Standard Model of particle physics, our mechanism prevents any of its vacuum energy, classical or quantum, from sourcing the curvature of the universe. The mechanism is consistent with the large hierarchy between the Planck scale, electroweak scale and curvature scale, and early universe cosmology, including inflation. Consequences of our proposal are that the vacuum curvature of an old and large universe is not zero, but very small, that wDE≃−1 is a transient, and that the universe will collapse in the future.

Fake hair for black holes

• Christos Charmousis (LPT Orsay)

Room: Pontecorvo room We will review modification of gravity theories and in particular scalar tensor theories the mildest of modifications where apart from the metric we have an additional interacting scalar field coupling to the metric tensor. By means of a theorem given by Horndeski back in 1974 we will show the most general of these theories. Horndeski theory is often dubbed Galileon theory. We will examine a particular sub class of Horndeski theory which has interesting properties with respect to the cosmological constant problem. We will then find black hole solutions of this subclass which in some cases will be identical to GR solutions. The novel ingredient will be the presence of a time and space dependent scalar. As we will see time dependence will bifucate no hair theorems and provide for the first time regular scalar tensor black holes with a non trivial scalar field.

Nonlocal infrared modifications of gravity and dark energy

• Michele Maggiore (Geneva University)

Room: Pontecorvo room We discuss a recently developed approach to infrared modifications of GR, in which a mass term is introduced as a coefficient of nonlocal operators. We discuss conceptual aspects and cosmological consequences of the proposal. Such nonlocal theories, involving retarded propagators, must be understood as effective classical theories derived from some more fundamental (and local) QFT. The theory only involve a mass parameter $m$, which replaces the cosmological constant in LCDM, and is highly predictive. At the background level, after fixing $m$ so as to reproduce the observed value of $\Omega_M$, we get a pure prediction for the equation of state of dark energy as a function of redshift, $w_{\rm DE}(z)$, with $w_{\rm DE}(0)$ in the range $[-1.165,-1.135]$ as $\Omega_M$ varies over the broad range $\Omega_M\in[0.20,0.36]$. We find that the cosmological perturbations are well-behaved, and the model fully fixes the dark energy perturbations as a function of redshift $z$ and wavenumber $k$. The nonlocal model provides a good fit to supernova data and predicts deviations from General Relativity in structure formation and in weak lensing at the level of 3-4\%, therefore consistent with existing data but readily detectable by future surveys.

Some results on bimetric theory and relation to conformal gravity

• Fawad Hassan (University of Stockholm)

Room: Pontecorvo room In a perturbative expansion in powers of curvatures, the equations of motion in bimetric theory can be expressed as an equation for a single metric with higher derivative corrections. Motivated by such an expansion we prove some exact results and discuss properties of bimetric theory that are of relevance to avoiding determinant singularities and superluminal modes. A particular bimetric model reduces to conformal gravity at the lowest order in such a derivative expansion. We discuss the relation between this model and the "partially massless'' theory around de Sitter backgrounds. In particular we show that the bimetric model has solutions that do not survive in the massive gravity limit.

The Vainshetein mechanism

• Kazuya Koyama (University of Portsmouth)

Room: Pontecorvo room