Introduction Room: 132:028

Extended Theories of Gravity

• Fawad Hassan (Fysikum)

Room: 132:028

Journal club (different avenues of modifying gravity) Room: 132:028 We will collect the different ideas that have been proposed so far in the literature to tackle the problems that Fawad has discussed in the morning session.

Polytropic stellar configurations in modified gravity

• Kaisa Henttunen

Room: 132:028 Modified gravity models arise from different motivations but in cosmology are studied because of their properties that allow inflation or the current accelerated expansion. We have concentrated on the local matter configurations and compared some modified gravity model properties to the general relativistic counterparts and observations. The studied gravity models are cosmologically motivated f(R) theories (CDTT and HS) and a scalar-tensor model class arising the O'Hanlon action. We have numerically analyzed the physically parametrized polytropic stellar configurations with the above models to study their viability.

Planck 2015 results. XIV. Dark energy and modified gravity Room: 132:028 http://www.cosmos.esa.int/documents/387566/522789/Planck_2015_Results_XIV_Dark_Energy_Mod_Gravity.pdf

On metric-affine variational principles

• Alexey Golovnev

Room: 132:028

Albanova colloquim: Claudia de Rham Room: Oskar Klein auditorium Albanova colloquim

Introduction to Supergravity

• Angnis Schmidt-May

Room: 132:028

Journal club (degravitation)

• Andrew Tolley (Case Western Reserve University)

Room: 132:028

Massive Gravity

• Claudia de Rham (Case Western Reserve University)

Room: FB53 If the particle carrier of the gravitational force had a mass, gravity could behave differently as GR on cosmological scales thereby potentially tackling the dark energy and cosmological constant problems. I will review the cosmological behavior of this model as well as some of its extensions. In particular I will present some exact FLRW solutions in generalized massive gravity where the mass parameters are naturally promoted to Lorentz-invariant functions of the Stuckelberg fields. The existence of stable self-accelerating solutions will be discussed as well as some relevant phenomenology.

Duality and Monopoles in Partially Massless Gravity

• Kurt Hinterbichler (Perimeter Institute for Theoretical Physics)

Room: FB53

Observational cosmology for theorists

• Edvard Mörtsell (Stockholm University)

Room: FB53 I will give an overview of the current status of observational cosmology with emphasis on the implications for gravitational theories.

Observations of the accelerating Universe: past, present and prospects

• Ariel Goobar (Stockholm University)

Room: FB53

The square-root matrix in ghost-free bimetric theory

• Angnis SCHMIDT-MAY (ETH Zürich)

Room: FB53 The recent development of ghost-free nonlinear massive gravity and its generalization to bimetric theory has created an exciting new research area within theoretical cosmology. Bimetric theory can be viewed as a deformation of general relativity by the presence of a massive spin-2 field. It is usually formulated in terms of two metrics with individual Einstein-Hilbert terms as well as an interaction potential whose structure is severely constrained by consistency. In particular, the interactions contain a matrix square-root which may not always be well-defined and unfortunately complicates many computations. In this talk, I will provide the conditions under which the square-root matrix is well-defined and discuss their relation to the vierbein formulation of bimetric theory. Moreover, I will suggest a method of dealing with the linear variation of the square root which can simplify the treatment of perturbation theory around curved backgrounds.

Scalarisation instability with disformally coupled matter

• Hannu Nyrhinen (Helsinki Institute of Physics)

Room: FB53

Non-linear stability of scalar graviton in bigravity theory

• Ryo Namba (Kavli IPMU, University of Tokyo)

Room: FB53 A fully non-linear massive gravity was recently constructed by de Rham, Gabadadze and Tolley (dRGT), removing the unwanted extra degree of freedom condemned by ghost instability, known as Boulware-Deser (BD) ghost. It was, however, soon found that homogeneous and isotropic solutions with accelerated expansion in this theory suffers new-type ghost instabilities at the non-linear level. The bigravity theories as an extension of the dRGT massive gravity promote the non-dynamical fiducial metric in the dRGT theory to a dynamical quantity. The class of these theories with each matter sector coupling to only one of the physical and fiducial metrics preserves the structure that guarantees the absence of the BD ghost, and it has been shown to have some parameter space for cosmological solutions. While these solutions are stable against perturbations for the graviton mass greater than the Hubble expansion rate, a linear analysis exhibits ghosts for the scalar graviton in the opposite regime, indicating an instability in the early universe. We extend the analysis including non-linear effects from self interactions of the scalar graviton. We study the stability of the non-linear solutions, exploring the possibility to evade the instability at both early and late times.

From configuration to dynamics

• Sjinji Mukohyama (Yukawa Institute for Theoretical Physics, Kyoto University)

Room: FB53 The Lorentzian metric structure used in any field theory allows one to implement the relativistic notion of causality and to define a notion of time dimension. In this talk we investigate the possibility that at the microscopic level the metric is Riemannian, i.e., locally Euclidean, and that the Lorentzian structure, that we usually consider as fundamental, is in fact an effective property that emerges in some regions of a four-dimensional space with a positive definite metric. In such a model, there is no dynamics nor signature flip across some hypersurface; instead, all the fields develop a Lorentzian dynamics in these regions because they propagate in an effective metric. It is shown that one can construct a decent classical field theory for scalars, vectors, and spinors in flat spacetime. It is then shown that gravity can be included but that the theory for the effective Lorentzian metric is not general relativity but of the covariant Galileon type. The constraints arising from stability, the equivalence principle, and the constancy of fundamental constants are detailed and a phenomenological picture of the emergence of the Lorentzian metric is also given. The construction, while restricted to classical fields in this article, offers a new view on the notion of time.

Status of Bimetric Cosmology I

• Yashar AKRAMI (Institute for Theoretical Physics, University of Heidelberg)

Room: FB53

Status of Bimetric Cosmology II

• Adam SOLOMON (DAMTP, University of Cambridge)

Room: FB53

UV Properties of Galileons

• Andrew Tolley (Case Western Reserve University)

Room: FB53 I will discuss the possible UV properties of Galileons and by extension Massive Gravity theories.

Cosmological implications of Lorentz violation

• Diego Blas (CERN)

Room: FB53 Recent approaches to quantum gravity suggest abandoning Lorentz invariance as a fundamental symmetry of Nature. This introduces new gapless degrees of freedom in the gravity sector. I will review the consequences for cosmology of the minimal set-up and present the constraints from different observations. If I have time, I’ll briefly comment on the successful merging of these ideas with massive gravity.

Horava gravity: Suppressing the Lorentz violations in the matter sector

• Emir Gumrukcuoglu (University of Nottingham)

Room: FB53 Horava gravity is a carefully constructed theory which is renormalizable in the power-counting sense. As the theory is founded on an anisotropic scaling between time and space, it does not respect the local Lorentz symmetry that interchanges time and space coordinates. Instead, it has a preferred time direction allowing the high energy theory to be finite without spoiling unitarity. A major obstacle for the theory is the restoration of Lorentz invariance at low energies in the matter sector, where constraints are very stringent. A promising mechanism, proposed by Pospelov and Shang (2010), restricts the Lorentz violations to the gravity sector at tree level and these percolate the matter sector only though graviton loops. In this set-up, one can keep the Lorentz-violating terms in the matter sector under control, although the gauge mode loops uncover a technical naturalness problem. A large portion of the talk will be devoted to the review of Horava gravity, with an emphasis on theoretical consistency requirements and observational constraints on the Lorentz breaking scales. I will conclude by a discussion of the mixed-derivative extension of the theory, designed to resolve the naturalness problems in the Pospelov-Shang model.

Towards the most general scalar-tensor theories of gravity

• Miguel ZUMALACARREGUI (ITP Heidelberg)

Room: FB53 Horndeski's theory of gravity (the most general scalar-tensor theory with second-order equations of motion) has recently attracted considerable attention as a framework to study gravitation and cosmology. I will challenge the generality of Horndeski's theory as the most general, physically reasonable scalar-tensor theory by analyzing the simplest theory beyond Horndeski and its equations of motion. Theories beyond Horndeski have unique characeristics and represent a new class of scalar-tensor theories of gravity. If time permits, I will present new results on bigravity, regarding the implications of growing modes in the tensor sector for CMB observations

Emergent non-linearity from alternative action principles

• Marit SANDSTAD

Room: FB53 C-theory is a framework which provides a smooth transition between metric and palatini variations of f(R) actions. In the paper 1305.0695 we proved an equivalence between a specific C-theory and a particular non-local theory of gravity. We will present this result and speculate a bit on the implictions it has for locality in the context of alternative action principles in general.

Maximal freedom at minimum cost in general scalar-tensor theories

• Emilio BELLINI (University of Barcelona)

Room: FB53 In this talk I shall discuss the properties of linear perturbations of scalar-tensor theories belonging to the Horndeski class of actions whilst evolving on a cosmological background. I will discuss the physical interpretation of five independent functions of time, which are sufficient in order to describe the exact linear dynamics of these models. Finally, I will briefly generalize these result to second-order perturbation theory.

The effective theory of fluids and dark energy

• Guillermo BALLESTEROS (IFT-UAM/CSIC)

Room: FB53 I will discuss the effective theory of fluids and some of its applications in cosmology. In particular, I will focus on the features of the theory that make it especially useful to model the acceleration of the Universe and search for departures from the standard LambdaCDM. I will also comment on the main differences with respect to popular single-field models.

Challenges for Bouncing Cosmologies

• Robert Brandenberger (McGill University)

Room: FB53 Since bouncing cosmologies trivially solve the horizon and entropy problems of Standard Cosmology, it is reasonable to hope that one might be able to construct alternatives to cosmological inflation using bounces. I will discuss key challenges which must be overcome in order to realize this hope.

SuperCool Inflation

• Douglas Spolyar (OKC)

Room: FB53 SuperCool Inflation is a new technically natural model which gives a graceful exit to old inflation. The Universe starts off hot and trapped in a false vacuum. The Universe supercools and inflates solving the horizon and flatness problems. The inflaton couples to a set of QCD like fermions. When the fermions' non-Abelian gauge group freezes, the Yukawa terms generate a tadpole for the inflaton, which removes the barrier. The model works at scales which can be probed by LHC. Finally, we introduce a light spectator field to generate density perturbations, which match the CMB.

Inflation from Galilean Genesis

• Luca SANTONI (Scuola Normale Superiore)

Room: FB53 I will discuss a class of scalar models that, once coupled to gravity, lead to cosmologies that smoothly and stably connect an inflationary quasi-de Sitter universe to a low, or even zero-curvature, maximally symmetric spacetime in the asymptotic past, strongly violating the null energy condition at intermediate times. The models are based on symmetries, both exact and approximate, that give rise to the quantum robustness of the whole picture. The resulting cosmological backgrounds can be viewed as regularized extensions of the galilean genesis scenario, or, equivalently, as 'early-time-complete' realizations of inflation

Dark energy and nonlocal gravity

• Michele Maggiore (Geneva University)

Room: FB53 I will discuss the motivations for a recently proposed nonlocal modification of GR, and I will perform a detailed comparison of its predictions with cosmological data

Special Finite Quantum Gravity

• Leonardo Modesto

Room: FB53 We hereby present a class of weakly non-local gravitational theories consistent at quantum level regardless of the spacetime dimension. These theories are unitary (ghost-free) and perturbatively super-renormalizable in any dimension. Moreover, we recently discover a simple extension of them that is finite at quantum level in even and odd dimension. Finally, we propose a super-renormalizable or finite theory for (super-)gravity coupled to gauge fields and matter laying the groundwork for a "finite standard model of particle physics" and/or a grand unified theory of all fundamental interactions (including a weakly nonlocal completion of 11-dimensional Supergravity/M-Theory.)

Black holes and scalar fields

• Thomas Sotiriou (The University of Nottingham)

Room: FB53

Towards understanding the ultraviolet behavior of quantum loops in infinite-derivative theories of gravity

• Spyridon Talaganis

Room: FB53 In this talk, I will consider quantum aspects of a non-local, infinite-derivative scalar field theory - a toy model depiction of a covariant infinite-derivative, non-local extension of Einstein’s general relativity which has previously been shown to be free from ghosts around the Minkowski background. The graviton propagator in this theory gets an exponential suppression making it asymptotically free, thus providing strong prospects of resolving various classical and quantum divergences. In particular, I will find that at 1-loop, the 2-point function is still divergent, but once this amplitude is renormalized by adding appropriate counter terms, the ultraviolet (UV) behavior of all other 1-loop diagrams as well as the 2-loop, 2-point function remains well under control. I will go on to discuss how one may be able to generalize our computations and arguments to arbitrary loops.

Inflation in higher derivative gravity theories

• Alexey Koshelev

Room: FB53 I will describe the stringy motivated non-local gravity and discuss the Starobinsky inflation in this model, the most attractive inflationary scenario from a phenomenological point of view. Computation of scalar and tensor spectral tilts will be discussed and a comparison to the existing data will be made. As an upshot the results show that the model is well-behaved and exhibits similarities with the local counterpart.

UV finiteness and exact solutions in non-local gravity

• Leslaw Rachwal

Room: FB53

The graviton and its masses

• Matteo Fasiello

Room: FB53

Energy in ghost-free massive gravity theory

• Michael Volkov (University of Tours)

Room: FB53 I calculate the energy in the dRGT massive gravity. This requires resolving the constraints, which can be done in the spherically symmetric sector. The energy is found to be positive for globally regular and asymptotically flat fields constituting the "physical sector" of the theory. There are also solutions of the constraints whose energy is negative and unbounded from below and which show tachyons. However, these solutions are either not globally regular or not asymptotically flat. They cannot describe initial data triggering ghost instability of the physical sector. This allows one to conjecture that the physical sector is protected from the instability by a potential barrier separating it from superluminal and negative energy states. I also consider cosmological applications.

Stability of massive gravity solutions for holographic conductivity

• Lasma ALBERTE (Ben Gurion University)

Room: FB53 We consider non-linear massive gravity with two Stueckelberg fields in which the diffeomorphism invariance is broken spontaneously in two of the spacetime directions. This theory admits a charged anti-de Sitter black brane solution and has recently been used in holographic context as a bulk description of a boundary field theory with momentum dissipation. Here we study the stability of the black brane solution. We identify the physical degrees of freedom and determine the healthy regions of the parameter space of the theory. We find that there is a region in parameter space, where the theory suffers from ghost instability. We recognise this as the reason for the instability found in the previous works on holographic conductivity. We also rederive the previous results for the holographic conductivity in a coordinate independent way.

Imprints of the dark sector on cosmic structure formation: insights from the "Full Universe Runs"

• Yann Rasera

Room: FB53 In this talk I will focus on the probes of the dark sector. I will present several non-trivial effects of cosmology on the non-linear regime of structure formation. These imprints have been studied in great details thanks to the large volume of the "Full Universe Runs" simulations combined with the high resolution of the "Dark Energy Universe Simulations Series". I will highlight some results from horizon scale down to halo scale: bias and dispersion of the angular distance due to lensing, shift and damping of the BAOs, non-universality of the dark matter halo mass function, variation of the "sparsity" of dark matter halo profile. If not accounted for properly, these non-linear imprints of cosmology on structure formation could bias the cosmological inference.

Fluids and gravity modifications

• Luigi Pilo (University of L'Aquila and LNGS)

Room: FB53 An overview of the recent progresses on massive gravity focusing on the available ghost free theories is given. The relation between self-gravitating generalized fluids and massive gravity will be discussed, focusing on its relevance to cosmology.

Consistent massive graviton on arbitrary backgrounds

• Cedric Deffayet (IAP, Paris)

Room: FB53

Inflationary Tensor Fossils in CMB and LSS

• Emanuela Dimastrogiovanni (Arizona State University)

Room: FB53 In some early universe scenarios, a correlation between a long wavelength tensor perturbations with two short wavelength scalar fluctuations may generate an observable signal in form of a quadrupolar power asymmetry. Alternatively, it may produce an off-diagonal contribution to the matter power spectrum which would represent a probe for primordial gravitational waves. We discuss how such intriguing possibilities comes about in specific inflationary models.

Bigravity from braneworld setup and doubly coupled matter

• Yamashita Yasuho (YITP, Kyoto University)

Room: FB53 Recently, a framework of covariant bimetric gravity with no ghost has been established by assuming a restricted form of mass interaction between two metrics derived by de Rham, Gabadadze and Tolley. However, it is not clear why the interaction term takes this specific form and whether the theory can be derived from a more natural setup as a low energy effective theory. In order to improve our understanding of ghost-free bigravity, we consider to reproduce this theory from a braneworld setup as a low energy effective theory. Although this attempt succeeds in the very limited low energy regime, there is a problem: doubly coupled matter. In my talk, I will discuss why doubly coupled matter anticipates difficulties in our attempt and what is our plan to work out this problem.

Disformal vectors and anisotropies on a warped brane

• Federico URBAN (Université Libre de Bruxelles)

Room: FB53 The Maxwell action is conformally invariant and classically ignorant of conformally flat metrics. However, if the vector lives in a disformal metric--- as it does if residing upon a moving brane---this is no longer true. The disformal coupling is then mediated by a Dirac-Born-Infeld scalar field. Here a systematic dynamical system analysis is developed for anisotropic Bianchi I cosmology with a massive disformally coupled vector field. Several new fixed points are found, including anisotropic scaling solutions. The presented formalism here presented can be conveniently applied to general scenarios with or without extra dimensional motivations. This is illustrated here by performing a complete analysis with simple assumption that both the potentials and the warp factor for the brane are (nearly) exponential. In that case, the anisotropic fixed points are either not attractors, do not describe accelerating expansion or else they feature too large anisotropies to be compati ble with observations. Nonetheless, viable classes of models exist where isotropy is retained due to rapid oscillations of the vector field, thus providing a possible realisation of disformally interacting massive dark matter.

Breaking spatial diffeomorphisms in the EFT of inflation

• Gianmassimo TASINATO (Swansea University)

Room: FB53

Born-Infeldizing gravity

• Jose Beltran Jimenez

Room: FP41 In order to regularize the energy of point-like charged particles, Born and Infeld introduced a modification of the Maxwell lagrangian that naturally imposes an upper bound on electromagnetic fields. This approach was later taken by Deser and Gibbons to propose an analogous modification for gravity. I will review some of these ideas as well as some extensions for the gravitational sector. Finally, a scenario where inflation could be supported by a set of massive particles within the context of these theories will be discussed.

Teleparallel, f(T) and other torsional modified gravities, and their cosmological applications

• Emmanuel SARIDAKIS (Baylor U.)

Room: FP41 Torsion has been proved to be crucial in gauging gravity, which is in turn a necessary step towards its quantization. On the other hand, almost all the efforts in modifying gravity has been performed in the usual curvature-based framework. We investigate the case where one modifies gravity based on its torsional-teleparallel formulation, namely the f(T) gravity paradigm, and its cosmological applications. Moreover, we analyze the perturbations of the theory examining the growth history, we construct a cosmological bounce, and we use solar system and cosmological observations in order to impose constraints on the f(T) forms. Additionally, we analyze the charged black hole solutions of the theory, performing a comparison between f(R) and f(T) modifications. Finally, we study the case where T is nonminimally coupled to a scalar field, as well as other extensions of the theory, using higher- order torsion invariants, or torsion-matter couplings.

Dark energy and neutrino physics

• Steen hannestad (Aarhus university)

Room: FD5 Neutrinos have an impact on cosmological structure formation which is similar to that of dark energy. I will discuss the importance of understanding neutrino physics in order to precisely measure the properties of dark energy with current and future cosmological surveys.

Generalized quadratic curvature, non-local infrared modifications of gravity and Newtonian potential

• Aindriu Conroy (Lancaster University)

Room: FD5

Cosmic phenomenology of massive bigravity

• Jonas Enander (Stockholm University)

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Completing Lorentz violating massive gravity at high energies

• Sergey SIBIRYAKOV (INR)

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Viable cosmology in bimetric theory and its extensions

• Norihiro TANAHASHI (DAMTP, University of Cambridge)

Room: 132:028 We study cosmological perturbations in bimetric theory focusing on the effect of coupling of matter fields to metrics. Firstly we review the results for the case that a matter field couples to the physical metric and the other one couples to the second metric, in which a stable cosmology can be realized once physically sensible conditions are satisfied. Next we study the case that a matter field couples to a composite metric made out of the two metrics, in which a matter couples to the two metrics simultaneously without introducing the Boulware-Deser ghost. We find that some cosmological solutions which is stable in the former case is destabilized in the latter case, while there is a class of solutions which is unstable in the former case but could be made viable in the latter case. We argue possible usages of this new class of stable solutions.

Searching for viable cosmological solutions in bigravity

• Frank KOENNIG (Institute for Theoretical Physics, University of Heidelberg)

Room: 132:028 I will discuss the constraints for the absence of gradient instabilities in the scalar sector and the Higuchi ghost. These will show that all infinite branches suffer from the Higuchi ghost at all times and provide a relation between ghost-like helicity-0 and helicity-2 dofs. Furthermore, I will briefly discuss "exotic" branches, like bouncings or asymptotic static universes, and comment on one promising way to solve the gradient instability problem in finite branches.

On the existence and construction of gauge invariant theories of p-forms with non-linear dependence on the second derivative of the gauge field

• Vishagan Sivanesan (Institut'd Astrophysique de Paris)

Room: 132:028 Galileon field theory plays an important role in modified theories of gravity. It arises as the scalar mode in Massive gravity, DGP etc and describes the leading order interactions thereby capturing the salient features of these theories, such as Vainshtein screening and strong coupling. The distinctive aspect of these theories are that they possess derivative interactions that are non-linear in the second derivative of the field. In [1] the authors generalised the Galileon field (0-form) to gauge invariant p-form in a 'direct' way and found that this approach yields trivial field equations when p is odd. In [2] the existence of theories with non-linearity in the second derivative of a gauge invariant vector field was investigated and it was proved that such theories do not exist . Here we investigate the existence of 'Galileon like' theories when p > 1 and odd, using tools from the representation theory of the permutation group. Remarkably, we find such theories do exist for p>1 in a subtle way, and construct them explicitly for some special cases. Moreover, this approach can be considered as a step towards the classification of all gauge invariant theories of p-forms. [1]: 'Arbitrary p-form Galileons' Phys.Rev. D82 (2010) 061501' [2]: 'A no-go theorem for generalized vector Galileons on flat spacetime' JHEP 1404 (2014) 082

Galileon from Weyl bi-connection!

• Nima Khosravi

Room: 132:028 The Weyl biconnection model manifests a natural framework to automatically produce the structure of the Galileon theory. It is shown that this framework can explain the scalar and vector Galileon as well as their interactions by generalizing the Weyl nonmetricity, and as such it can be interpreted as a geometrical realization for Galileons. The nonmetricity part enjoys a U(1) gauge invariance. The result is interestingly nontrivial since the structure of the Galileon theory appears spontaneously, and not by demanding the absence of the Ostrogradsky ghost. This fact suggests a possible deeper conceptual relation between the Weyl biconnection model and the absence of the Ostrogradsky ghost.

Vector fields in modified gravity and cosmology

• Jose Beltran Jimenez

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Summary Room: 132:028