Dark Energy and Modified-Gravity cosmologies: DARKMOD

11 Sep 2017, 09:30 → 6 Oct 2017, 17:00 Europe/Paris

Institut de Physique Théorique, CEA Saclay

Grégoire MISGUICH, Karolina Kolodziej, Patrick VALAGEAS, Philippe BRAX

Dark Energy and Modified-Gravity cosmologies: DARKMOD
The acceleration of the expansion of the Universe at low redshifts has not received a satisfactory theoretical explanation yet. The value of a possible cosmological constant remains a puzzle and alternative approaches, dark energy or modified gravity, are still being developed.
In view of the future observations by galaxy surveys, such as EUCLID for instance, it is timely to gather experts on both the theoretical aspects of dark energy and modified gravity, together with phenomenologists and observers.
This 4-week workshop will provide an opportunity to discuss most aspects of both dark energy and modified gravity, from their theoretical construction to their link with observational cosmology.

Monday, 11 September

09:30 → 11:00 Betoule: "SNeIa, constraints on cosmology"

Slides betoule_supernovae.pdf

12:30 → 14:00 lunch

14:00 → 15:30 Blanchard: Formation of large-scale structures, clusters of galaxies

Slides Blanchard1.pdf

16:00 → 17:30 Langlois: Introduction to modified gravity theories

Slides Langlois-Lectures_Modified_Gravity.pdf

Tuesday, 12 September

09:00 → 10:30 Blanchard: Formation of large-scale structures, clusters of galaxies

Slides Blanchard2.pdf

11:00 → 12:30 Rich: "BAO - Lyman-alpha, constraints on cosmology"

Slides bao2017.pdf

13:00 → 14:15 lunch

14:15 → 15:45 Betoule: "SNeIa, constraints on cosmology"

16:15 → 17:15 Discussion: "Background versus perturbation constraints on models"

Wednesday, 13 September

10:00 → 11:30 Langlois: Introduction to modified gravity theories

12:30 → 14:00 lunch

14:00 → 15:30 Kilbinger: Weak lensing

Slides wl_darkmod_day1.pdf

16:00 → 17:00 Burtin: Redshift-space distortions

Slides Burtin-2017-darkmod.pdf

17:00 → 18:00 Discussion: "Which observables to detect DE or MG ?"

Thursday, 14 September

10:00 → 11:30 Kilbinger: Weak lensing

Slides wl_darkmod_day2.pdf

12:30 → 14:00 lunch

14:00 → 15:30 Rich: "BAO - Lyman-alpha, constraints on cosmology"

16:00 → 17:30 Langlois: Introduction to modified gravity theories

Slides Lectures_MG_17.pdf

17:30 → 18:30 "Linear versus non-linear scales"

Friday, 15 September

09:30 → 11:00 Kilbinger: weak lensing

Slides wl_darkmod_day3.pdf wl_darkmod_td_cfhtlens.pdf

11:30 → 11:50 Mina: dark matter

Slides Mina_adm.pdf

12:30 → 14:00 lunch

Monday, 18 September

11:00 → 12:00 Tolley: Massive gravity

Slides Lectures_ATolley.pdf

12:30 → 14:00 lunch

14:00 → 15:00 Vernizzi: Effective Field Theory of Dark Energy

Tuesday, 19 September

11:00 → 12:00 Tolley: Massive gravity

12:30 → 14:00 lunch

14:00 → 15:00 Vernizzi: Effective Field Theory of Dark Energy

15:30 → 16:30 Senatore:

Slides seminar_EFT_in_LSS_v3_senatore.pdf

Wednesday, 20 September

11:00 → 12:00 Tolley: Massive gravity

12:30 → 14:00 lunch

14:00 → 15:00 Vernizzi: Effective Field Theory of Dark Energy

Thursday, 21 September

11:00 → 12:00 Tolley: Massive gravity

12:30 → 14:00 lunch

14:00 → 15:00 Vernizzi: Effective Field Theory of Dark Energy

15:00 → 16:00 Discussion: Constraints on Galileon models and other scenarios

16:00 → 17:00 Happy hour: beer, wine and crisps

Friday, 22 September

11:00 → 12:00 Tolley: Massive gravity

12:30 → 14:00 lunch

14:00 → 15:00 Vernizzi: Effective Field Theory of Dark Energy

Monday, 25 September

09:30 → 10:05 Creminelli: "Relaxation of the Cosmological Constant"

10:05 → 10:40 Tolley: "Positivity Bounds on Effective Field Theories of Gravity"

I will discuss recently developed positivity bounds for the scattering amplitudes of massive particles of any spin following from unitarity, Lorentz invariance and Analyticity and how these can be used to constrain various EFTs of gravity.

Slides Tolley.pdf

10:40 → 11:10 coffee

11:10 → 11:35 Bellini: "Towards a unified description of theories with a single scalar degree of freedom"

Models propagating a single extra degree of freedom are very popular as Dark Energy candidates. Examples are Scalar-Tensor, Vector-Tensor, and bimetric theories of gravity. Despite their common properties, these theories are usually described in different frameworks. I will show a  method to construct general parametrizations to describe the linear dynamics of all these models. I will present the results in a unified formalism, and discuss under what circumstances it is possible to have a unified description for all these theories.

Slides Bellini.pdf

11:35 → 12:10 Vernizzi: "More on the Effective Theory of Dark Energy"

Slides Vernizzi.pdf

12:10 → 12:30 Kennedy: "Reconstructing Horndeski models from the effective field theory of dark energy"

With the next generation of cosmological surveys about to commence, it is important to understand how one can use the information obtained from such surveys to constrain the space of dark energy and modified gravity theories. The effective field theory (EFT) of dark energy has proven to be a useful approach in this regard, allowing an efficient exploration of the parameter space. However, in order to gain a deeper understanding of the mechanism underlying cosmic acceleration one would like to interpret constraints on the EFT parameters in terms of fundamental covariant theories. In this talk I will discuss a method to reconstruct the class of covariant Horndeski theories that reproduce the same background dynamics and linear perturbations as a given EFT action. I will then present a number of examples and applications. The work was done in collaboration with Lucas Lombriser and Andy Taylor.

Slides Kennedy.pdf

12:30 → 14:00 lunch

14:00 → 14:35 Khosravi: "Uber-Gravity and H0 tension"

Recently, the idea of taking ensemble average over gravity models has been introduced. Based on this idea, we study the ensemble average over (effectively) all the gravity models dubbing the name ubergravity.

Slides Khosravi.pdf

14:35 → 14:55 Cespedes: "Detecting modified gravity using B modes experiment"

In this talk We will discuss the effects that a modified sector tensor  can produce on the polarisation CMB power spectra. Alternative theories  of gravity can give the graviton a mass but also modify the dispersion  relation of the gravitational waves. by A mass for the graviton can  affect the large angular scales and in general a modified dispersion  relation could change the position of the acoustic peaks. These effect  modifies either the CMB power spectrum and bispectrum, with respect to  the canonical case. Thus given the precision of the CMB experiments, if  detected, primordial gravitational waves can help to constraint gravity  at a much higher accuracy.

Slides Cespedes.pdf

14:55 → 15:20 Winther: "COLA with scale-dependent growth"

N-body simulations is the main method we have to accurately compute observables in the non-linear regime of structure formation. Such simulations are too computationally expensive for many purposes, for example when one needs to generate large ensembles of galaxy mocks to model observables and their covariances for future surveys. Over the last few years a wide range of approximate methods for simulating structure formation have been proposed. I will first present a fast approximate method for including screening in numerical simulations of modified gravity theories and then show how one can extend the COmoving-Lagrangian-Acceleration (COLA) method to general models where the growth-rate is scale-dependent. These two methods can be combined and leads to a very fast method, often O(100) times faster than N-body, to compute clustering observables accurate down to fairly non-linear scales for a large class of models. Another application of the scale-dependent COLA method is that it allows for a simple way of including the effects of massive neutrinos on structure formation. The numerical code developed for this purpose is made available to the public and comes with several models already implemented.

Slides Winther.pdf

15:20 → 15:40 Ebrahimi: "Clustering of Cold Dynamical Dark Energy Models"

The nature of dark energy can be investigated not only by equation of state but also through clustering and sound speed. In this research, we examine clustering of some dynamical dark energy models namely, power-law (PL), Chavelier-Polarski-Linder (CPL) and Feng-Shen-Li (FSL). We will go beyond zero-order and Free parameter of models constraints by using current available data including Planck DR2, Baryonic Acoustic Oscillation (BAO) and Supernovae type Ia (SNIa) observation and Hubble space telescope (HST). We investigate that PL as early dark energy has different behavior rather than semi-LCDM models such as rapid potential changes, higher matter density contrast due to crossing LCDM equation of state and matter behavior at early universe. We quantify the importance of uncoupled dark energy clustering, PL exhibits strong clustering with δP L > 0 which can grow faster than δm and semi-LCDM models produce void of dark energy with powerless amplitude around 10−12. In the linear regime, density contrast, growth rate index f(σ8) and gravitational potential computed. Furthermore, temperature anisotropy and matter power spectrum by modification of CAMB for dynamical models obtained and ISW effect. PL shows more ISW effect, more value for matter power spectrum at large scale around k=0.01.Tension between HST and CMB for H0 disappears for all models.

Slides Ebrahimi.pdf

15:40 → 16:10 coffee

16:10 → 16:30 Naik: Simulating Milky Way-like galaxies under f(R) gravity

There is a growing body of work utilising N-body simulations with alternative gravity theories, but simulations at a galactic scale remain a relatively unexplored area. In this work, we present the first simulated galaxy rotation curves under f(R) gravity to our knowledge. The f(R) gravity solver MG-GADGET (Puchwein et al. 2013) is employed statically on pre-simulated spiral galaxies, leading to solutions for the rotation curves and screening radii under HuSawicki f(R) gravity. It is shown that under certain parameter regimes, the so-called 'fifth force' yields observable signatures, which can be exploited to constrain departures from classical gravity.

Slides Naik.pdf

16:30 → 16:55 Armengaud: Ultra-light scalar Dark Matter probed with the Lyman-alpha forest

I will introduce the main features of the so-called "fuzzy dark matter" (FDM) model, in which a (pseudo)scalar field with mass in the 10^{-24} - 10^{-20} eV range behaves as DE at high redshift, and DM at low z. I will review the growth of LSS in this scenario, in the linear as well as non-linear regimes. FDM is a good candidate to solve the so-called small-scale crisis of the CDM paradigm. I will then present constraints on FDM models derived from measurements of the small-scale Lyman-alpha forest flux power spectrum. Using a combination of SDSS-BOSS and higher-resolution spectra, we exclude FDM masses in the range 10^-22 - 3x10^-21 eV.

16:55 → 17:15 Moniez: Galactic Structure Measured from Microlensing

Tuesday, 26 September

09:30 → 10:05 Kilbinger: Weak gravitational lensing peak counts as probe of dark energy and modified gravity

To include the full non-Gaussian information about cosmology and the
large-scale structure from measurements of weak gravitational lensing, one
needs to go beyond the second-order shear power spectrum. In my talk I will
then focus on weak-lensing peak counts, an indirect probe of the halo mass
function. I present a new fast and flexible model of peak counts. This model
allows us to use new statistical inference methods that do not require
assumptions about the functional form of the likelihood of the observables,
e.g. Gaussianity. In particular, I will present results using Approximate
Bayesian Computation (ABC), a new technique that has recently gained momentum
in the astrophysics community. I will show some results and forecasts how peak
counts can constrain dark energy and f(R) models.

10:05 → 10:40 Schmidt: Bias and modified gravity

Galaxy clustering is one of the key probes of large-scale structure, and hence of interest to tests of gravity as well. However, in order to make this probe robust, we need to understand the relation between the galaxy density and the underlying stress-energy and spacetime perturbations, i.e. bias. I will discuss the general bias expansion in the context of GR, and what changes in the case of frequently studied modified gravity scenarios.

Slides Schmidt.pdf

10:40 → 11:05 Pujol: Mass mapping and galaxy bias in the Dark Energy Survey

The study of the large scale structures of the Universe is an important tool to learn about cosmology. The relation between the clustering of matter and the clustering of galaxies is known as galaxy bias. On the other hand,  gravitational lensing allows us to measure the dark matter distribution from the shapes of the source galaxies. In this talk I will present the largest weak lensing mass maps to date obtained from the Dark Energy Survey Year 1 data which covers around 1500 square degrees. 
I will also present a method to measure tomographic galaxy bias in observations from the combination of galaxy density and weak lensing maps, developed in Pujol et al. 2016 and applied to the Dark Energy Survey in Chang et al. 2016.

11:05 → 11:30 coffee

11:30 → 12:05 Li: Cosmological tests of gravity - some recent updates

Testing gravity on cosmological scales has received a great deal of interest in recent years in light of big galaxy surveys such as Eucild, LSST and DESI. In this talk I will describe some of the recent compelte and ongoing works in this development. The talk will begin with a description of the models begin studied and the rationales behind them, especially the generality of the results that would be derived from them. I will then talk about three main cosmological probes - weak gravitational lensing, galaxy clustering and clusters of galaxies - and their possible benefits and limitations in such tests.

Slides Baojiu_Li.pdf

12:05 → 12:25 Spurio Mancini: Testing Horndeski gravity with 3D weak lensing

We forecast the ability of future Euclid-like experiments to constrain Horndeski theories of gravity using a fully 3D cosmic shear analysis. We use the parametrization of the Horndeski Lagrangian density first proposed by Bellini&Sawicki(2014) based on four functions of time only, to constrain them by means of a Fisher matrix analysis. The constraints imply a fixed background cosmology, purely reflecting the change in structure growth, to which 3D weak lensing is particularly sensitive. This is due to the spherical Fourier-Bessel decomposition of the shear field that characterises the 3D formalism, which allows for inclusion and retention of the sources redshifts along the entire analysis. This has the advantage of following the evolution of structures in a non-averaged way, retaining more information on structure growth and how this is influenced by gravity.

Slides Spurio-Mancini.pdf

12:25 → 12:50 Achitouv: "Probing dark energy with cosmic voids"

Cosmic voids can supply powerful insights into the nature of dark energy, provided that we have accurate models of their statistical properties. In this talk I will present a new method to predict void density profiles, and show how we can reduce systematic errors when probing the growth rate around voids. This will be crucial for testing non-standard dark energy models with upcoming large survey campaigns. Finally, I will also present a self-consistent measurement of the growth rate around galaxies and around voids, in the 6- degree Field Galaxy Survey, as a test of the standard model of gravity.

Slides Achitouv.pdf

12:50 → 13:15 Durrive: Constraining scalar field Dark Energy models

Slides Durrive.pdf

13:15 → 14:45 lunch

14:45 → 15:10 Pierre: The cosmological analysis of X-ray cluster surveys

We recall the bases of cosmological studies with clusters of galaxies. We present an original method that aims at determining the cosmological parameters, including the DE equation of state, by considering only the statistics of raw X-ray observables from a well defined cluster population. We illustrate the method with the XXL survey, which is the largest programme of XMM, the X-ray observatory of the European Space Agency.

15:10 → 15:35 Peel: Investigating a dark core: sparse reconstruction of the A520 cluster merger

Slides Peel.pdf

15:35 → 16:10 Davis: Gravitational Waves in Doubly Coupled Bigravity

In doubly coupled bigravity the two gravitons are coupled by a non-diagonal mass matrix and show birefringence. One of the two gravitons propagates with a speed which differs from one. This deviation is tightly constrained by both the gravitational Cerenkov effect and the energy loss of binary pulsars. When emitted from astrophysical sources, the wave form displays beats, which could be detected in future nano-Hertz interferometric experiments.

Slides Davis.pdf

16:10 → 16:35 Lombriser: Challenges to Cosmic Self Acceleration in Modified Gravity from Gravitational Waves and Large-Scale Structure

Scalar-tensor modifications of gravity have long been considered as an alternative explanation for the late-time accelerated expansion of our Universe. I will first show that a rigorous discrimination between acceleration from modified gravity and from a cosmological constant or dark energy is not possible with observations of the large-scale structure alone. I will then demonstrate how gravitational wave observations break this dark degeneracy and how the combination of the two challenges the concept of cosmic acceleration from a genuine scalar-tensor modification of gravity.

Slides Lombriser.pdf

16:35 → 17:00 coffee

17:00 → 17:25 Blas: Using binary pulsars to test the dark sector

Slides Blas.pdf

17:25 → 17:50 Zumalacarregui: Testing Dark Energy with Gravitational Waves

Scalar-tensor theories of gravity can provide viable models to explain cosmic acceleration and several discrepancies of standard cosmology such as the value of H0. I will present recent progress to understand these theories and determine the properties of gravity using gravitational waves and the large scale structure of the universe: The speed of gravitational waves will provide the most stringent test for a large class of theories, while a new generation of galaxy surveys will be sensitive to new relativistic effects on the largest cosmological scales. I will also introduce hi_class (www.hiclass-code.net), an accurate, fast and flexible code to compute cosmological predictions in a very large class of gravitational theories.

Slides Zumalacarregui.pdf

Wednesday, 27 September

09:30 → 10:05 Bartolo: "Large Scale Structure in mimetic Horndeski gravity"

Within the so called mimetic gravity I will discuss cosmological perturbations for mimetic Horndeski models. I will focus on a proposal for a  model of the dark universe, where both cold dark matter (CDM) and dark energy phenomena are described by a single component, the mimetic field.   For the simpler minimal model it turns out that if the background expansion history is chosen to be  identical to a perfect fluid dark energy (PFDE), then the mimetic model predicts the same power spectrum for the gravitational potentials as the PFDE. I will also discuss the distinguishability between these mimetic models and other popular models for DE. Finally I will present some preliminary results about the  extraction of some parameters commonly used to quantify modifications w.r.t. to standard GR, like for example modifications of the Poisson equation.

Slides Bartolo.pdf

10:05 → 10:30 Crisostomi: Higher Order Scalar-Tensor Theories

Scalar-tensor theories of gravity play an essential role for building models of inflation and dark energy. For a very long time the focus has been placed on those theories leading to second order field equations, believing all the others plagued by instabilities. In this talk I will present new degenerate theories that, despite higher order in the equations of motion, are free from such a kind of pathology. Moreover, I will show that some higher order scalar-tensor theories are a useful tool to study parity violating effects in the gravity sector.

Slides Crisostomi.pdf

10:30 → 10:55 Saltas: "Challenging the consistency of scalar—field theories with derivative interactions from a fundamental viewpoint"

In this talk, I will discuss the consistency of a large class of theories for dark energy and primordial inflation from a fundamental point of view. In particular, focusing on the effective theory of a single scalar field exhibiting non—linear derivative interactions, I will discuss the ultraviolet properties of the theory at the 1—loop level and beyond, and argue about the significance of understanding the initial conditions and consistency of phenomenologically succesfull theories at sufficiently high energies.

Slides Saltas.pdf

10:55 → 11:30 coffee

11:30 → 11:55 Motohashi: "Healthy degenerate theories with arbitrary higher-order derivatives"

Scalar-tensor theories serve models for inflation and dark energy. Many efforts have been made recently for constructing most general scalar-tensor theories with higher-order derivatives in their Lagrangian. Since higher-derivative theories are typically associated with Ostrogradsky ghost which causes unbounded Hamiltonian, it is important to clarify how to evade it. In this talk, I will discuss construction of healthy degenerate theories with higher-order derivatives which circumvent Ostrogradsky ghost. Generalizing the previous works on construction of ghost-free theories with second-order derivatives in Lagrangian, we clarify how to construct healthy theories with arbitrary higher-order derivatives. The necessary and sufficient condition for the absence of ghost degrees of freedom for theories with second-order derivatives is to eliminate linear dependence of canonical momenta in Hamiltonian. We find that this condition is not sufficient for theories with higher-than-second-order derivatives, and that linear dependence of canonical variables are also needed to be removed appropriately. We provide specific examples of healthy higher-order theories as well as ghost-free conditions for general theories with arbitrary higher-order derivatives to remove all the ghost degrees of freedom, under which we show that the Euler-Lagrange equations are reducible to a second-order system.

Slides Motohashi.pdf

11:55 → 12:30 Vikman: Canonical Exorcism for Cosmological Ghosts

Slides Vikman.pdf

12:30 → 12:55 Barreira: "Galileon gravity: self-acceleration consistent with current cosmological data"

Slides Barreira.pdf

12:55 → 14:20 lunch

Thursday, 28 September

09:30 → 10:05 Saridakis: "Torsional Modified Gravity and Cosmology"

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 theories of modified gravity based on its torsional-teleparallel formulation, and their 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 torsional modifications. Finally, we study the case where torsion is nonminimally coupled to a scalar field or its derivatives, as well as other extensions of the theory, using higher-order torsion invariants, or various torsion-matter couplings.

Slides Saridakis.pdf

10:05 → 10:40 Heisenberg: “Vector-Tensor Theories”

10:40 → 11:15 Pilo: "Cosmology of self-gravitating media"

The low-energy dynamics of a generic self-gravitating media can be studied by using effective field theory in terms four derivatively coupled scalar fields. Imposing SO(3) internal spatial invariance, the theory describes fluids, superfluids, solid and supersolids. Dynamical and thermodynamical properties of the medium are dictated by internal symmetries of the effective theory. In the unitary gauge, where the scalar fields' fluctuations are gauged away, the most general medium can be equivalently described as rotational invariant massive gravity. From the analysis of cosmological perturbations it emerges that in the scalar sector, besides the gravitational potential, a non-adiabatic mode corresponding to the perturbations of entropy per particle σ is present. Perfect fluids and solids are adiabatic with constant in time σ while for superfluids and supersolids σ has non-trivial dynamics. Tensor perturbations are massive for solid and supersolid. Such an effective approach can be used to give a very general modelling of the dark sector based on symmetries.

Slides Pilo.pdf

11:15 → 11:45 coffee

11:45 → 12:20 Nunes: Dark energy couplings

Slides nunesA.pdf

12:20 → 12:55 Van de Bruck: "Interactions in the dark sector: the cosmology of conformally and disformally coupled dark matter"

A fifth force between the standard model particles is highly constrained. On the other hand, dark matter particles could interact for example via a force mediated by dark energy scalar field. Such interactions are currently constrained only by cosmological observations. In this talk I will discuss scalar-tensor theories of interacting dark matter-dark energy, allowing for both conformal and disformal couplings. I will discuss that while conformally coupled dark matter is strongly constrained by observations of CMB anisotropies and large scale structures, this is not the case for disformally coupled dark matter or mixtures between conformal and disformal couplings. After discussing the cosmological consequences of a disformal coupling term, I will present the latest constraints on both conformal and disformal couplings coming from diverse cosmological probes.

12:55 → 13:15 Nunes: "The cosmology of the de Sitter Horndeski models"

Slides NunesB.pdf

13:15 → 14:45 lunch

14:45 → 15:20 Deffayet: "Partially Massless Graviton on Beyond Einstein Spacetimes"

We show that a partially massless graviton can propagate on a large set of spacetimes which are not Einstein spacetimes. Starting from a recently constructed theory for a massive graviton that propagates the correct number of degrees of freedom on an arbitrary spacetime, we first give the full explicit form of the scalar constraint responsible for the absence of a sixth degree of freedom. We then spell out generic conditions for the constraint to be identically satisfied, so that there is a scalar gauge symmetry which makes the graviton partially massless. These simplify if one assumes that spacetime is Ricci symmetric. Under this assumption, we find explicit non-Einstein spacetimes (some, but not all, with vanishing Bach tensors) allowing for the propagation of a partially massless graviton. These include in particular the Einstein static Universe.

15:20 → 15:55 Burrage: The effect of screening on galactic dynamics

Slides Burrage.pdf

15:55 → 16:15 Wong: "Contemplating Atoms: What hydrogen teaches us about modified gravity"

Considering simple systems can often be helpful in elucidating the most important features of a theory. I provide a recent example by revisiting the behaviour of hydrogen atoms in chameleon-like modified gravity, highlighting some recent advancements in our understanding. A careful analysis uncovers additional interaction terms in the Hamiltonian previously unaccounted for, which can dominate the chameleon-induced fine-structure corrections of certain spectral lines. Effects on hyperfine structure and on violations of the Einstein equivalence principle are also presented for the first time. While these developments enable modest updates to chameleon constraints based on atomic precision tests, they remain largely uncompetitive with other experimental bounds. Of greater value are the general insights gained from this study, and their possible implications for future work will be discussed.

Slides Wong.pdf

16:15 → 16:45 coffee

16:45 → 17:10 Kading: "Chameleon-induced decoherence in an atom interferometer"

Many modified gravity theories lead to the introduction of a new scalar field which would be expected to cause an additional gravity-like force. However, so far no solar system based observation could confirm the existence of such a fifth force. A way of explaining a force’s apparent absence is by a screening mechanism. For example, chameleon scalar fields obey to a mechanism by which they adapt their mass to the mass density of their environment. In this way, a force caused by chameleons is screened in our solar system due to the large effective mass of its carrier. Nevertheless, there are possible manners of observing chameleons even in earth based experiments. One way would be to look at the effect of the chameleon force on the decoherence of two parts of a wave function in an atom interferometer. I will present the method of calculating the time of chameleon-induced decoherence in such an experiment and current results of this calculation.

Slides Kading.pdf

17:10 → 17:30 Ip: "Solar System Constraints on Disformal Gravity Theories"

[Coauthors: Jeremy Sakstein, Fabian Schmidt] (arXiV: 1507.00568) We wanted to see if (Horndeskian) disformal theories of gravity can be a driving mechanism of cosmic acceleration. Local test constraints on PPN parameters encoding preferred-frame effects (due to the SS moving against the cosmological background) require these theories to have a cosmology indistinguishable from LCDM, under the assumption of the equivalence principle.

17:30 → 17:55 Nishimichi: "Response function and reconstruction for accurate tests of dark energy using clustering statistics on BAO scale"

Slides Nishimichi.pdf

Friday, 29 September

09:30 → 10:05 Liguori: "Constraining K-mouflage models with CMB data"

I will discuss the implementation of K-mouflage models of Modified Gravity in the EFTCAMB solver and its application to constraining K-mouflage theories using Planck data. I will show that, 
since K-mouflage presents a non-trivial expansion history at high-redshift, interesting bounds on relevant parameters can already be placed by looking at primordial CMB spectra. Finally, I will briefly discuss CMB-galaxy cross-correlation in K-mouflage scenarios, showing that also in this case an interesting phenomenology arises, due to the evolution of the growth factor in the matter dominated regime.

Slides Liguori.pdf

10:05 → 10:30 Fasiello: LSS probes of late-time cosmic acceleration

After a brief introduction on perturbative large scale structure probes of dark energy/modified gravity, I will describe a newly proposed phenomenological framework to account for screening mechanisms at mildly-non-linear scales.

Slides Fasiello.pdf

10:30 → 11:00 coffee

11:00 → 11:35 Dhawan: "Narrowing down the possible explanations of cosmic acceleration with geometric probes"

Recent re-calibration of the Type Ia supernova (SNe~Ia) magnitude-redshift relation combined with cosmic microwave background (CMB) and baryon acoustic oscillation (BAO) data have provided excellent constraints on the standard cosmological model. Here, we examine particular classes of alternative cosmologies, motivated by various physical mechanisms, e.g. scalar fields, modified gravity and phase transitions to test their consistency with observations of SNe~Ia and the ratio of the angular diameter distances from the CMB and BAO. Using a model selection criterion for a relative comparison of the models (the Bayes Factor), we find moderate to strong evidence that the data prefer flat Λ CDM over models invoking a thawing behaviour of the quintessence scalar field. However, some exotic models like the growing neutrino mass cosmology and vacuum metamorphosis still present acceptable evidence values. The bimetric gravity model with only the linear interaction term can be ruled out by the combination of SNe~Ia and CMB/BAO datasets whereas the model with linear and quadratic interaction terms has a comparable evidence value to standard Λ CDM. Thawing models are found to have significantly poorer evidence compared to flat Λ CDM cosmology under the assumption that the CMB compressed likelihood provides an adequate description for these non-standard cosmologies. We also present estimates for constraints from future data and find that geometric probes from oncoming surveys can put severe limits on non-standard cosmological models.

Slides Dhawan.pdf

11:35 → 12:00 Karmakar: "Cosmic structures in ghost-free scalar-tensor theories of gravity"

In this article, we study the cosmic structures in the quadratic Degenerate Higher Order Scalar Tensor (qDHOST) model, which has been recently proposed as the most general scalar tensor theory without ghosts (up to quadratic dependence on the covariant derivatives of the scalar field).  We explicitly provide the covariant field equations of general Higher Order Scalar Tensor (qHOST) and qDHOST. We then study a static, spherically symmetric object embedded in de Sitter space-time for the qDHOST model. This model exhibits breaking of the Vainshtein mechanism inside matter and Schwarzschild-de Sitter space-time outside the matter, where GR can be recovered. (To appear on arxiv soon.)

12:00 → 12:35 Marinoni: "E pur si muove! A real-time detection of our acceleration through space"

High-precision astrometric experiments will allow to detect our proper acceleration  through  space via real-time observations of the change in the aberration of  sources at cosmic distances.   This `Aberration Drift’  effect is a powerful consistency test of  FLRW metric,  it may set independent constraints on the amplitude of the Hubble constant and the linear growth rate of  cosmic structures,   and it  may also be instrumental in searching for evidences of new physics beyond the standard model.  I will present the formalism  of this novel  test of cosmology, discuss the physics to which it is sensitive   and  show simulated forecasts  of the accuracy with which it can be implemented by   ongoing satellite missions such as Gaia or upgraded future experiments.

12:35 → 14:00 lunch

14:00 → 14:35 Sawicki: Vorticity in the time of galaxy surveys

Vorticity in the time of galaxy surveys

14:35 → 15:00 Lehebel: "The hair of compact objects in Horndeski theory"

Compact astrophysical objects, like black holes and neutron stars, are going to open a new and enlightening window on gravity in the forthcoming years. The study of such objects in strong curvature regimes will a priori allow to test alternative gravity theories, like scalar-tensor models. I will discuss spherically symmetric and static objects in the framework of Horndeski theory, proving the absence of scalar hair for most models. I will also carefully classify the possible ways to escape this no-hair theorem.

Slides Lehebel.pdf

15:00 → 15:25 Llinares: "Weighted density fields as improved probes of modified gravity models"

Slides Llinares.pdf

15:25 → 15:50 Lewandowski: "EFT of Large-Scale Structure and EFT of Dark Energy"

In the next few years, large-scale structure (LSS) surveys will probe the low-redshift universe with unprecedented accuracy. Because there are more modes available at smaller scales, the amount of information about the the universe that we can extract from these surveys depends on our ability to theoretically understand these small, non-linear scales. Among other things, we hope to learn about the early universe (primordial non-Gaussianities) and the late universe (dark energy). The effective field theory of large-scale structure (EFTofLSS) is a theoretical framework that has allowed us to make accurate predictions in the mildly non-linear regime. In this talk, I will briefly review recent progress in the EFTofLSS and explore how it can be used to constrain interactions in the effective field theory of dark energy.

Slides LEWANDOWSKI.pdf

15:50 → 16:20 coffee

16:20 → 16:45 Ilic: "Cosmology of the Galileon extension of the TeVeS theory"

For more than a decade, many attempts have been made at developing a fully relativistic theory with Milgrom's MOND-like phenomenology. Many of these theories have a similar ambiguity in that all possess a function in the Lagrangian that must be chosen by hand. Babichev et al. (2011) proposed a model that avoids the use of such an unspecified function : this theory uses Bekenstein’s TeVeS theory as basis but extends it with the addition of a Galileon-type term (generating a convenient Vainshtein screening in high-curvature environment such as the solar system) and the removal of the free function. However, the viability of the theory in a cosmological context has yet to be proven. In this talk, I will present the results of a recent work where we tested this theory both at the level of the cosmological background evolution as well as the perturbed sector, using constraints from the latest cosmological microwave background observations.

Slides Ilic.pdf

16:45 → 17:10 de Fromont: "Testing modified gravity with cosmic voids and compensated structures"

These last years, cosmic voids have been used as original probes for gravity and dark energy. This is motivated by the fact that these regions are expected to be safe from both baryonic and non linear physics unlike galaxy clusters or DM halos. However, these approaches are limited by the absence of a fully consistent model for cosmic voids. In this talk, I will introduce a new model that provides predictions for both shape and statistics of these regions for any cosmology and redshift. Using this model, I will discuss their sensitivity to modified gravity such as f(R) theory and discuss the possibilities to constraint efficiently the nature of gravity in upcoming surveys.

Slides deFromont.pdf

Monday, 2 October

11:00 → 12:00 Davis: Screening mechanisms in modified-gravity scenarios

Slides Davis-1.pdf

12:30 → 14:00 lunch

14:00 → 15:00 Winther: "COLA algorithm for faster simulations"

Tuesday, 3 October

11:00 → 12:00 Charmousis: Compact objects in scalar-tensor and vector-tensor theories

Slides Charmousis.pdf

12:30 → 14:00 lunch

14:00 → 15:00 Davis: Screening mechanisms in modified-gravity scenarios

Slides Davis-2.pdf Davis-2-Vainshtein.pdf

15:30 → 16:00 Suroso: "Four Dimensional Cosmology with Nonminimal Derivative Coupling of External Scalar Field"

The nonminimal derivative coupling (NMDC) model is a class of tensor scalar theory of gravity that contains a coupling between curvature tensor and derivative of scalar field. This model has been used in various aspects of gravity and cosmology including inflation, late time acceleration, and black hole physics. The dynamics of the model is described by the Einstein equations (which is obtained by varying the action with respect to the metric) and the scalar field equation of motion (which comes from the variation of the action with respect to the scalar field). In this talk, the scalar field is considered as an external scalar field, which means that we do not take a variation of the action with respect to the scalar field, and the field equations are derived from the Einstein equations and the conservation of energy momentum tensor. Then their solution is studied for some particular potential functions of scalar field.

Wednesday, 4 October

11:00 → 12:00 Puchwein: "Numerical simulations of screened modified gravity models"

12:30 → 14:00 lunch

14:00 → 15:00 Charmousis / Babichev: Compact objects in scalar-tensor and vector-tensor theories

Slides Babichev.pdf

15:30 → 16:30 Sawicki + Vikman : "constraints on Horndeski theories"

Thursday, 5 October

11:00 → 12:00 Davis: Screening mechanisms in modified-gravity scenarios

Slides Davis-3.pdf

12:30 → 14:00 lunch

14:00 → 15:00 Babichev: "Black holes in massive gravity"

Slides Babichev-2.pdf

15:00 → 16:00 Zumalacarregui: "scalar fields: local vs cosmological"

16:00 → 17:00 Happy hour: beer, wine and biscuits