Name: 50 Years of Horndeski Gravity: Exploring Modified Gravity
Start: 2024-07-15T08:00:00-04:00
End: 2024-07-19T17:00:00-04:00
Location: Perimeter Institute for Theoretical Physics

50 Years of Horndeski Gravity: Exploring Modified Gravity

from Monday, July 15, 2024 (8:00 a.m.) to Friday, July 19, 2024 (5:00 p.m.)

Monday, July 15, 2024

8:30 a.m. Registration
Registration
8:30 a.m. - 9:00 a.m.
Room: University of Waterloo - Federation Hall

9:00 a.m. Welcome Remarks
Welcome Remarks
9:00 a.m. - 9:15 a.m.
Room: University of Waterloo - Federation Hall

9:15 a.m. Opening Talk
Opening Talk
9:15 a.m. - 9:30 a.m.
Room: University of Waterloo - Federation Hall
9:30 a.m. Horndeski Gravity in Cosmology - Alessandra Silvestri (Leiden University)
Horndeski Gravity in Cosmology
- Alessandra Silvestri (Leiden University)
9:30 a.m. - 10:15 a.m.
Room: Federation Hall In this talk I will review how Horndeski gravity made its way in Cosmology and why it became a very popular framework for tests of gravity on cosmological scales.
10:15 a.m. Break
Break
10:15 a.m. - 11:00 a.m.
Room: Federation Hall
11:00 a.m. How we rediscovered Horndeski gravity - Cédric Deffayet (École Normale Supérieure)
How we rediscovered Horndeski gravity
- Cédric Deffayet (École Normale Supérieure)
11:00 a.m. - 11:45 a.m.
Room: Federation Hall I will review the steps that led to the rediscovery and the wide use of Horndeski theories as well as stress some left over open questions and ideas.
11:45 a.m. Black holes in Horndeski theories - Christos Charmousis (IJCLAB CNRS)
Black holes in Horndeski theories
- Christos Charmousis (IJCLAB CNRS)
11:45 a.m. - 12:30 p.m.
Room: Federation Hall We will discuss black hole solutions in Horndeski and beyond Horndeski theories. Starting from the no hair paradigm in GR we will elaborate on one of the first black hole solutions with secondary hair. We will then start by introducing stealth solutions, in other words GR metrics endowed with a non trivial scalar field, their regularity properties, shortcomings etc. We will then go on to construct black holes with primary scalar hair which can be regular black holes and modify usual GR static metrics. We will discuss their properties and the status of explicit stationary metrics to conclude.
12:30 p.m. Lunch
Lunch
12:30 p.m. - 1:30 p.m.
Room: Federation Hall

1:30 p.m. Free Time
Free Time
1:30 p.m. - 2:30 p.m.
2:30 p.m. Hi-COLA: Horndeski Goes Non-linear - Tessa Baker (University of Portsmouth)
Hi-COLA: Horndeski Goes Non-linear
- Tessa Baker (University of Portsmouth)
2:30 p.m. - 3:15 p.m.
Room: Federation Hall Simulating non-linear scales of structure formation is essential to make use of frontier data from Stage IV galaxy surveys. Performing these simulations in modified gravity theories introduces additional challenges, and further forces us to make choices about which theories we deem ‘worth’ the computational investment. Horndeski Gravity is very helpful in this regard, as it encompasses a large swathe of models of major interest. I’ll introduce Hi-COLA, a software suite which simulates large-scale structure formation in the class of luminal Horndeski theories. Hi-COLA was designed to be: i) flexible — it avoids hard-coded models and instead receives a user-specified Lagrangian; ii) consistent — the background expansion history, linear growth and nonlinear screening are solved consistently with one another; iii) efficient — using the COLA method, large sets of simulations can be generated at low cost. I’ll explain how Hi-COLA can be used to make robust predictions for scalar-tensor theories on nonlinear scales. If time permits, we’ll also dip a toe into constraining the Horndeski framework with gravitational waves.
3:15 p.m. Break
Break
3:15 p.m. - 3:45 p.m.
Room: Federation Hall
3:45 p.m. Contributed Talks (Session 1)
Contributed Talks (Session 1)
3:45 p.m. - 5:00 p.m.
Room: Federation Hall
Contributions

3:45 p.m. Photon Rings and Shadow Size for General Axi-Symmetric and Stationary Integrable spacetimes - Kiana Salehi (Perimeter Institute/University of Waterloo)

4:00 p.m. Modified gravity getting to the one-point clustering statistics - Cora Uhlemann (Universität Bielefeld)

4:15 p.m. Generalized disformal Horndeski theories: consistency of matter coupling and cosmological perturbations - Kazufumi Takahashi (Yukawa Institute for Theoretical Physics, Kyoto University)

4:30 p.m. Quantum Energy Conditions and Bouncing Cosmologies - Brayden Hull (UWaterloo & PI)

4:45 p.m. Spontaneous scalarization in scalar-Gauss-Bonnet gravity and beyond - Stoytcho Yazadjiev (Sofia University)
5:00 p.m. Poster Session
Poster Session
5:00 p.m. - 6:00 p.m.
Room: Federation Hall
Tuesday, July 16, 2024
9:00 a.m. Do Neutron stars k-mouflage? - Dina Traykova (Max Planck Institute for Gravitational Physics)
Do Neutron stars k-mouflage?
- Dina Traykova (Max Planck Institute for Gravitational Physics)
9:00 a.m. - 9:45 a.m.
Room: Federation Hall Scalar-tensor theories with a screening mechanism can serve as a viable model of cosmic acceleration, while still passing existing local tests of gravity. The validity of different screening mechanisms, such as kinetic screening (or k-mouflage) has been studied extensively in static and weak field regimes. However, only recently have works started to focus on determining whether these would work as expected near extremely compact objects. In this talk I will discuss some recent efforts to characterise kinetic screening mechanisms in these highly dynamical and non-linear regimes using numerical relativity in the case of a single oscillating neutron star, as well as how this picture may change for a binary neutron star configuration in a quasi-equilibrium state.
9:45 a.m. Scalarized black holes - from equilibrium models to nonlinear dynamics - Daniela Doneva (University of Tuebingen)
Scalarized black holes - from equilibrium models to nonlinear dynamics
- Daniela Doneva (University of Tuebingen)
9:45 a.m. - 10:30 a.m.
Room: Federation Hall Black holes in Horndeski theories of gravity are a perfect playground for exploring possible deviations from General Relativity in a theory-specific manner and studying their astrophysical manifestation. I will review the recent advances in constructing stationary hairy black hole models in Gauss-Bonnet theories. Special attention will be paid to their nonlinear dynamics when isolated or put in a binary, and the resulting astrophysical implications. The potential loss of hyperbolicity will also be discussed.
10:30 a.m. Break
Break
10:30 a.m. - 11:00 a.m.
Room: Federation Hall
11:00 a.m. Nonlinear dynamics of compact object mergers in Einstein-scalar-Gauss-Bonnet gravity - Maxence Corman
Nonlinear dynamics of compact object mergers in Einstein-scalar-Gauss-Bonnet gravity
- Maxence Corman
11:00 a.m. - 11:45 a.m.
Room: Federation Hall In recent years, gravitational wave observations of compact objects have provided new opportunities to test our understanding of gravity in the strong-field, highly dynamical regime. To perform model-dependent tests of General Relativity with these observations, one needs accurate inspiral-merger-ringdown waveforms in alternative theories of gravity. In this talk, we will discuss the nonlinear dynamics of compact object mergers in a class of modified theories of gravity, as well as the challenges in numerically obtaining those solutions. The theory we focus on is Einstein-scalar-Gauss-Bonnet gravity, which is a representative example of a Horndeski gravity theory and is interesting because it admits scalar hairy black hole solutions.
11:45 a.m. Black hole binaries in Einstein-scalar-Gauss-Bonnet gravity and their effective-one-body description - Félix-Louis Julié (Max Planck Institute for Gravitational Physics)
Black hole binaries in Einstein-scalar-Gauss-Bonnet gravity and their effective-one-body description
- Félix-Louis Julié (Max Planck Institute for Gravitational Physics)
11:45 a.m. - 12:30 p.m.
Room: Federation Hall I will show how to derive libraries of semi-analytic gravitational waveforms for coalescing “hairy” black hole binaries, focusing on the example of Einstein-scalar-Gauss-Bonnet gravity (ESGB). To do so, I will start from the state-of-the-art, effective-one-body waveform model “SEOBNRv5PHM” in general relativity, and deform it with ESGB corrections to infer inspiral-merger-ringdown waveform estimates.
12:30 p.m. Lunch
Lunch
12:30 p.m. - 1:30 p.m.
Room: Federation Hall

1:30 p.m. Poster Session
Poster Session
1:30 p.m. - 2:15 p.m.
Room: Federation Hall
2:15 p.m. Contributed Talks (Session 2)
Contributed Talks (Session 2)
2:15 p.m. - 3:30 p.m.
Room: Federation Hall
Contributions

2:15 p.m. Constructing rotating black holes in Horndeski gravity - Jibril BEN ACHOUR (Arnold Sommerfeld Center (Germany) / ENS de Lyon (France))

2:30 p.m. Assessing observational constraints on dark energy - David Shlivko (Princeton University)

2:45 p.m. Quasinormal modes and non-linear behaviour - Taillte May (Perimeter Institute)

3:00 p.m. Non-linear gravitational waves in Horndeski gravity - Hugo Roussille (École Normale Supérieure de Lyon)

3:15 p.m. Formulating the complete initial boundary value problem in numerical relativity to model black hole echoes - Conner Dailey (UWaterloo & PI)
3:30 p.m. Break - Presenters should remove posters
Break - Presenters should remove posters
3:30 p.m. - 4:00 p.m.
Room: Federation Hall
4:00 p.m. Town Hall - Modified gravity strong field regime - Enrico Barausse Suvendu Giri Laura Bernard Maxence Corman
Town Hall - Modified gravity strong field regime
- Enrico Barausse
- Suvendu Giri
- Laura Bernard
- Maxence Corman
4:00 p.m. - 5:00 p.m.
Room: Federation Hall Lead: Will East
Wednesday, July 17, 2024
9:00 a.m. Gravitational wave generation in effective field theories of dark energy - Enrico Barausse (International School for Advanced Studies (SISSA))
Gravitational wave generation in effective field theories of dark energy
- Enrico Barausse (International School for Advanced Studies (SISSA))
9:00 a.m. - 9:45 a.m.
Room: PI/1-100 - Theatre I will present the class of effective field theories of dark energy, which aim to reproduce a dark energy-like phenomenology by modifying general relativity with the addition of a scalar graviton. I will review how non-linearities can "screen" local scales from scalar effects, therefore allowing these theories to pass existing solar-system experimental tests. I will then present fully relativistic simulations of gravitational wave generation in these theories in 1+1 dimensions (stellar oscillations and collapse) and 3+1 dimensions (binary neutron stars). I will show that screening tends to suppress the (subdominant) dipole scalar emission in binary neutron star systems, but it fails to quench monopole scalar emission in gravitational collapse, and quadrupole scalar emission in binaries. This opens the way to the exciting possibility of testing dark energy with gravitational wave data.
9:45 a.m. Beyond Horndeski theories - David Langlois (APC/Université Paris Cité, CNRS)
Beyond Horndeski theories
- David Langlois (APC/Université Paris Cité, CNRS)
9:45 a.m. - 10:30 a.m.
Room: PI/1-100 - Theatre This talk will introduce scalar-tensor theories of gravity that contain a single scalar degree of freedom in addition to the usual tensor modes. These theories constitute the very broad family of Degenerate Higher-Order Scalar-Tensor (DHOST) theories, which include and extend Horndeski theories. Cosmological aspects of these theories will then be discussed. Finally, I will also present some results concerning black hole perturbations in the context of these models of modified gravity.
10:30 a.m. Break
Break
10:30 a.m. - 11:00 a.m.
Room: PI/1-119 - Atrium
11:00 a.m. Non-linear dark energy simulations - Martin Kunz (University of Geneva)
Non-linear dark energy simulations
- Martin Kunz (University of Geneva)
11:00 a.m. - 11:45 a.m.
Room: PI/1-100 - Theatre The coming years will see an amazing increase in data on the large-scale structure of the Universe, ushering in a new phase for "precision cosmology". One of the major questions in fundamental physics concerns the nature of the dark energy, and the new data may help to shed light on this issue. But in order to unlock the full power of the future data to test alternative models like Horndeski Gravity, we need theoretical predictions that are as accurate as the new observations on all scales, including non-linear scales. In my presentation I will introduce our relativistic N-body code for cosmological simulations, gevolution, and how we are using it to look at non-linear effects in the Universe. In particular I will discuss our k-essence simulations, how to use them for cosmology, and what can happen when dark energy clustering becomes non-linear in models with low speed of sound.
11:45 a.m. Testing screened modified gravity models - Kazuya Koyama (University of Portsmouth)
Testing screened modified gravity models
- Kazuya Koyama (University of Portsmouth)
11:45 a.m. - 12:30 p.m.
Room: PI/1-100 - Theatre Since the discovery of the accelerated expansion of the universe, significant progress has been made to develop modified gravity theories as alternatives to dark energy and these have been developed into tests of General Relativity itself via cosmological observations. These models share common properties such as screening mechanisms they use to evade the stringent Solar System tests. In this talk, I will review recent status of observational tests of screened modified gravity models and discuss the prospect of cosmological tests of gravity from ongoing surveys such as Euclid.
12:30 p.m. Lunch
Lunch
12:30 p.m. - 1:30 p.m.
Room: PI/2-251 - Upper Bistro

1:30 p.m. // Free Time
// Free Time
1:30 p.m. - 2:15 p.m.
2:15 p.m. Contributed Talks (Session 3)
Contributed Talks (Session 3)
2:15 p.m. - 3:30 p.m.
Room: PI/1-100 - Theatre
Contributions

2:15 p.m. Constraining modified gravity models using galaxy cluster masses - Minahil Adil Butt (SISSA)

2:30 p.m. mochi_class: A tool to streamline cosmological analyses of Horndeski's gravity - Matteo Cataneo (University of Bonn)

2:45 p.m. Testing gravity with the Dark Energy Survey - Jessica Muir (Perimeter Institute)

3:00 p.m. A Cosmic Glitch in Gravity - Yunfei Wen (California Institute of Technology)

3:15 p.m. Cosmological implications from DESI Y1 BAO and Future Forecasts - Hanyu Zhang
3:30 p.m. Break
Break
3:30 p.m. - 4:00 p.m.
Room: PI/1-119 - Atrium
4:00 p.m. Town Hall - New generation of LSS surveys and modified gravity/dark energy - Jessie Muir Hanyu Zhang Martin Kunz Kazuya Koyama
Town Hall - New generation of LSS surveys and modified gravity/dark energy
- Jessie Muir
- Hanyu Zhang
- Martin Kunz
- Kazuya Koyama
4:00 p.m. - 5:00 p.m.
Room: PI/1-100 - Theatre Lead: Alex Krowlewski
5:00 p.m. // Free Time
// Free Time
5:00 p.m. - 6:00 p.m.

6:00 p.m. Banquet
Banquet
6:00 p.m. - 8:00 p.m.
Room: PI/2-251 - Upper Bistro
Thursday, July 18, 2024
9:00 a.m. Post-Newtonian limit of Lorentz-violating scalar-tensor theories - Tsutomo Kobayashi (Rikkyo University)
Post-Newtonian limit of Lorentz-violating scalar-tensor theories
- Tsutomo Kobayashi (Rikkyo University)
9:00 a.m. - 9:45 a.m.
Room: PI/1-100 - Theatre We study the weak-gravity regime of higher-order scalar-tensor theories that are degenerate in the unitary gauge. In a certain subset of theories analogous to Lorentz-violating scalar-tensor theories, we show that the Vainshtein mechanism due to nonlinear derivative interactions does not work. For this family of theories we determine all the PPN parameters in terms of the EFT of dark energy parameters and discuss the experimental bounds.
9:45 a.m. Galileon Duality and its Generalizations - Andrew Tolley
Galileon Duality and its Generalizations
- Andrew Tolley
9:45 a.m. - 10:30 a.m.
Room: PI/1-100 - Theatre
10:30 a.m. Break
Break
10:30 a.m. - 11:00 a.m.
Room: PI/1-119 - Atrium
11:00 a.m. Extending EFT of inflation/dark energy to arbitrary background with timelike scalar profile - Shinji Mukohyama (Kyoto University)
Extending EFT of inflation/dark energy to arbitrary background with timelike scalar profile
- Shinji Mukohyama (Kyoto University)
11:00 a.m. - 11:45 a.m.
Room: PI/1-100 - Theatre I extend EFT of inflation/dark energy to arbitrary background with timelike scalar profile. In this framework a set of consistency relations among EFT coefficients ensures the spatial diffeo invariance. Some applications to will also be discussed.
11:45 a.m. Against Horndeski - Cliff Burgess
Against Horndeski
- Cliff Burgess
11:45 a.m. - 12:30 p.m.
Room: PI/1-100 - Theatre The Horndeski program is motivated by arguing that scalar-tensor modifications to gravity should have two properties: effective interactions that are at most second-order in time derivatives and only a single scalar. I will argue against both of these criteria. First I argue why the low-energy limit of known well-behaved theories can have more than two-derivative field equations. Second I argue why the scalar-tensor interactions most likely to be found competing with gravity at very low energies typically are those with two derivatives, at least when semiclassical methods are justified, and this suggests exploring multiple-scalar models.
12:30 p.m. Group Photo
Group Photo
12:30 p.m. - 12:40 p.m.
Room: PI/1-119 - Atrium

12:40 p.m. Lunch
Lunch
12:40 p.m. - 1:30 p.m.
Room: PI/2-251 - Upper Bistro

1:30 p.m. Free Time
Free Time
1:30 p.m. - 2:15 p.m.
2:15 p.m. Contributed Talks (Session 4)
Contributed Talks (Session 4)
2:15 p.m. - 3:30 p.m.
Room: Federation Hall
Contributions

2:15 p.m. Bouncing cosmology; a solution to the singularity problem and more. - Amirhossein Dehghanizadeh (The University of Waterloo)

2:30 p.m. Gauss-Bonnet Gravity in 4D and the connection to Horndeski's Theory - Pedro Fernandes (Southern Denmark University)

2:45 p.m. How to study modified gravity as a particle theory and not collapse in the process - Sergio Sevillano (Durham University)

3:00 p.m. Scattering amplitudes in high-energy limit of projectable Horava gravity - Jury Radkovski (Perimeter Institute, McMaster University)

3:15 p.m. Theoretical status of Horava gravity - Sergey Sibiryakov (McMaster U. & Perimeter Inst.)
3:30 p.m. Break
Break
3:30 p.m. - 4:00 p.m.
Room: PI/1-119 - Atrium
4:00 p.m. Town Hall - Fundamental aspects of Modified gravity - Adam Solomon Astrid Eichhorn Sergey Sibiryakov Andrew Tolley
Town Hall - Fundamental aspects of Modified gravity
- Adam Solomon
- Astrid Eichhorn
- Sergey Sibiryakov
- Andrew Tolley
4:00 p.m. - 5:00 p.m.
Room: PI/1-100 - Theatre Lead: Jerome Quintin
Friday, July 19, 2024
9:00 a.m. Simulating a Quantised Black Hole - Ruth Gregory (King's College London)
Simulating a Quantised Black Hole
- Ruth Gregory (King's College London)
9:00 a.m. - 9:45 a.m.
Room: PI/1-100 - Theatre
9:45 a.m. Probing quantum gravity at all scales - Astrid Eichhorn
Probing quantum gravity at all scales
- Astrid Eichhorn
9:45 a.m. - 10:30 a.m.
Room: PI/1-100 - Theatre
10:30 a.m. Break
Break
10:30 a.m. - 11:00 a.m.
Room: PI/1-119 - Atrium
11:00 a.m. Production of Solar Scalars - Anne-Christine Davis (University of Cambridge)
Production of Solar Scalars
- Anne-Christine Davis (University of Cambridge)
11:00 a.m. - 11:45 a.m.
Room: PI/1-100 - Theatre I will first introduce screened modified gravity theories and then discuss the chameleon mechanism. Light scalars can be produced from the sun and detected on earth. I will discuss the production of chameleons, including novel production channels, and discuss potential detection in helioscopes.
11:45 a.m. Effective cuscuton theory - Maria Mylova (Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU))
Effective cuscuton theory
- Maria Mylova (Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU))
11:45 a.m. - 12:15 p.m.
Room: PI/1-100 - Theatre Cuscuton field theory is an extension of general relativity that does not introduce additional propagating degrees of freedom, or violate relativistic causality. We construct a general geometric description of the cuscuton field theory by introducing curvature corrections to both the volume (potential) and the surface (kinetic) terms in the original cuscuton action. Our assumptions involve a stack of spacelike branes, separated by 4-dimensional bulks. We conjecture that the cuscuton, initially a discrete field, becomes continuous in the limit, there are many such transitions. From this we derive an effective action for the cuscuton theory and show that at the quadratic level our theory propagates only the two tensorial degrees of freedom.
12:15 p.m. Closing Remarks
Closing Remarks
12:15 p.m. - 12:30 p.m.
Room: PI/1-100 - Theatre

12:30 p.m. // Free time
// Free time
12:30 p.m. - 12:45 p.m.

12:45 p.m. Lunch
Lunch
12:45 p.m. - 1:45 p.m.
Room: PI/2-251 - Upper Bistro

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