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Some effective field theories popular in cosmology exhibit superluminality; they contain fluctuations that propagate faster than the speed of light around non-trivial backgrounds. There is controversy as to whether this kind of superluminality is actually a trustable physical feature of the effective theory, or merely an artifact of using an incomplete effective theory or of asking the wrong questions. Even if it is trustable, there is controversy as to whether this spells trouble for a theory, i.e. ruling out a UV completion, or whether it is harmless. This workshop will aim to bring people on all sides to Perimeter, with the goal of making progress on these issues.
Sponsorship for this workshop has been provided by:
- Lasha Berezhiani, University of Pennsylvania
- Paolo Creminelli, International Centre for Theoretical Physics
- Cedric Deffayet, Laboratoire Astroparticule et Cosmologie (APC)
- Sergei Dubovsky, New York University
- Gregory Gabadadze, New York University
- Fawad Hassan, Stockholm University
- Timothy Hollowood, Swansea University
- Keisuke Izumi, National Taiwan University
- Alberto Nicolis, Columbia University
- Antonio Padilla, University of Nothingham
- Massimo Porrati, New York University
- Raquel Ribeiro, Queen Mary University of London
- Andrew Tolley, Case Western Reserve University
- Alex Vikman, Ludwig-Maximillian University
- Andrew Waldron, University of California, Davis
- George Zahariade, University of California, Davis
- Lasha Berezhiani, University of Pennsylvania
- Alejandro Cardenas Avendano, Universidad Nacional de Colombia
- Paolo Creminelli, International Centre for Theoretical Physics
- Cedric Deffayet, Laboratoire Astroparticule et Cosmologie (APC)
- Sergei Dubovsky, New York University
- Matteo Fasiello, Stanford University
- Antonia Frassino, Perimeter Institute
- Gregory Gabadadze, New York University
- Mariana Gonzalez, Perimeter Institute
- Garret Goon, Cambridge University
- Fawad Hassan, Stockholm University
- Kurt Hinterbichler, Perimeter Institute
- Timothy Hollowood, Swansea University
- Lam Hui, Columbia University
- Keisuke Izumi, National Taiwan University
- Austin Joyce, University of Chicago
- Justin Khoury, University of Pennsylvania
- Andrew Matas, Case Western Reserve University
- John Moffat, Perimeter Institute
- Antonio Padilla, University of Nothingham
- Riccardo Penco, Columbia University
- Massimo Porrati, New York University
- Raquel Ribeiro, Queen Mary University of London
- Rachel Rosen, Columbia University
- Alexandra Terrana, Perimeter Institute
- Andrew Tolley, Case Western Reserve University
- Mark Trodden, University of Pennsylvania
- Alex Vikman, Ludwig-Maximillian University
- Andrew Waldron, University of California, Davis
- George Zahariade, University of California, Davis
Thursday, April 9, 2015
Time |
Event |
Location |
8:30 – 9:00am |
Registration |
Reception |
9:00 – 9:15am |
Welcome and Opening Remarks |
Bob Room |
9:15 – 9:45am |
George Zahariade, University of California, Davis |
Bob Room |
9:45 – 10:30am |
Keisuke Izumi, National Taiwan University |
Bob Room |
10:30 – 11:00am |
Coffee Break |
Bistro – 1st Floor |
11:00 – 12:30pm |
Andrew Tolley, Case Western Reserve University |
Bob Room |
12:30 – 2:30pm |
Lunch |
Bistro – 2nd Floor |
2:30 – 3:15pm |
Gregory Gabadadze, New York University |
Bob Room |
3:15 – 3:45pm |
Lasha Berezhiani, University of Pennsylvania |
Bob Room |
3:45 – 4:30pm |
Coffee Break |
Bistro – 1st Floor |
4:30 – 5:30pm |
Andrew Tolley, Case Western Reserve University |
Bob Room |
Friday, April 10, 2015
Time |
Event |
Location |
9:00 – 9:45am |
Timothy Hollowood, Swansea University |
Bob Room |
9:45 – 10:30am |
Sergei Dubovsky, New York University |
Bob Room |
10:30 – 11:00am |
Coffee Break |
Bistro – 1st Floor |
11:00 – 11:45am |
Alex Vikman, Ludwig-Maximillian University |
Bob Room |
11:45 – 12:30pm |
Massimo Porrati, New York University |
Bob Room |
12:30 – 2:30pm |
Lunch Break |
Bistro – 2nd Floor |
2:30 – 3:15pm |
Cedric Deffayet, Laboratoire Astroparticule et Cosmologie |
Bob Room |
3:15 – 4:00pm |
Antonio Padilla, University of Notthingham |
Bob Room |
4:00 – 4:15pm |
Conference Photo |
TBA |
4:15 – 4:45pm |
Coffee Break |
Bistro – 1st Floor |
4:45 – 5:30pm |
Alberto Nicolis, Columbia University |
Bob Room |
6:00pm Onwards |
Banquet |
Bistro – 2nd Floor |
Saturday, April 11, 2015
Time |
Event |
Location |
9:00 – 9:45am |
Raquel Ribeiro, Queen Mary University of London |
Bob Room |
9:45 – 10:30am |
Paolo Creminelli, International Centre for Theoretical Physics |
Bob Room |
10:30 – 11:00am |
Coffee Break |
Bistro – 1st floor |
11:00 – 11:30am |
Fawad Hassan, Stockholm University |
Bob Room |
11:30 – 12:15pm |
Andrew Waldron, University of California, Davis |
Bob Room |
12:15pm |
Lunch |
Bistro – 1st floor |
Lasha Berezhiani, University of Pennsylvania
Subluminal Vainshtein Screening in Massive Gravity
I will discuss the Vainshtein mechanism in massive gravity. I will show that the spherically symmetric backgrounds that were believed to have superluminal sound speed are in fact unstable. Instead, there is a new class of phenomenologically relevant solutions with stable and subluminal perturbations.
Paolo Creminelli, International Centre for Theoretical Physics
Galileon dualities and superluminality
Cedric Deffayet, Laboratoire Astroparticule et Cosmologie (APC)
Galileon p-forms
I will discuss generalizations and no-go theorems for generalizations to p-forms of Galileon actions
Sergei Dubovsky, New York University
Asymptotic fragility, superluminality and pi
Fawad Hassan, Stockholm University
Some recent results in Bimetric Theory
This talk will summarize some recent results in bimetric theory, including the existence of the square-root matrix, possible connection to partial masslessness and conformal gravity, the structure of constraints and finally, the cosmological implications of the theory.
Timothy Hollowood, Swansea University
Causality constraints and the lightcone
It is an attractive idea that effective theories admitting a consistent UV completion require quanta to propagate sub-luminally in non-trivial backgrounds. However, there is a counter example to this proposition in the form of QED in a curved geometry, a theory that is certainly causal. Nevertheless, Drummond and Hathrell showed that there is always at least one choice of polarization for which low frequency photons propagate super-luminally. Conventional arguments involving dispersion relations would then normally imply that the high frequency phase velocity would also exceed c yielding a contradiction with the UV completion. We show how the contradiction is avoided by a mechanism that relies on the subtle behaviour of the lightcones in the geometry and that, in the end, super-luminal low frequency propagation is perfectly consistent with causality. In particular, time machines cannot be constructed using the effect. The lesson is that causality constraints in low energy effective theories need to be treated with some caution.
Keisuke Izumi, National Taiwan University
Causal structures in Massive gravity and Gauss-Bonnet gravity
In General Relativity, gravitons propagate to null directions, because of its well-organized structures. Modifying the gravity theory slightly, meanwhile, the beautiful structure is broken and gravitons can easily propagate superluminaly. Here, applying the characteristic method, which is the well-established powerful way to analyze causal structures, the causal structures in Massive gravity and Gauss-Bonnet gravity are analyzed. We discuss the superluminality, acausality and black holes.
Antonio Padilla, University of Nottingham
The Cosmological Constant Problem (and its sequester)
I will review the notorious cosmological constant problem, sometimes described as the worst fine tuning problem in Physics. I will explain the true nature of the problem, which is one of radiative instability against any change in the effective description. I will recall Weinberg’s venerable no-go theorem that prohibits certain attempts to “solve” this problem before going on to explain a new mechanism that circumvents Weinberg. This is the vacuum energy sequester, a global modification of GR that results in the cancellation of large vacuum energy contributions from a protected matter sector (taken to include the Standard Model) at each and every order in the perturbative loop expansion. Cosmological consequences are a Universe which has finite space-time volume, will ultimately crunch, and for which dark energy can only be a transient. Furthermore, using a linear scalar potential within the sequestering set-up, I will show that dark energy today can be intimately related to the trigger that brings about cosmological collapse in the not too distant future, at the same time providing a possible solution to the “Why Now?” problem.
Massimo Porrati, New York University
Superluminalities in Galileon theories
Subluminal Vainshtein Screening in Massive Gravity
I will discuss the Vainshtein mechanism in massive gravity. I will show that the spherically symmetric backgrounds that were believed to have superluminal sound speed are in fact unstable. Instead, there is a new class of phenomenologically relevant solutions with stable and subluminal perturbations.
TBA
Superluminalities in Galileon theories
Causal structures in Massive gravity and Gauss-Bonnet gravity
In General Relativity, gravitons propagate to null directions, because of its well-organized structures. Modifying the gravity theory slightly, meanwhile, the beautiful structure is broken and gravitons can easily propagate superluminaly. Here, applying the characteristic method, which is the well-established powerful way to analyze causal structures, the causal structures in Massive gravity and Gauss-Bonnet gravity are analyzed. We discuss the superluminality, acausality and black holes.
TBA
Pages
Scientific Organizers:
Kurt Hinterbichler, Perimeter Institute
Rachel Rosen, Columbia University