Before the workshop, a number of the participants brainstormed some topics for discussion.

Effective Field Theory

  • In what types of physical situations is EFT valid?
  • Are we guaranteed that an EFT treatment is appropriate for gravity?
  • Is physics in the IR/UV of some theory sensitive/insensitive to the opposite regime?
  • Are there theories in which the IR/UV somehow “mix” and affect physics in the opposite regime?
  • I like this line of questioning. I have some examples of gravitational “theories” that exhibit UV/IR mixing, and potentially defy the EFT dictum.

Cosmological frameworks / parametrizations

  • Can PPF (Friedmannian) or other cosmological parametrizations be extended into the non-linear regime? Or would this miss out lots of relevant physics/terms?
  • Are perturbative approaches like “EFT of Large Scale Structure” helpful in connecting large and small scales?
  • What is the relationship between parameterizations and screening effects?
  • What are the best variables to use in an EFT-of-DE-like framework? Can one generalize the variables in [1404.3713] to other regimes?

PPN and PPK frameworks

  • What are the assumptions behind PPN?
  • Does it work around curved background (like FLRW)?
  • Is PPK an extension of PPN?
  • Is there a way to deal with screening?
  • How are violations of the equivalence principle encoded?

PPE framework

  • Does PPE include screening?
  • More about gravitational waves in general: can modified gravity have effects on large scale (i.e. not necessarily at the source, but more during the propagation)?

Bumpy black holes

  • Can we get BH solution in scalar tensor theories such as Horndeski’s? Even around FLRW?
  • How dependent are bumpy BH solutions on assumptions about the boundary conditions of the spacetime? What do they look like if embedded in expanding spacetimes?
  • Are there observable signatures of firewalls/fuzzballs, e.g. in gravitational wave data?

Cosmological observables

  • Of all the typical cosmological signatures of modified gravity (RSD, cosmic shear, cluster/void lensing, etc), which stand the best chance of being detected? Which possess the most discriminatory power (e.g. distinguish between different screening mechanisms)?
  • Do any cosmological observables probe aspects of theories that are also interesting to compact object people? For example, would a detection of Lorentz violation on cosmological scales also have implications for (e.g.) BH stuff? Would the discovery of a background scalar field (quintessence etc.) be important to scalar no-hair calculations? Or are the distance scales so far apart that the theoretical implications aren’t really connected?

Screening effects

  • Departures from spherical symmetry - still not well understood?
  • Is it possible to make a parametrisation of screening mechanisms?
  • How does screening work inside composite objects like galaxies? If the galaxy is screened, does that mean everything inside the galaxy is also screened?
  • Do screening mechanisms that rely on spontaneous symmetry breaking produce topological defects or other exotic objects?
  • Can PPN/PPK/PPF/EFT/etc. parameters be related to any properties of screening mechanisms? Or does screening depend on essentially different properties of the relevant theories?
  • Vainshtein mechanism in curved ST (i.e. broken Galilean symmetry)?
  • The current status of N-body cosmological simulations of non-GR theories, and their importance in understanding screening effects on structure formation.
  • I would personally be interested in a (quick) review on the different screening mechanisms.

Proposed overlap scenarios

  • Can we trace violations of the equivalence principle throughout different scales?
  • Brown dwarfs could be good candidates for overlap as the screening effects might not be as efficient there.
  • Can the dynamics of stars in galaxies be treated using a PPN approach?
  • Can the dynamics of galaxies in clusters be treated using a PPN approach?
  • Can the dynamics of galaxies in clusters be treated using a PPF/EFT approach?
  • The necessity to embed non-linear structures in a FLRW spacetime can put strong conditions on alternative theories of gravity in cosmology (e.g. see [1404.3713]). Do similar things apply for compact objects (e.g. bumpy BHs must be embedded in some background)?
  • Gravitational waves are caused by the merging of compact objects, but travel over cosmological distances before being detected. They are therefore sensitive to modifications of GR in both the strong gravity and cosmological regimes. Is this useful for connecting the two? Or do we have to use two separate parameterizations at once!? Are there degeneracies between the cosmological/PPF and PPE parameters?
  • Some details of large-scale structure formation depend on “feedback mechanisms” such as the injection of energy into the ISM/IGM by SMBHs at the centre of galaxies. Could modifications to GR affect the SMBH drastically enough to alter the energy injection, and thus impact some large-scale observables too?

Theoretical motivations to modify GR

  • Cosmological Constant Problem
    • Can modifying GR address the cosmological constant problem(s)?
    • If so, can such modifications be tested in Cosmology, Black Holes, Neutron Stars, etc.?
    • Can we test how/if quantum vacuum gravitates?
    • Can Dark Energy couple to Black Holes?
    • Gravitational theories can be constructed that are similar (observationally) to GR, except that they are technically natural/degravitate
  • Black Hole Information Paradox
    • Can tests of BH physics (e.g. GW’s from BH mergers) shed light on the BH information paradox/ firewall controversy? Niayesh
  • Quantum gravity
    • Do some aspects of QG theories (e.g. extra dimensions) suggest certain modifications to GR?
    • Can modifications to GR be introduced to enable (e.g.) renormalizability or other properties desirable for unifying gravity with the standard model of particle physics?
  • Are there any shared motivations between the cosmological and compact object communities?

Reading list

We asked the lecturers to recommend some references to background reading which would inform the discussion.


Connecting large and small scales

Screening mechanisms

Observations and observables