WGCPcall20180209

Next call

Call: 27 August 2018

Meeting Agenda

• move meetings to 1st Monday of each month
• reminder: EU ETK Workshop and School in Lisbon, 10-13 September
• future directions
• Q & A discussion session

Call: 1 October 2018

Meeting Agenda

• [IC] discussion on constraint evolution vs free evolution

Previous calls

Call: 25 June 2018

Meeting Agenda

• [BW] 3+1 formulation and hyperbolicity of f(R) theories

Minutes

Participants: Katy Clough, Isabel Cordero, Michele Grasso, Bishop Mongwane, Daniele Vernieri, Helvi Witek

• Reminder: EU Einstein Toolkit and school in Lisbon, September 2018
  • Website: https://centra.tecnico.ulisboa.pt/network/grit/einsteintoolkit2018/
  • Deadlines: 1 July 2018 (abstract submission); 15 July 2018 (registration)

• Summer calls:
  • NO call in July or August
  • next regular working group call: September 2018

• Discussion: Bishop Mongwane on 3+1 formulation and hyperbolicity of f(R) theories
  • based on papers:
    • https://arxiv.org/abs/1610.07224
    • https://arxiv.org/abs/1707.05757
  • motivation
    • f(R) gravity captures class of interesting cosmological models,.
    • e.g. Starobinsky model of inflation
    • under certain conditions equivalent to scalar-tensor theories ($f'(R) \sim \phi$)
  • results
    • derive time evolution formulation for f(R) gravity, specifically ADM and BSSNOK formulation
    • neat trick: use the 4-dimensional Ricci scalar $^{(4)}R$ and its time derivative $\psi=\p_{t} ^{(4)}R$ as independent variables
    • showed strong hyperbolicity for BSSN-type formulation
  • numerical tests still problematic, maybe future work
  • Comments
    • BSSN only valid for $\det\tilde{\gamma}=1$; maybe reference metric formulation for spherically symmetric spacetimes
    • numerical issues related to gauge choice?
    • future numerical tests (a la ``apples with apples) of non-trivial cosmological or black hole spacetimes
    • references for non-trivial or cosmological boundary conditions:
      • https://arxiv.org/abs/1409.3476
      • https://arxiv.org/abs/1509.08354
      • https://arxiv.org/abs/1204.2019
  • general discussion on constraint evolution vs free evolution in future working group call (Isa)

Call: 23 April 2018

Meeting Agenda

• [KC] introduction to GRChombo (http://www.grchombo.org/)

Minutes

Participants:

Call: March 19th, 2018

Meeting agenda

• [DV] introduction to Horndeski theories
• [HW] regular meeting time

Minutes

Participants: Sante Carloni, Katy Clough, Michele Grasso, Bishop Mongwane, Erik Schnetter, Daniele Vernieri, Helvi Witek

• Timing for regular meeting
  • Agreed that we will aim to meet once a month on Mondays at 4.30pm GMT / 5.30pm CET time
  • Agreed that having a 20 minute discussion on a specific topic at each call is a good idea to give a focus to the call
  • Next meeting will be on 23rd April, with 20 minute discussion on GRChombo numerical relativity code

• Horndeski theories
  • DV gave an introduction to Horndeski theories, which are the most general theories with an additional scalar field leading to second order field equations.
  • The original Horndeski paper was published in 1973 but the theory was rediscovered in 2010 in the the equivalent context of Galileons, after which it became more popular in research
  • The field equations can be simplified (a little!) by writing them in terms of 4 arbitrary functions of the field phi and the kinetic term X.
  • Covers a number of common modified gravity theories - f(R), Brans-Dicke, Gauss-Bonnet.
  • Recent measurements of GWs have been used to show (PRL 119 251302, 2017) that, given that the speed of propagation of GWs is c (to very high precision), several terms in the most general action must be set to zero. This may rule out the Vainstein screening mechanism.
  • Some work is being done to investigate the well-posedness of the equations, (see https://arxiv.org/abs/1710.10155, https://arxiv.org/pdf/1705.04370.pdf), which is crucial for their numerical formulation and evolution, but so far they have only been shown to be weakly hyperbolic. This may, however, be due to issues around gauge choices.

Call: February 9th, 2018

Meeting agenda

• [HW] introduction and scope of the working group

Minutes

Participants: Eloisa Bentivegna, Sante Carloni, Katy Clough, Isabel Cordero, Michele Grasso, Daniele Vernieri, Helvi Witek

• Introduction
  • ET open community, combine efforts on similar topics in working groups
  • wide scope to use numerical relativity in less traditional contexts including, e.g. cosmology (e.g., inflation, structure formation,...), modified gravity (e.g. effective 4D models, cosmological models, higher-D, ...), black hole properties (e.g. superradiance, stability properties,...)
  • soft deliverables: initiate collaborations; contact for potential common science proposals

• Feedback and common interests
  • important to combine expertise of currently distinct communities, both on a scientific and methodological level (e.g cosmology, modified gravity, compact objects; numerical relativity, maths, (perturbation) theory and phenomenology)
  • development of new mathematical methods for numerical schemes
  • apply numerical methods to tackle increasingly complicated problems in perturbation theory in modified gravity
  • understanding perturbations crucial for extraction and interpretation of gravitational wave signatures
  • cross-checking and benchmarking with independent codes, e.g., Einstein Toolkit and GR Chombo
  • common scientific goals:
    • evolution and perturbations in cosmological backgrounds
    • early universe cosmology
  • black hole stability and superradiance / axion dark matter physics

• Frequency of meetings and next meeting
  • once per month or every two months
  • next meeting in the beginning of March; Helvi will create doodle closer to the date
  • please consider attending the workshop ``Numerical relativity beyond GR in Benasque/ Spain, 3-9 June 2018