[Cosmoparticle-wg] ET "Cosmology and particles" meeting minutes 19 March 2018

[helvi witek]

Dear all,

please find the minutes for today's meeting below and on our wiki:
https://docs.einsteintoolkit.org/et-docs/Cosmology_and_Particles
@Katy: thanks for taking them!

We decided to have a call once a month on a Monday around 4:30pm (GMT),
i.e., after the regular Einstein Toolkit call.
The next call takes place on 23rd April 2018 where Katy will give us an
introduction to GRChombo (see http://www.grchombo.org/ ) which is now
publicly available.

cheers,
Helvi

-----
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.



===========================================
Dr. Helvi Witek
Royal Society University Research Fellow
Theoretical Particle Physics and Cosmology
Department of Physics
King's College London
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