<div dir="ltr"><div><div><div><div><div>Dear all,<br><br></div>please find the minutes for today's meeting below and on our wiki:</div><div><a href="https://docs.einsteintoolkit.org/et-docs/Cosmology_and_Particles">https://docs.einsteintoolkit.org/et-docs/Cosmology_and_Particles</a><br></div>@Katy: thanks for taking them!<br><br></div>We decided to have a call once a month on a Monday around 4:30pm (GMT), i.e., after the regular Einstein Toolkit call. <br></div><div>The next call takes place on 23rd April 2018 where Katy will give us an introduction to GRChombo (see <a href="http://www.grchombo.org/" class="external gmail-free" title="http://www.grchombo.org/" rel="nofollow">http://www.grchombo.org/</a> ) which is now publicly available.<br></div></div><div><div><br><div><div><div><div><div>cheers,</div><div>Helvi</div><div><br></div><div>-----</div><div><h3> Minutes </h3>
<p>Participants: Sante Carloni, Katy Clough, Michele Grasso, Bishop Mongwane, Erik Schnetter, Daniele Vernieri, Helvi Witek
</p>
<ul><li> Timing for regular meeting
</li></ul>
<ul><li><ul><li> Agreed that we will aim to meet once a month on Mondays at 4.30pm GMT / 5.30pm CET time
</li><li> 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
</li><li> Next meeting will be on 23rd April, with 20 minute discussion on GRChombo numerical relativity code
</li></ul>
</li></ul>
<ul><li> Horndeski theories
<ul><li> DV gave an introduction to Horndeski theories, which are the
most general theories with an additional scalar field leading to second
order field equations.
</li><li> 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
</li><li> 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.
</li><li> Covers a number of common modified gravity theories - f(R), Brans-Dicke, Gauss-Bonnet.
</li><li> 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.
</li><li> Some work is being done to investigate the well-posedness of the equations, (see <a href="https://arxiv.org/abs/1710.10155" class="external gmail-free" title="https://arxiv.org/abs/1710.10155" rel="nofollow">https://arxiv.org/abs/1710.10155</a>, <a href="https://arxiv.org/pdf/1705.04370.pdf" class="external gmail-free" title="https://arxiv.org/pdf/1705.04370.pdf" rel="nofollow">https://arxiv.org/pdf/1705.04370.pdf</a>),
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.
</li></ul>
</li></ul></div><div><br></div><div><br clear="all"><div><div class="gmail_signature"><div dir="ltr"><div><div dir="ltr"><span><div><div><div><span>===========================================<br>
Dr. Helvi Witek<br></span></div><span>Royal Society University Research Fellow<br></span></div><span>Theoretical Particle Physics and Cosmology <br></span></div><div><span>Department of Physics<br></span></div><span>King's College London</span></span><span></span><br><span></span><span><span></span></span><span><div><div><div><div><span>
===========================================</span></div></div></div></div></span></div></div></div></div></div>
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