'''Working Group: Cosmology and Particles'''

'''Date Approved:''' December 4th, 2017

'''Leads:''' Eloisa Bentivegna, Helvi Witek

'''Approval date:''' ET call on [http://lists.einsteintoolkit.org/pipermail/users/2017-November/005927.html 2017-11-27]

'''Background:''' The study of relativistic effects and strong-field dynamics is playing an increasingly important role in many disciplines related to, but not necessarily overlapping with, gravitational-wave astronomy. The full numerical integration of Einstein’s equation has now been adopted to describe structure formation in the late Universe, high-energy processes near black holes, gravitational-wave emission by exotic objects, and many other phenomena. However, numerical implementations of extensions to GR -- be it in the form of additional fields serving as dark matter candidates, more involved extensions motivated by quantum gravity paradigms or novel initial conditions for inflation -- are still scarce. Many of these effects are expected to be testable with advanced LIGO/Virgo or the space-based LISA mission. In order to match theory with observations it is crucial to prepare the necessary waveform models, which is one of the goals of this working group.

This Working Group should serve as the first arena to discuss extensions of Numerical Relativity towards new physical applications and their implementation in the Einstein Toolkit, enabling the modelling of these spacetimes beyond the traditional canons of GW-related NR. Given the exploratory character of many of these applications, the discussion will include not only algorithmic and infrastructural aspects, but also foundational issues connected to the physical and mathematical traits of the theories involved (such as how to best formulate the constitutive equations, construct initial data, define observables, and so on).

'''Activities:''' The Working Group will hold a monthly or bimonthly brainstorming call. Practical developments arising from the discussions may be subsequently formalized as Development Teams and attached to this WG’s activities. The calls may also contain short seminars. The development of an interface to the many experiments and observational missions within the WG’s scope should be among the top priorities.

'''Milestones:''' The main objective of this Working Group is that of collecting all the interested parties in a single community, give it an official structure, promote brainstorming and knowledge-sharing in this vast new field, and provide a public interface to the growing number of non-GW relativists who are turning to the Einstein Toolkit to extend the reach of their investigations in their respective fields.

'''Deliverables:''' This WG will deliver a yearly document, representing the Group’s outlook on the year’s developments in the field such as major publications, conferences, code releases, grant awards and their possible ramifications, the topics discussed during the calls, and any other issue of interest. This document will be the standard reference both for existing ETers looking to join new projects and for new members from other areas of Theoretical Physics where the ET may be employed.
Furthermore, we expect new add-ons to the Einstein Toolkit as deliverables of this working group. Ensuring the quality and and performance of these thorns, as well as cross-checking new code, will be part of the WG’s responsibility.

'''Engagement:''' New members can join the working group by subscribing to the mailing list [mailto:cosmoparticle-wg@einsteintoolkit.org cosmoparticle-wg@einsteintoolkit.org].

== Working group calls ==
'''Link for the calls:'''
https://hangouts.google.com/hangouts/_/tce4j4gzcfcormnadoauiqcvg4e

* Monday, 1 October 2018, 4:30pm BST (10:30am CDT): [[WGCPcall20180209 | agenda]]
* Monday, 27 August 2018, 4:30pm BST (10:30am CDT): [[WGCPcall20180209 | agenda and minutes]]
* Monday, 25 June 2018, 4:30pm BST (10:30am CDT): [[WGCPcall20180209 | agenda and minutes]]
* Monday, 23 April 2018, 4:30pm BST (10:30am CDT): [[WGCPcall20180209 | agenda and minutes]]
* Monday, 19 March 2018, 4:30pm UTC (10:30am CST): [[WGCPcall20180209 | agenda and minutes]]
* Friday, 9th February 2018, 10pm UTC (4pm CST / Saturday 7am JST): [[WGCPcall20180209 | agenda and minutes]]

WGCPcall20180209

2018-08-27T16:11:48Z

HelviWitek:

=Next call=
== Call: 1 October 2018 ==
=== Meeting Agenda ===
* [IC] discussion on constraint evolution vs free evolution

= Previous calls =

== Call: 27 August 2018 ==
=== Meeting Agenda ===
* [HW] future directions
* [HW] Q & A discussion session

=== Minutes ===
Participants:
Katy Clough, Daniele Vernieri, Helvi Witek

* Reminder: EU Einstein Toolkit workshop and school in Lisbon, 10-13 September 2018
* Meetings:
** move calls to first Monday of month
** next meeting 1st October (Isabel)
* future directions
** decide on more concrete deliverables / projects
** code review for modified gravity thorns; needs reviewers

* Q&A: simulations in (asymptotically) de Sitter spacetimes
** e.g. inflation:
*** bottleneck can be blow-up of scale factor/conformal factor
*** still allows to evolve for 40 e-folds
*** possible way out: rescaling of conformal factor
** e.g. BH binaries:
*** bottleneck: in McVittie coordinates, the cosmological horizon shrinks (pure coordinate effect), i.e., requires large number of additional refinement levels and increasing resolution
*** maybe use different conformal decomposition

== 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 [http://benasque.org/2018relativity/ ``Numerical relativity beyond GR''] in Benasque/ Spain, 3-9 June 2018

WGCPcall20180209

2018-08-27T15:27:36Z

HelviWitek:

=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 [http://benasque.org/2018relativity/ ``Numerical relativity beyond GR''] in Benasque/ Spain, 3-9 June 2018