Nick Gnedin FCPA Retreat Computational Astrophysics and Cosmology.

Nick GnedinFCPA Retreat

Computational Astrophysics and Computational Astrophysics and CosmologyCosmology

ContentsContents

• Dreams and reality• Science• Methods and Tools• Computational Cosmology in the World…• … and @ FNAL

Ultimate Dream ofUltimate Dream ofEvery CosmologistEvery Cosmologist

Noah’s Ark of the Universe:a quantitative model of theuniverse in a large enough volume to represent all typesof galaxies in sufficient detail.

“Every cosmologist carries a digital universe in his pack.” Napoleon Bonaparte

… … and Realityand Reality

If we cannothave this…

Large-scale simulations Small-scale simulations

Science:Science:Computational Cosmology for Computational Cosmology for

Fundamental PhysicsFundamental Physics

Weak lensing:Directly probes the matter distribution.Unfortunately, we exist.Baryonic effects can be calibrated by simulations.The required computational effort is massive.

(Rudd et al 2007)



Baryon Acoustic Oscillations:Most clean test for darkenergy (potentially).Galaxies are biased.Baryonic effects can be calibrated by simulations.We are still very far frompredicting galaxy colorsfrom simulations reliably.

(Tegmark et al 2006)



Galaxy Clusters:Sensitive probes to DE(live on exponential tail).Complex physical systems.Baryonic effects can be calibrated by simulations.It is not clear how much physicswe need to include to modelclusters with high precision(even ignoring central 20%).

(Nagai et al 2007)


Fundamental AstrophysicsFundamental Astrophysics

Galaxy Formation:Direct comparison withmost observations.Important physics (such asstar formation) will not be understood any time soon.There exist simple scaling laws in the ISM.Models based on those lawsmake wrong galaxies.



Reionization & 1st stars:Physics is reasonably wellunderstood.Radiative Transfer is a hardcomputational problem.Substantial progress hasbeen achieved, though.A very large dynamic rangeis required (1kpc – 100Mpc).



Supermassive Black Holes:Probably the most important unsolved astrophysical problem now.Very large dynamic rangeand complex physics.AMR can reach it now.We may not have eventhe most rudimentary,conceptual understandingof the physics of AGN.

Methods and ToolsMethods and Tools

• Dark Matter:similar to plasma simulationsparticle methods (N-body)

• Gas Dynamics:mostly Lagrangian methodslittle use of engineering expertise

• Radiative Transfer:6D problemuseful approximations existfull computational solution is still lacking

Cosmological N-bodyCosmological N-body

The problem of cosmological N-body simulations isessentially solved

• PM = particle-mesh• P3M = particle-particle particle-mesh• Tree• Hybrid codes: TreePM, Adaptive P3M• Adaptive Mesh Refinement (AMR)

10 Billion Particles Simulation10 Billion Particles Simulation

Cosmological Gas DynamicsCosmological Gas Dynamics

Little use of engineering expertise:• very high resolution required• complex physics• gas is gravitating• no solid boundaries

2020thth Century Century(1(1stst Generation Codes) Generation Codes)

• Eulerian schemes: R ~ 1000, N~109, const

• Simple Path to Hell (SPH): R ~ 30,000, N~108, const

• Arbitrary Lagrangian-Eulerian (ALE): R ~ 10,000, N~107, const

2121stst Century Century(2(2ndnd Generation Codes) Generation Codes)

• Adaptive Mesh Refinement (AMR): R > 100,000, N ~ 109, variable

can follow fragmentation non-uniform initial conditions spatially variable resolution

Our ToolsOur Tools• We (FNAL+KICP) have several cosmological codes• The main one is the Adaptive Refinement Tree (ART) code

• Implementation of Adaptive Mesh Refinement method• Refines on a cell-by-cell basis• Full 4D adaptive• Includes - dark matter dynamics - gas dynamics and chemistry - radiative transfer - star formation, etc

Computational Cosmology in Computational Cosmology in the Worldthe World

Virgo Consortium [>30 people](Germany + UK)

• Europe: Research mostly done by large groups:

Project Horizon [>20 people](France)

• USA/Canada: Large number of small groups (10-12 people) Chicago (FNAL+KICP), UWashington, Harvard, CITA, SLAC, LANL, Princeton, UCLA, UIUC, Berkeley, UMass…

Computing NeedsComputing Needs• Modern state-of-the art cosmological simulations require in excess of 100,000 CPU-hours (130 CPU-months). Biggest ones use > 1,000,000 CPU-hours.

• A single simulation produces a Terabyte-scale data set.

• Even rudimentary analysis requires intermediate-level computing capability.

The time of “this is a workstation to analyze your simulation” is over.

Solution (a-la LQCD)Solution (a-la LQCD)

• National consortium/collaboration (the level of integration may vary)

• Powerful local, intermediate-level resources (10,000 CPU, can be spread over a few separate places: FNAL, SLAC, ANL, LANL)

• United approach to funding agencies and national supercomputer centers.

Our PlanOur Plan• build-up local resources on par with other US groups• create a Chicago-wide collaboration (FNAL, KICP, ANL)• charm Mont, PAC, and Director• overtake other US groups in local computing resources (reach 1,000 – 2,000 cores; there are good reasons why FNAL is the best place for that)• move towards a national consortium/collaboration:

join with DOE labs (LANL, LBL, SLAC) invite universities to join

• get lots of money along the way… (people, software support)

Nick Gnedin FCPA Retreat Computational Astrophysics and Cosmology.

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