# Presentation: 519475

## Nick Gnedin
FCPA Retreat

- FCPA Retreat

- Computational Astrophysics and Cosmology

## Contents

**Dreams and reality**
**Science**
**Methods and Tools**
**Computational Cosmology in the World...**
**... ****and @ FNAL**

## Ultimate Dream ofEvery Cosmologist

- Noah’s Ark of the Universe:
*a quantitative model of the*
*universe in a large enough *
*volume to represent all types*
*of galaxies in sufficient detail.*

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

## ... and Reality

- If we cannot
- have this...

- Large-scale simulations

- Small-scale simulations

## Science:Computational Cosmology for Fundamental Physics

- Science:Computational Cosmology for Fundamental 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)

## Science:Computational Cosmology for Fundamental Physics

- Science:Computational Cosmology for Fundamental Physics

**Baryon Acoustic Oscillations**:
- Most clean test for dark
- energy (potentially).
- Galaxies are biased.
*Baryonic effects can be *
*calibrated by simulations*.
- We are still very far from
- predicting galaxy colors
- from simulations reliably.

- (Tegmark et al 2006)

## Science:Computational Cosmology for Fundamental Physics

**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 physics
- we need to include to model
- clusters with high precision
- (even ignoring central 20%).

- (Nagai et al 2007)

## Science:Computational Cosmology for Fundamental Astrophysics

**Galaxy Formation**:
- Direct comparison with
- most observations.
- Important physics (such as
- star formation) will not be
- understood any time soon.
- There exist simple scaling
- laws in the ISM.
- Models based on those laws
- make wrong galaxies.

## Science:Computational Cosmology for Fundamental Astrophysics

**Reionization & 1****st**** stars**:
- Physics is reasonably well
- understood.
- Radiative Transfer is a hard
- computational problem.
- Substantial progress has
- been achieved, though.
- A very large dynamic range
- is required (1kpc – 100Mpc).

## Science:Computational Cosmology for Fundamental Astrophysics

**Supermassive Black Holes**:
- Probably the most important unsolved astrophysical
- problem now.
- Very large dynamic range
- and complex physics.
- AMR can reach it now.
- We may not have even
- the most rudimentary,
- conceptual understanding
- of the physics of AGN.

## Methods and Tools

-  Dark Matter:
- 	similar to plasma simulations
- 	particle methods (N-body)
-  Gas Dynamics:
- 	mostly Lagrangian methods
- 	little use of engineering expertise
-  Radiative Transfer:
- 	6D problem
- 	useful approximations exist
- 	full computational solution is still lacking

## Cosmological N-body

***The problem of cosmological N-body simulations is***
***essentially solved***

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

## 10 Billion Particles Simulation

## Cosmological Gas Dynamics

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

## 20th Century(1st 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

## 21st Century(2nd Generation Codes)

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

-  *can follow fragmentation*
* **non-uniform initial conditions*
* **spatially variable resolution*

## Our Tools

-  We (FNAL+KICP) have several cosmological codes
-  The main one is the _**A**_**daptive **_**R**_**efinement **_**T**_**ree (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 the 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 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)

-  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 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)