Large Numerical Simulations for Dark Energy Surveys
@ Marenostrum Supercomputer
Science Case
The universe is observed to expand at an accelerated rate, what is
thought to be caused by the so-called dark-energy, a form of energy
effectively acting as a gas "pressure" that dominates gravity and
determines the fate of the universe. We are carrying out the largest
dark-matter simulations to date to understand the role of dark-energy
in the evolution of the universe. In particular, we propose to measure
the effect on the spatial and time distribution of matter, galaxies
and clusters of galaxies, and how this information will be used by
upcoming galaxy surveys, such as the
Dark-Energy Survey (DES) and the
Wide-Field Multi-Object Spectrograph (WFMOS), to measure dark-energy properties with unprecedented precision.
These simulations are intended to provide input into the design and the interpretation
of planned galaxy surveys; They are one of the key ingredients in the series of data challenges
of increasing complexity and data volume that are required to build the data reduction and analysis
pipelines for these upcoming surveys (see below schematic view).
The DES is a 4 band digital optical survey with unprecedented depth (up to z~1.5) for its
wide coverage (~5000 sq. deg) that will collect more than 300 million galaxies and about
2000 Type Ia SNe from observations in the southern galactic sky between 2009 and 2014 using
a new CCD camera, DECam, on top of the 4-meter Blanco telescope at the CTIO in Chile.
The DES main goal is to measure the dark-energy equation of state with 5% accuracy using
four different techniques: angular galaxy clustering, galaxy clusters, weak lensing and Type Ia
supernovae (see astro-ph/0510195 and
astro-ph/0510346 ).
The WFMOS has been proposed for the Subaru 8m class telescope to carry out two surveys to measure
acoustic oscillations in the galaxy power spectrum and constrain dark energy. The first survey
covers 0.5< z < 1.3 including 2 million galaxies up to R=22.7 over 2000 sq.deg., and the
second survey includes 2.5 < z < 3.5 using 0.6 million galaxies up to R=24.5 over 300 sq.deg.;
The volume covered by WFMOS is about twice the volume of the DES.
These simulations will make unique contributions to a whole range of critical science questions,
in addition to providing the most precise predictions for the visibility of acoustic oscillations.
The other scientific applications of the simulation include:
- probe the high mass end of the mass function and constrain the presence of rare events such as
superclusters in local surveys
- huge sample of high resolution clusters to study their structure and make predictions for
forthcoming Sunyaev-Zeldovich surveys (SPT, SZA, ACT, SuZIE, etc.)
- in combination with galaxy formation models, predictions for galaxy clustering at different
epochs to constrain the physical models
- produce detailed synthetic galaxy catalogs which are essential for the full interpretation
of future surveys
- probing the time evolution of the gravitational potential on large scales what is a further
constrain on dark-energy
Supercomputing Facilities
Given the large amount of CPU memory and storage (of order ~few TB)
required to simulate the gravitational interaction between billions of
dark-matter particles that fill our expanding universe, it is critical
to have access to forefront computing facilities.
We have been awarded computing time to develop these challenging
simulations using the Marenostrum computer cluster at the Barcelona
Supercomputing Center. Marensotrum. Marenostrum is the most powerful supercomputer in Europe and
the fifth in the world, according to the last
Top500 list.
As of November 2006, it has increased the calculation capacity
reaching 94.21 Teraflops and 10.240 processors.
Software
We use a freely available C code for cosmological N-body/SPH simulations, GADGET,
developed by V.Springel @ MPA.
It performs well on massively parallel computers with distributed memory.
GADGET uses an explicit communication model that is implemented with the standardized MPI communication
interface. The code can be used for studies of isolated systems, or for simulations that include
the cosmological expansion of space, both with or without periodic boundary conditions.
In all these types of simulations, GADGET follows the evolution of a self-gravitating
collisionless N-body system, and, if required, allows gas dynamics to be optionally included.
Both the force computation and the time stepping of GADGET are fully adaptive, with a dynamic range
which is, in principle, unlimited. GADGET computes gravitational forces with a
hierarchical tree algorithm (optionally in combination with a particle-mesh scheme for
long-range gravitational forces) and represents fluids by means of smoothed particle hydrodynamics (SPH).
Funding Resources
The activity proposed in this proposal is an essential part of our
research project related to the DES, which is funded by the following National Grants:
- Title: Large-Scale Structure and Cosmology: The Dark-Energy Survey.
P.I: E.Gaztanga (IEEC/CSIC). Funding Institution: Spanish Ministry of Science (MEC).
Code:AYA2005-09414-C02-01. Period: 2006
- Title: Observational Cosmology. P.I: F. Castander (IEEC/CSIC).
Funding Institution: Generalitat de Catalunya. Code:2005SGR-00728. Period: 2005-2008
- The Dark-Energy Survey. P.I: E.Gaztanaga (IEEC/CSIC).
Funding Institution: MEC, Spanish government. Code:AYA2006-06341. Period: 2006-2008
LCDM Simulations
Production phase: "Intermediate Simulation" (N=1024, L=1536)
- Full Parameter List: L_Box=1536 Mpc/h, Npart=1024^3,
PMGrid=2048^3, DM density = 0.25, DE density = 0.75, baryon density =
0.044, h = 0.7, sigma8 = 0.8, L_soft = 50 kpc/h
This simulation is at the top of the international "Simulation Ranking"
in terms of number of particles used. We have submitted a proposal to
develop a simulation that is one order of magnitude larger than
any cosmological simulation produced to date: this simulation will comprise
100000 million DM particles what will allow us to study the formation and evolution
of large-scale structures in the universe with an unparalleled level of detail.
Lightcone simulation
The lightcone simulation produces a realization of how the observer
would see the universe. Here we present a figure of the closest volume
(64 Mp/h size) to the observer who is in the right bottom corner.
Snapshots in cosmological time
The universe seen at different times through its evolution according to our simulation.
Images display 25% of the celestial sphere.
Time is labelled by the corresponding cosmological
redshift, z. For reference, light emitted at z=1 reach
us the observers about 7000 million years later and thus it provides us with a picture of the universe at that time.
Zooming into these different snapshots one can see how structures develop in the unverse.
In particular, filamentary structures that connect
clusters of galaxies (red points) in the real universe. These phenomena can be explained by the
gravitational instability picture that describes the way large-scale structures develop
under gravity in an expanding universe.
Images produced with CMBView, developed by J.Portsmouth.
Animations
Link to the animations page
Simulations Download
Link to the simulations download page (restricted access)
Last modified: 8 Dec 2006 by Pablo Fosalba