page_title: Bolshoi Cosmological Simulation
section_title: Methods
hierarchical_section_title: Bolshoi Cosmological Simulation / Methods
caption_attribution_description: English: Key Cosmological Parameters σ8 and ΩM from WMAP and Planck Observations Compared with Millennium, Bolshoi, and Bolshoi-Planck Simulations
context_page_description: The Bolshoi simulation, a computer model of the universe run in 2010 on the Pleiades supercomputer at the NASA Ames Research Center, was the most accurate cosmological simulation to that date of the evolution of the large-scale structure of the universe.
  The Bolshoi simulation used the now-standard ΛCDM model of the universe and the WMAP five-year and seven-year cosmological parameters from NASA's Wilkinson Microwave Anisotropy Probe team. "The principal purpose of the Bolshoi simulation is to compute and model the evolution of dark matter halos, thereby rendering the invisible visible for astronomers to study, and to predict visible structure that astronomers can seek to observe." “Bolshoi” is a Russian word meaning “big.”
  The first two of a series of research papers describing Bolshoi and its implications were published in 2011 in the Astrophysical Journal.
  The first data release of Bolshoi outputs has been made publicly available to the world's astronomers and astrophysicists.
  The data include output from the Bolshoi simulation and from the BigBolshoi, or MultiDark, simulation of a volume 64 times that of Bolshoi.
context_section_description: The Bolshoi simulation follows the evolving distribution of a statistical ensemble of 8.6 billion particles of dark matter, each of which represents about 200 million solar masses, in a cube of 3-dimensional space about 1 billion light years on edge. Dark matter and dark energy dominate the evolution of the cosmos in this model. The dynamics are modeled with the ΛCDM theory and Albert Einstein's general theory of relativity, with the model including cold dark matter (CDM) and the Λ cosmological constant term simulating the cosmic acceleration referred to as dark energy.
  The first 100 million years (Myr) or so of the evolution of the universe after the Big Bang can be derived analytically. The Bolshoi simulation was started at redshift z=80, corresponding to about 20 Myr after the Big Bang.  Initial parameters were calculated with linear theory as implemented by the CAMB  tools, part of the WMAP website.  The tools provide the initial conditions, including a statistical distribution of positions and velocities of the particles in the ensemble, for the much more demanding Bolshoi simulation of the next approximately 13.8 billion years. The experimental volume thus represents a random region of the universe, so comparisons with observations must be statistical.
  The Bolshoi simulation employs a version of an adaptive mesh refinement (AMR) algorithm called an adaptive refinement tree (ART), in which a cube in space with more than a predefined density of matter is recursively divided into a mesh of smaller cubes. The subdivision continues to a limiting level, chosen to avoid using too much supercomputer time.  Neighboring cubes are not permitted to vary by too many levels, in the case of Bolshoi by more than one level of subdivision, to avoid large discontinuities. The AMR/ART method is well suited to model the increasingly inhomogeneous distribution of matter that evolves as the simulation proceeds. “Once constructed, the mesh, rather than being destroyed at each time step, is promptly adjusted to the evolving particle distribution.”
  As the Bolshoi simulation ran, the position and velocity of each of the 8.6 billion particles representing dark matter was recorded in 180 snapshots roughly evenly spaced over the simulated 13.8-billion-year run on the Pleiades supercomputer. Each snapshot was then analyzed to find all the dark matter halos and the properties of each (particle membership, location, density distribution, rotation, shape, etc.).  All this data was then used to determine the entire growth and merging history of every halo.  These results are used in turn to predict where galaxies will form and how they will evolve.  How well these predictions correspond to observations provides a measure of the success of the simulation. Other checks were also made.
caption: Two key cosmological parameters, σ8 and ΩM, with values and 1-σ uncertainties from observations and values used in three cosmological simulations. The parameter σ8 represents the amplitude of the fluctuation spectrum on the scale of clusters of galaxies, and the parameter ΩM is the dark + ordinary matter fraction of the cosmic density. The observations represented by the shapes on the figure are from X-ray and gravitational lensing studies of clusters of galaxies. The observations with error bars are from cosmic microwave background data combined with other data from the Wilkinson Microwave Anisotropy Probe (WMAP) five-year (2009), seven-year (2011), and nine-year (2013) publications and the Planck (2013) data release. The simulations are the Millennium I, II, and XXL simulations (which all used the same cosmological parameters consistent with the WMAP first-year data release 2003), and the Bolshoi (2011) and Bolshoi-Planck (2014) simulations.
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