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 (Lambda-CDM) 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. The Bolshoi-Planck simulation, with the same resolution as Bolshoi, was run in 2013 on the Pleiades supercomputer using the Planck satellite team's cosmological parameters released in March 2013. The Bolshoi-Planck simulation is currently being analyzed in preparation for publication and distribution of its results in 2014.
Bolshoi simulations continue to be developed as of 2018.
Contributors
Joel R. Primack's team at the University of California, Santa Cruz, partnered with Anatoly Klypin's group at New Mexico State University, in Las Cruces to run and analyze the Bolshoi simulations. Further analysis and comparison with observations by Risa Wechsler's group at Stanford and others are reflected in the papers based on the Bolshoi simulations.
Rationale
A successful large-scale simulation of the evolution of galaxies, with results consistent with what is actually seen by astronomers in the night sky, provides evidence that the theoretical underpinnings of the models employed, i.e., the supercomputer implementations ΛCDM, are sound bases for understanding galactic dynamics and the history of the universe, and opens avenues to further research. The Bolshoi Simulation isn't the first large-scale simulation of the universe, but it is the first to rival the extraordinary precision of modern astrophysical observations.
The previous largest and most successful simulation of galactic evolution was the Millennium Simulation Project, led by Volker Springel. Although the success of that project stimulated more than 400 research papers, the Millennium simulations used early WMAP cosmological parameters that have since become obsolete. As a result, they led to some predictions, for example about the distribution of galaxies, that do not match very well with observations. The Bolshoi simulations use the latest cosmological parameters, are higher in resolution, and have been analyzed in greater detail.
Methods
The Bolshoi simulation follows the evolving distribution of a statistical ensemble of 8.6 billion particles of dark matter, each of which represents about 100 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.