The Millennium Run, or Millennium Simulation (referring to its size ) is a computer

N-body simulation In physics and astronomy, an ''N''-body simulation is a simulation of a dynamical system of particles, usually under the influence of physical forces, such as gravity (see n-body problem, ''n''-body problem for other applications). ''N''-body ...

used to investigate how the distribution of matter in the

Universe The universe is all of space and time and their contents, including planets, stars, galaxies, and all other forms of matter and energy. The Big Bang theory is the prevailing cosmological description of the development of the universe. ...

has evolved over time, in particular, how the observed population of galaxies was formed. It is used by

scientists A scientist is a person who conducts scientific research to advance knowledge in an area of the natural sciences. In classical antiquity, there was no real ancient analog of a modern scientist. Instead, philosophers engaged in the philosophica ...

working in

physical cosmology Physical cosmology is a branch of cosmology concerned with the study of cosmological models. A cosmological model, or simply cosmology, provides a description of the largest-scale structures and dynamics of the universe and allows study of f ...

to compare observations with theoretical predictions.

Overview

A basic scientific method for testing

theories A theory is a rational type of abstract thinking about a phenomenon, or the results of such thinking. The process of contemplative and rational thinking is often associated with such processes as observational study or research. Theories may be ...

in cosmology is to evaluate their consequences for the

observable In physics, an observable is a physical quantity that can be measured. Examples include position and momentum. In systems governed by classical mechanics, it is a real-valued "function" on the set of all possible system states. In quantum phy ...

parts of the universe. One piece of observational evidence is the distribution of matter, including galaxies and intergalactic gas, which are observed today.

Light Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 te ...

emitted from more distant matter must travel longer in order to reach

Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...

, meaning looking at distant objects is like looking further back in

time Time is the continued sequence of existence and events that occurs in an apparently irreversible succession from the past, through the present, into the future. It is a component quantity of various measurements used to sequence events, ...

. This means the evolution in time of the matter distribution in the universe can also be observed directly. The Millennium Simulation was run in 2005 by the

Virgo Consortium The Virgo Consortium was founded in 1994 for '' Cosmological Supercomputer Simulations'' in response to the UK's High Performance Computing Initiative. Virgo developed rapidly into an international collaboration between a dozen scientists in the U ...

, an international group of

astrophysicists The following is a list of astronomers, astrophysicists and other notable people who have made contributions to the field of astronomy. They may have won major prizes or awards, developed or invented widely used techniques or technologies within a ...

from

Germany Germany,, officially the Federal Republic of Germany, is a country in Central Europe. It is the second most populous country in Europe after Russia, and the most populous member state of the European Union. Germany is situated betwe ...

, the

United Kingdom The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom (UK) or Britain, is a country in Europe, off the north-western coast of the European mainland, continental mainland. It comprises England, Scotlan ...

Canada Canada is a country in North America. Its ten provinces and three territories extend from the Atlantic Ocean to the Pacific Ocean and northward into the Arctic Ocean, covering over , making it the world's second-largest country by tot ...

, Japan and the

United States The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 states, a federal district, five major unincorporated territori ...

. It starts at the epoch when the

cosmic background radiation Cosmic background radiation is electromagnetic radiation from the Big Bang. The origin of this radiation depends on the region of the spectrum that is observed. One component is the cosmic microwave background. This component is redshifted p ...

was emitted, about 379,000 years after the universe began. The cosmic background radiation has been studied by satellite experiments, and the observed inhomogeneities in the cosmic background serve as the starting point for following the evolution of the corresponding matter distribution. Using the physical laws expected to hold in the currently known cosmologies and simplified representations of the astrophysical processes observed to affect real galaxies, the initial distribution of matter is allowed to evolve, and the simulation's predictions for formation of galaxies and black holes are recorded. Since the completion of the Millennium Run simulation in 2005, a series of ever more sophisticated and higher fidelity simulations of the formation of the galaxy population have been built within its stored output and have been made publicly available over the internet. In addition to improving the treatment of the astrophysics of galaxy formation, recent versions have adjusted the parameters of the underlying cosmological model to reflect changing ideas about their precise values. To date (mid-2018) more than 950 published papers have made use of data from the Millennium Run, making it, at least by this measure, the highest impact astrophysical simulation of all time.

Size of the simulation

For the first scientific results, published on June 2, 2005, the Millennium Simulation traced 2160, or just over 10 billion, "particles." These are not particles in the

particle physics Particle physics or high energy physics is the study of fundamental particles and forces that constitute matter and radiation. The fundamental particles in the universe are classified in the Standard Model as fermions (matter particles) an ...

sense – each "particle" represents approximately a billion solar masses of

dark matter Dark matter is a hypothetical form of matter thought to account for approximately 85% of the matter in the universe. Dark matter is called "dark" because it does not appear to interact with the electromagnetic field, which means it does not a ...

. The region of space simulated was a cube with about 2 billion light years as its length. This volume was populated by about 20 million "galaxies". A super computer located in Garching, Germany executed the simulation, which used a version of the GADGET code, for more than a month. The output of the simulation needed about 25

terabytes The byte is a unit of digital information that most commonly consists of eight bits. Historically, the byte was the number of bits used to encode a single character of text in a computer and for this reason it is the smallest addressable uni ...

of storage.

First results

The Sloan Digital Sky Survey had challenged the current understanding of cosmology by finding black hole candidates in very bright quasars at large distances. This meant that they were created much earlier than initially expected. In successfully managing to produce quasars at early times, the Millennium Simulation demonstrated that these objects do not contradict our models of the evolution of the universe.

Millennium II

In 2009, the same group ran the 'Millennium II' simulation (MS-II) on a smaller cube (about 400 million light years on a side), with the same number of particles but with each particle representing 6.9 million solar masses. This is a rather harder numerical task since splitting the computational domain between processors becomes harder when dense clumps of matter are present. MS-II used 1.4 million CPU hours over 2048 cores (i.e. about a month) on the Power-6 computer at Garching; a simulation was also run with the same initial conditions and fewer particles to check that features in the higher-resolution run were also seen at lower resolution.

Millennium XXL

In 2010, the 'Millennium XXL' simulation (MXXL) was performed, this time using a much larger cube (over 13 billion light years on a side), and 6720 particles each representing 7 billion times the mass of the Sun. The MXXL spans a cosmological volume 216 and 27,000 times the size of the Millennium and the MS-II simulation boxes, respectively. The simulation was run on JUROPA, one of the top 15 supercomputers in the world in 2010. It used more than 12,000 cores for an equivalent of 300 years CPU time, 30 terabytes of RAM and generated more than 100 terabytes of data. Cosmologists use the MXXL simulation to study the distribution of galaxies and dark matter halos on very large scales and how the rarest and most massive structures in the universe came about.

Millennium Run Observatory

In 2012, the Millennium Run Observatory (MRObs) project was launched. The MRObs is a theoretical virtual observatory that integrates detailed predictions for the dark matter (from the Millennium simulations) and for the galaxies (from semi-analytical models) with a virtual telescope to synthesize artificial observations. Astrophysicists use these virtual observations to study how the predictions from the Millennium simulations compare to the real universe, to plan future observational surveys, and to calibrate the techniques used by astronomers to analyze real observations. A first set of virtual observations produced by the MRObs have been released to the astronomical community for analysis through the MRObs Web portal. The virtual universe can also be accessed through a new online tool, the MRObs browser, which allows users to interact with the Millennium Run Relational Database where the properties of millions of dark matter halos and their galaxies from the Millennium project are being stored. Upgrades to the MRObs framework, and its extension to other types of simulations, are currently being planned.

References

External links

Millennium Simulation Data PagePress release of the June 2 results (MPG)VIRGO home pageSimulating the joint evolution of quasars, galaxies and their large-scale distributionThe Millennium Run Observatory PageThe Millennium Run Relational Database
Physical cosmology Simulation Cosmological simulation