* Astronomy

One of the biggest computer simulations ever run is illuminating the deepest mysteries of the universe

The simulation by an international team is the biggest ever attempted and shows how structures in the Universe changed and grew over billions of years.
The Millennium Run, as it is dubbed, could help explain observations made by astronomers and shed more light on the Universe's elusive dark energy field.
"What's unique about the simulation is its scope and the level of detail"
Prof Carlos Frenk, University of Durham

The group, dubbed the Virgo Consortium—a name borrowed from the galaxy cluster closest to our own—is creating the largest and most detailed computer model of the universe ever made. While other groups have simulated chunks of the cosmos, the Virgo simulation is going for the whole thing. The cosmologists' best theories about the universe's matter distribution and galaxy formation will become equations, numbers, variables, and other parameters in simulations running on one of Germany's most powerful supercomputers, an IBM Unix cluster at the Max Planck Society's Computing Centre in Garching, near Munich.
The machine, a cluster of powerful IBM Unix computers, has a total of 812 processors and 2 terabytes of memory, for a peak performance of 4.2 teraflops, or trillions of calculations per second.

The fundamental challenge for the Virgo team is to approximate that reality in a way that is both feasible to compute and fine-grained enough to yield useful insights. The Virgo astrophysicists have tackled it by coming up with a representation of that epoch's distribution of matter using 10 billion mass points, many more than any other simulation has ever attempted to use.
THE DIMENSIONLESS POINTS have no real physical meaning; they are just simulation elements, a way of modelling the universe's matter content. Each point is made up of normal and dark matter in proportion to the best current estimates, having a mass a billion times that of our sun, or 2000 trillion trillion trillion (239) kilograms. (The 10 billion particles together account for only 0.003 percent of the observable universe's total mass, but since the universe is homogeneous on the largest scales, the model is more than enough to be representative of the full extent of the cosmos.)



"We have learned more about the Universe in the last 10 or 20 years than in the whole of human civilisation," - Professor Carlos Frenk, Ogden professor of fundamental physics at the University of Durham and co-author on the Nature report.
"We are now able, using the biggest, fastest supercomputers in the world, to recreate the whole of cosmic history," - Professor Carlos Frenk.
The researchers looked at how the Universe evolved under the influence of the mysterious material called dark matter.

TO BEGIN CREATING THEIR MODEL of the universe, the researchers faced two basic questions: at what moment, precisely, should they start the simulation? And what are the universe's conditions at that very moment? Fortunately, cosmologists believe they have these answers.
According to the inflationary universe theory put forward in the 1980s by Alan Guth, of the Massachusetts Institute of Technology, and Andrei Linde, of Stanford University, the universe swelled at an extraordinarily rapid rate during a tiny fraction of a second immediately after the Big Bang. This exponentially fast expansion amplified minute, quantum-scale fluctuations that existed in the primordial energy field that filled the very early universe. These fluctuations caused matter to clump, and later, gravity created denser and denser aggregates.

The result is that these aggregates, which began as unimaginably small energy fluctuations in that primeval universe much smaller than a proton, ultimately evolved into the giant structures that compose the universe's sponge like web of matter. Even more surprising, most of the mass in this web of matter is not the ordinary stuff we know that makes up galaxies, stars, planets, and people.
After many experiments and calculations throughout the past decade, most cosmologists now agree on the astounding fact that some 85 percent of the matter in the universe consists of a mysterious substance known as dark matter that cannot be seen directly. They infer its presence by tracking the motions of stars and galaxies: stars are attracted to the centres of galaxies, and galaxies to the centres of galaxy clusters, by gravitational forces that are far greater than visible matter alone can possibly account for. Something else must be out there.

This shadowy substance is made up not of the familiar quarks, electrons, and their derivatives—atoms and molecules—but of some particle that has so far eluded experimenters. Candidates include axions, photinos, neutralinos—all yet to be discovered—among other particles predicted by theorists. The upshot is that, because dark matter is not visible, what astronomers have observed are the contours of the universe's great web, revealed by the light of the stars and galaxy clusters that formed onto the web's nodes, the junctions at which large amounts of matter accumulate.
It's kind of like inferring the shape of a Christmas tree in a pitch-dark room from the positions of the lights strung on it. These stars, galaxies, and other objects that we can see were born from dense aggregates of normal matter embedded in the dark matter of the web.

Dark matter model
According to cosmological theory, soon after the Big Bang, cold dark matter formed the first large structures in the Universe, which then collapsed under their own weight to form vast halos.
The gravitational pull of these halos sucked in normal matter, providing a focus for the formation of galaxies.
The simulation tracked some 10 billion dark matter particles over roughly 13 billion years of cosmic evolution. It incorporated data from satellite observations of the heat left over from the Big Bang, information on the make-up of the Universe and current understanding of the laws of physics on Earth.
"What's unique about the simulation is its scope and the level of detail with which we can re-create the cosmic structures we see around us," - Professor Carlos Frenk.
"Now we have the Millennium Run simulations, we have the predictions of the theory in enough detail that we can see if there is a meshing together of how the world looks on the larger scale and the way we expect it should look according to our theories. It's a way to check our theories." - Sir Martin Rees, Astronomer Royal.

Energy problem
Comparisons between the results of the simulation and astronomical observations are already helping shed light on some unsolved cosmic mysteries.
Some astronomers have previously questioned how radio-sources in distant galaxies called quasars could have formed so quickly after the Big Bang under the cold dark matter model.

The Millennium Run simulation demonstrates that such structures form naturally under the model in numbers consistent with data from the Sloan Digital Sky Survey.
The virtual universe may also shed light on the nature of dark energy, which makes up about 73% of the known Universe, and which, Frenk says, is the "number one unsolved problem in physics today - if not science itself".

"Our simulations tell us where to go looking for clues to learn about dark energy. If we want to learn about this we need to look at galaxy clusters, which encode information about the identity of dark energy," - Professor Carlos Frenk.
One of the major cosmological features that the Virgo team relies on to formulate its simulations is something called the cosmic microwave background, a feeble radiation remnant of the Big Bang that astronomers have now studied in great detail. This radiation, a key piece of evidence supporting the inflation theory, was emitted 380 000 years after the Big Bang when protons combined with free electrons to form neutral hydrogen atoms.

If the cosmic particle soup were absolutely smooth, with evenly distributed hydrogen atoms, this radiation would also be smooth all over the place—always the same wherever you look. But as cosmologists pointed their detectors to different parts of the sky, they found small variations in the cosmic microwave background.
These variations were recently minutely detailed by NASA's Wilkinson Microwave Anisotropy Probe, whose first scientific results were made public early last year. In a triumph of modern cosmology, the measured variations correspond precisely with the predictions of inflation theory.

The cosmic microwave background, therefore, gives cosmologists a fairly good picture of the distribution of matter when the universe, with an estimated current age of 13.7 billion years, was still in its infancy, only 380 000 years old. That's the starting point the Virgo group has chosen for its simulations. The main one, the first of a series, dubbed the Millennium Run, was completed this past June.
When data is fully processed within the next few months, the Millennium Run will reveal with unprecedented detail how the cosmos's broad distribution of matter came to be.




http://www.virgo.dur.ac.uk/