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Topic Name: Scientists solve cosmological puzzle using supercomputer simulations

Category: STAR (Space, Telecommunications & Radioscience)

Research persons: Sergey Mashchenko

Location: McMaster University, Canada

Details

Hamilton, ON. November 29, 2007 – Researchers using supercomputer simulations have exposed a very violent and critical relationship between interstellar gas and dark matter when galaxies are born – one that has been largely ignored by the current model of how the universe evolved.

The findings, published today in Science, solve a longstanding problem of the widely accepted model – Cold Dark Matter cosmology – which suggests there is much more dark matter in the central regions of galaxies than actual scientific observations suggest.

“This standard model has been hugely successful on the largest of scales—those above a few million light-years—but suffers from several persistent difficulties in predicting the internal properties of galaxies,” says Sergey Mashchenko, research associate in the Department of Physics & Astronomy at McMaster University. “One of the most troublesome issues concerns the mysterious dark matter that dominates the mass of most galaxies.”

Supercomputer cosmological simulations prove that indeed, this problem can be resolved. Researchers modeled the formation of a dwarf galaxy to illustrate the very violent processes galaxies suffer at their births, a process in which dense gas clouds in the galaxy form massive stars, which, at the ends of their lives, blow up as supernovae.

“These huge explosions push the interstellar gas clouds back and forth in the centre of the galaxy,” says Mashchenko, the lead author of the study. “Our high-resolution model did extremely accurate simulations, showing that this ‘sloshing’ effect – similar to water in a bathtub— kicks most of the dark matter out of the centre of the galaxy.”

Cosmologists have largely discounted the role interstellar gas has played in the formation of galaxies and this new research, says Mashchenko, will force scientists to think in new terms and could lead to a better understanding of dark matter.

The simulations reported in the research paper were carried out on the Shared Hierarchical Academic Research Computing Network (SHARCNET).

In figure, It is a picture of a dwarf galaxy forming one billion years after the Big Bang. The background image shows the large-scale cosmic context (the panel is approximately 100,000 light years across); the inset shows the central 2,000 light years of the dwarf galaxy where powerful feedback from newly born star clusters drives bulk motions in the gas. Stars are shown in yellow; colours from violet to blue to green to white correspond to gas of increasing density.

Note for Dark matter

In astrophysics and cosmology, dark matter is matter of unknown composition that does not emit or reflect enough electromagnetic radiation to be observed directly, but whose presence can be inferred from gravitational effects on visible matter. According to present observations of structures larger than galaxies, as well as Big Bang cosmology, dark matter accounts for the vast majority of mass in the observable universe. The observed phenomena consistent with dark matter observations include the rotational speeds of galaxies, orbital velocities of galaxies in clusters, gravitational lensing of background objects by galaxy clusters such as the Bullet cluster, and the temperature distribution of hot gas in galaxies and clusters of galaxies. Dark matter also plays a central role in structure formation and galaxy evolution, and has measurable effects on the anisotropy of the cosmic microwave background. All these lines of evidence suggest that galaxies, clusters of galaxies, and the universe as a whole contain far more matter than that which interacts with electromagnetic radiation: the remainder is called the "dark matter component".

Note for Cold dark matter

Cold dark matter (or CDM) is a refinement of the big bang theory that contains the additional assumption that most of the matter in the Universe consists of material that cannot be observed by its electromagnetic radiation and hence is dark while at the same time the particles making up this matter are slowly moving and hence are cold. As of 2006, most cosmologists favor the cold dark matter theory as a description of how the universe went from a smooth initial state at early times (as shown by the cosmic microwave background radiation), to the lumpy distribution of galaxies and their clusters we see today — the large-scale structure of the universe.

In the cold dark matter theory, structure grows hierarchically, with small objects collapsing first and merging in a continuous hierarchy to form more and more massive objects. In the hot dark matter paradigm, popular in the early eighties, structure does not form hierarchically (bottom-up), but rather forms by fragmentation (top-down), with the largest superclusters forming first in flat pancake-like sheets and subsequently fragmenting into smaller pieces like our galaxy the Milky Way. The predictions of hot dark matter strongly disagree with observations of large-scale structure, whereas the cold dark matter paradigm is in general agreement with the observations.

Note for Interstellar medium

In astronomy, the interstellar medium (or ISM) is the gas and dust that pervade interstellar space. The interstellar medium is the matter that exists between the stars within a galaxy. The energy, in the form of electromagnetic radiation, that occupies the same volume is the interstellar radiation field.

The interstellar medium consists of an extremely dilute (by terrestrial standards) mixture of ions, atoms, molecules, larger dust grains, cosmic rays, and (galactic) magnetic fields. The matter consists of about 99% gas and 1% dust by mass. It fills interstellar space, and blends smoothly into the surrounding intergalactic medium. The ISM is usually extremely tenuous, with densities ranging from a few thousand to a few hundred million particles per cubic meter, and an average value in the Milky Way Galaxy of a million particles per cubic meter. As a result of primordial nucleosynthesis, the gas is roughly 90% hydrogen and 10% helium by number of nuclei, with additional heavier elements ("metals" in astronomical parlance) present in trace amounts.

About Researcher

Sergey Mashchenko
Department of Physics and Astronomy 
ABB-241, McMaster University 
1280 Main Street West 
Hamilton, ON L8S 4M1 
Canada 
Phone: +1-905-5259140, ext. 27663 
E-mail: syam@physics.mcmaster.ca

Research Interests:
Computational/theoretical cosmology, 
structure formation in the universe, 
interstellar medium physics, 
physics of star formation, 
numerical methods.

Education:
1996 Ph.D. in Astrophysics 
Main Astronomical Observatory of the Ukrainian Academy of Sciences, Kiev, Ukraine 
Dissertation: 
Three-dimensional models of galactic supershells 
Advisor: Prof. S. A. Silich 
1992 M.S. in Astrophysics 
Department of Physics, Odessa State University, Odessa, Ukraine 
1990 B.S. in Physics (summa cum laude) 
Department of Physics, Odessa State University, Odessa, Ukraine