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Scientists see “Cosmic Dawn”

BY DR EMILY BALDWIN

ASTRONOMY NOW

Posted: 12 February, 2009

Using computer simulations, scientists have predicted what the Universe would have looked like 500 million years after the big bang.

The simulations, carried out by scientists at Durham University, yield clues as to how the Universe’s first big galaxies formed. This so-called cosmic dawn marks the era when galaxies began to coalesce out of the debris of massive stars which had died explosively shortly after the big bang. The simulations track the evolution of these galaxies from their birth through to the present day some 13 billion years later.

"Our research predicts which galaxies are growing through the formation of stars at different times in the history of the Universe and how these relate to dark matter," says researcher Dr Carlton Baugh at Durham University. "We give the computer what we think is the recipe for galaxy formation and we see what is produced which is then tested against observations of real galaxies."

The development of the first big galaxies in the Universe. The green swirls represent dark matter and the circles show the star formation rate in galaxies. The different colour circles represent the varying luminosity of star formation with yellow being brightest. Clockwise, Z=8.5 (590 million years after the big bang); Z=5.7 (1 billion years); Z=3.3 (1.9 billion years); Z=0 (13.6 billion years, i.e. today). Images: Alvaro Orsi, Institute for Computational Cosmology, Durham University.

Dark matter is a mysterious substance thought to make up around 23 percent of the Universe, while 73 percent is dark energy, and just four percent ‘normal’ matter. Although it is invisible, dark matter is known to play a central role in the formation of galaxies, and its presence can be inferred by studying its gravitational effects on visible matter.

"We are effectively looking back in time and by doing so we hope to learn how galaxies like our own were made and to understand more about dark matter," says Alvaro Orsi, a research postgraduate in Durham University's Institute for Computational Cosmology (ICC), and lead author of the paper describing the work. "The presence of dark matter is the key to building galaxies – without dark matter we wouldn't be here today."

The research combined a simulation showing how structures grow in dark matter with a model showing how normal matter, such as gas, behaves. The results showed that gas feels the pull of gravity from dark matter and is heated up before cooling by releasing radiation and turning into stars. The simulations also show which galaxies are forming stars most vigorously at a given time. Although the galaxies are biggest at the present day, the rate at which they are making new stars has dropped greatly compared with the rate in the early Universe.

Furthermore, the calculations can be tested against new observations reaching back to early stages in the history of the Universe almost one billion years after the Big Bang. "Computational cosmology plays an important part in our understanding of the Universe," adds Professor Keith Mason, Chief Executive of the Science and Technology Facilities Council. "Not only do these simulations allow us to look back in time to the early Universe but they complement the work and observations of our astronomers."

The paper appears in the Monthly Notices of the Royal
Astronomical Society
and the research was funded by the Science and Technology Facilities Council (STFC) and the European Commission.

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