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Home > iSGTW 02 April 2008 > iSGTW Image of the week - LICORICE, photons and the end of the Dark Ages


Image of the week - LICORICE, photons and the end of the Dark Ages


This grid-powered simulation shows radiative transfer of Lyman-alpha photons during the Epoch of Reionization: results that are necessary to compute the hydrogen 21-cm line emissions that will be observed by the future SKA telescope. The box is 2563 particles wide. The color ramp codes the number of Ly-alpha scatterings per atom per second.
Image courtesy of Benoit Semelin

Once upon a time, when the Universe was very young and hot, matter existed only as protons and electrons. Then, one day, things cooled off; the ions got together to form neutral atoms.

This recombination marked the beginning of what cosmologists call the “Dark Ages of the Universe,” a period of about one billion years for which there is no observational data.

Beyond the Dark Ages

Towards the end of these Dark Ages, the Universe entered a new phase—the Epoch of Reionization—when UV and X-ray emissions from stars and quasars began to reionize the newly formed neutral atoms.

We have very few ways to gather observational information about the epoch of reionization, because the photons emitted by stars and quasars are absorbed by the intergalactic medium before they reach us,” explains Benoit Semelin, an astrophysicist at the Paris Observatory in France.

But there is, says Semelin, one way forward.

One emission to view the past

“There is one kind of radiation to which the universe is transparent, even during the epoch of reionization: the 21-cm emission of neutral hydrogen,” Semelin explains.

To detect these emissions, the world is building a billion dollar tool: the international SKA radio-telescope, scheduled for around 2012.

“To optimize the design of this instrument we need to have the best possible models of the 21-cm emission,” says Semelin. “This is achieved by numerical simulations.”

Semelin and his team are using their grid-enabled LICORICE code to run Monte Carlo simulations of the radiative transfer of photons, including up to 109 virtual photons per simulation. These simulations are scheduled to run on EGEE grid. 

“The Monte-Carlo technique is well suited to grid technology because several computers can, to some extent, propagate photons independently,” explains Semelin. “The results from different computers are then summed, reducing statistical noise.”

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