Space bubbles help probe early cosmos
[ UniNews Vol. 13, No. 22
29 November - 13 December 2004 ] By Elaine Mulcahy
Space bubbles from the cosmic dark ages have provided scientists with the key to the design of a new radio telescope that, by simply tuning to the right frequency, can map the early universe in three dimensions.
University of Melbourne scientist Dr Stuart Wyithe (Physics) and Professor Abraham Loeb (Astronomy, Harvard University) describe in a recent edition of the journal Nature (11 November) a novel way to map the universe in the era when the first galaxies appeared (about 12 billion years ago).
The findings will be used in the design of a new telescope made of chicken wire and many pairs of tunable radio dipoles being designed by the Massachusetts Institute of Technology in collaboration with University of Melbourne, Harvard University and Australian National University research teams.
Dr Wyithe says, Current-day observations of the early universe are limited to single snapshots. We know that the universe is not two-dimensional but consists of ripples and bubbles, and it is difficult to get a true idea of how the universe looked during a specific time without looking at it in three dimensions.
The new telescope will enable us to view the early universe at the crucial era when the cosmic dark ages ended and the first galaxies were formed.
The research is based on the changing state of the cosmic gas, or the intergalactic medium, in the transition from a smooth simple universe to a complicated one that was filled with galaxies and stars.
Researchers think that 300,000 years after the Big Bang the universe was primarily filled with neutral hydrogen gas and dark matter. It was one billion years after the Big Bang that the first galaxies had appeared and by this time most of the hydrogen had been ionised, or charged.
The time in between is known as the cosmic dark ages, which towards the end, consisted of a period of re-ionisation of the hydrogen gas. This is the era that Dr Wyithe and his colleagues are primarily interested in.
Dr Wyithe and Professor Loebs finding is an extension of previous research by the pair (Nature, 26 February, 2004) that looked at stars and quasars (black holes) that first appeared a few million years after the beginning of the dark ages. These stars and quasars reacted with the gas surrounding them, ionizing the nearby hydrogen atoms. As more of these stars and quasars appeared, the universe started to fill with bubbles of ionised hydrogen embedded in the surrounding neutral gas.
Professor Loeb says that over a few tens of million years, the bubbles started to overlap, until eventually the whole universe had been re-ionised.
By simply mapping where the neutral hydrogen gas existed during the time before complete re-ionisation, we can get a picture of where the bubbles were (because they would appear like holes in the map) and what the universe looked like in this era, he says.
The researchers needed to firstly consider how the expansion of the universe could have affected the size of the wavelength in the time it took to reach Earth today, and secondly how they could map the three-dimensionality of the universe using this knowledge.
Dr Wyithe says, We know that the radio waves emitted by neutral hydrogen gas have a wavelength of about 21cm. However as cosmic radiation passes through the expanding universe, its wavelength is stretched a phenomenon known as redshift. Therefore, the further the emitting hydrogen is from the Earth, the more its wavelength will be stretched.
We estimate that radiation emitted by neutral hydrogen at about the time when re-ionisation was taking place will have a wavelength of about 1.5 metres when it reaches Earth today.
And, because of this changing dimension, by looking at different wavelengths we can effectively get a picture of the universe at a specific distance and then step through many different wavelengths to build a three-dimensional view.
Teams from MIT, Harvard University, Australian National University and CSIRO recently visited the University of Melbourne to discuss plans for the telescope.
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