The CHIME Project
Ken Tapping, February 6, 2013
A few days ago heavy earth-moving equipment arrived at our observatory site and starting moving snow and soil. Now there is a square patch of ground with assorted piles of soil and snow, pieces of orange plastic, heavy equipment and men laying out exactly what is to be built and where.
This work marks the starting point for the Canadian Hydrogen Intensity Mapping Experiment (CHIME), the development of a special radio telescope custom made for looking at structure in the early universe. When completed it will look like a 100m square array of snowplough blades mounted close to the ground and facing upwards. Although it will be at our observatory, which is part of the National Research Council of Canada, this instrument is a project by the University of British Columbia. It will be located here to make use of our most important asset, very low levels of man-made radio interference.
Our universe began just under 14 billion years ago, in an event we often refer to as the Big Bang. In the beginning it was small, unimaginably hot, and incredibly dense. As it expanded and cooled, it became a hot, dense fog of elementary particles, the fundamental building blocks of atoms. A few minutes after the Big Bang the universe was cool enough for protons, neutrons and electrons to form from the particles. Then, for the next 380,000 years or so, they joined together and separated again. By that time, the temperatures and densities had fallen enough for atoms to form and stay together. Because hydrogen atoms are very stable, they formed first, leading to a universe that is mostly hydrogen, even today. Hydrogen is the main raw material for making stars. It provides their energy and produces as waste products all the other elements needed to make planets and us.
At some point, what is believed to have been a featureless, homogenous expanding universe started to become unstable. The material started to clump, and the clumps collapsed into sub-clumps, which then collapsed further to form the first galaxies and stars, the beginning of the sort of universe we see around us today.
The best gift ever given to astronomers is that the cold hydrogen gas in space gives off a radio signal, at frequency of exactly 1420.40575177 MHz (MegaHertz). We can use the strength of that signal to map the abundance of hydrogen in clouds, galaxies and beyond. Even better, if that material is moving towards or away from us, that frequency is increased or decreased by the Doppler Effect, which is what makes sirens on emergency vehicles sound higher when approaching us and lower when the vehicle has gone past. Because the universe is expanding, that hydrogen signal is strongly shifted down in frequency, and because that expansion increases with distance, measuring the frequency, tells us how far away that hydrogen lies.
In that way, it will be possible to identify the signal from the hydrogen in the young universe and use it to map in 3D the beginnings of the first structures and get an idea of the forces moulding them.
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