A Look at CHIME
Ken Tapping, July 17, 2013
From my office window I can see the antennas for the CHIME radio telescope under construction. This acronym is short for Canadian Hydrogen Intensity Mapping Experiment. The University of British Columbia leads the project, and its primary purpose is to map the beginnings of structure in the young universe. That such a thing is possible is due to two factors: light, although it travels very quickly, does not get there instantaneously, and the universe is very big.
Light, along with all the other categories of electromagnetic waves, such as radio waves, travels about 300 metres in a millionth of a second. In our everyday lives on Earth, we can ignore this, but at cosmic distances the situation changes.
The Moon lies about 384,000 kilometres from us. To cover that distance its light takes about 1.25 seconds. That is, we see it as it was 1.25 seconds ago, when the light started on its journey. The Sun is further away, roughly 150 million kilometres, and its light takes over eight minutes to get here. When we look at the Sun we are looking over eight minutes into the past. This is interesting but not particularly exciting. However, for stellar and galactic distances the situation is very different.
For example the Andromeda Galaxy lies about 2.5 million light years away. Since a light year is the distance light travels in a year, the light from that galaxy takes 2.5 million years to get here, so when we look at it, we see that galaxy as it was 2.5 million years ago. The most distant galaxies we can see lie almost 13 billion light years away, and when observing them we are looking back to our universe’s early history.
The expansion of our universe remains one of the most important discoveries in astronomy. Moreover, as it became possible to measure the distances of more and more distant galaxies, and these measurements could be combined with the measurements of the speed with which the expansion is carrying those galaxies away from us, a startling thing emerged. Just under 14 billion years ago, everything we can see around us in the universe was concentrated in one very small, unbelievably dense and extremely hot lump. Then, at a moment now often described as the Big Bang, the universe started to expand and cool. We can still see evidence of that time, as the Cosmic Microwave Background radiation (CMB). By careful mapping of the CMB it has been possible to start reading this part of the universe’s story.
For almost 400,000 years the expanding universe was very hot and featureless. Then small temperature and density differences developed, which led to material collapsing to form the first stars and galaxies. This period, the beginning of structure in the universe, is what CHIME is intended to observe.
This will not be a trivial task. The possibility that we can even do such a thing comes from three main factors. Firstly the improvements in electronic devices over the last few years have been immense, and secondly, since many of the devices used in CHIME are also manufactured in the millions for consumer devices; they have become comparatively cheap. Finally, because the signals being mapped are very weak, manmade signals could obliterate them. That is where DRAO comes in. Our site is one of the most electromagnetically clean locations available, thanks to teamwork with local municipalities, constant efforts by observatory staff and collaboration with radio spectrum managers at Industry Canada. So when the first data starts to roll, there are many of us who can feel proud of what we did to make this possible.
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