STARGAZING: CHIME, the Canadian Hydrogen Intensity Mapping Experiment

Ken Tapping is an astronomer with the National Research Council's Dominion Radio Astrophysical Observatory, Penticton.

Anyone visiting our observatory at the moment cannot fail to notice a massive construction project going on to the right of the road onto the site.

Gradually taking shape are four immense metal lattice troughs or parts of cylinders, facing vertically into the sky. These are the antennas for a project called CHIME, the Canadian Hydrogen Intensity Mapping Experiment. This antenna is the biggest single signal collector in Canada and a contender for the biggest in the world. It looks very different from the other radio telescopes on the site because it is not a general-purpose instrument. It is aimed primarily at several projects, with the main ones based on the technique of Hydrogen Intensity Mapping – mapping the distribution of hydrogen in space. Because hydrogen is by far the most common element in the universe, and almost every structure contains a lot of it, mapping it is good way to map the structure of the universe.

In the 1940’s, Dutch astrophysicist Hendrik van de Hulst deduced that the cold hydrogen atoms in cosmic gas clouds have a signature, a radio signal with a wavelength of 21 centimetres. In 1951 the emission was successfully detected by Harold Ewen and Edward Purcell. This was an exciting discovery because this emission is at a specific and precisely known wavelength, which means we can measure the velocity with which the hydrogen is approaching or receding from us by exploiting the Doppler Effect.

We have all heard the Doppler Effect in action. It makes sounds from things approaching us have a higher pitch and sounds from things receding from us have a lower pitch. Motorcycles and train horns show this well. It might sound strange but having a huge universe is a boon to astronomers. It is so big that even light takes a long time to get here from distant cosmic objects. Sunlight takes over eight minutes to get here, so we can say that it is eight ìlight minutesî away. The most distant parts of the observable universe are billions of ìlight yearsî away. The universe began just under 14 billion years ago, so if we were to look out to a distance of just under 14 billion light years, we would see the beginning of the universe. Unfortunately that precise moment is hidden from us. The earliest light and radio waves we can observe started on their way when the universe had cooled and spread out enough for light and radio waves to pass through it, about 380,000 years after the Big Bang. Thanks to the expansion of the universe, that hydrogen is receding from us at high speed, so that its radio emission has been Doppler shifted in wavelength from 21 cm to between 40 and 70 cm, making it distinguishable from the other (nearer) hydrogen.

At the earliest time we can see, the universe was a big mass of hydrogen. However, there had to be a moment when some parts got denser than other parts, starting the process of forming the first stars and galaxies. However, what gave rise to these density enhancements? One possible cause is Baryonic Acoustic Oscillations, which are essentially huge sound waves moving to and fro in the rapidly-expanding young universe. Because the subsequent structure of the universe was moulded by the density enhancements due to those waves, we should see their signature in the distribution of hydrogen. We should also see the signature of dark energy, which also affected the formation of those density enhancements.

That brings us back to CHIME. This instrument will be the most powerful mapper of hydrogen in the universe we have ever had, particularly for the hydrogen in the young universe. That exciting new instrument is taking shape here for two reasons. Firstly we have the technical capability to do such things, and secondly, we have a site that is comparatively free of electromagnetic pollution.

The western evening sky is dominated by Venus (lower and incredibly bright) and Jupiter (higher and less bright). Saturn lies low in the southeast. The Moon will be New on the 16th.

Ken Tapping is an astronomer with the National Research Council’s Dominion Radio Astrophysical Observatory, Penticton.


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