ASTRONOMY FROM THE FAR SIDE

THERE'S ONLY ONE PLACE TO GO IF WE WANT TO CATCH SIGHT OF THE COSMIC DAWN

- by DR ALASTAIR GUNN

Astronomers love a challenge. They place their observatories on the highest mountains, in the driest deserts, on the coldest ice shelves, beneath the deepest oceans, in orbit around Earth and the Sun, and at the farthest-flung outposts of the Solar System. But now, they're planning to build telescopes on the far side of the Moon.

These instruments will probe one of the last unexplored windows on the Universe. Here, astronomers hope to get a glimpse of the elusive Cosmic Dawn, the moment when the Universe emerged from darkness, and stars and galaxies started to form (see 'Chasing the Cosmic Dawn', opposite).

But why take on the huge technical challenges and costs of building an observatory on the Moon? The reason is that, when it comes to detecting the Cosmic Dawn, nowhere else will do.

THE 21CM LINE

The all-important sign of the awakening Universe comes from neutral hydrogen atoms. Occasionally, the electron in a hydrogen atom flips over, releasing a photon with a telltale wavelength of 21cm (8.2in). If astronomers look at the radio waves being emitted by a cosmic gas cloud and they see a narrow spike in radio waves that are 21cm long (known as a 'spectral line'), they know the cloud contains neutral hydrogen. Although caused by an extremely rare (and random) transition, there's enough neutral hydrogen in the Universe to make the 21cm line easy to spot.

The 21cm line is extremely important to astronomers. Not only does it trace a large fraction of the gas that makes up galaxies, it can also penetrate clouds of dust that obscure the Universe at other wavelengths. It was observations of the 21cm line that first revealed the spiral structure of our galaxy, the Milky Way.