It’s all about photons. Two main things determine what you can learn from a new telescope array such as the Alma telescope in Chile. How many photons will it collect, and what is their wavelength?
Alma is big – the largest astronomical project in the world – so it can collect lots of photons. In particular, it can collect enough to see things that are very faint; sources of light such as galaxies, stars and cosmic dust billions of light years away. Since photons travel at the speed of light, when you look at very distant objects you are also looking back in time because the light takes millions or billions of years to reach you.
Alma will be able to see dust which is about 13bn light years away. The universe is about 13.5bn years old, so this dust is “only” about half a billion years after the big bang. The universe was different then. Well, you would expect it to be. Certainly a 13-and-a-half-year-old boy is rather different from a six-month-old baby.
Back then, any stars or galaxies were “first generation”. They were the first nuclear reactors which fired up and said “let there be light” in a dark universe. These stars started the process of creating heavy elements, such as iron, and when they died as supernovae (colossal explosions) they shared the goodness around. Then second or third generation stars, like our own sun, could start up and have planets with iron cores and carbon-based life forms. Alma will see the dust from which the first stars formed and the galaxies they made.
Then the wavelength. This is how we know what the stars are made of. We have never been out there to bring bits back. We have never even sent a robot to analyse them and tell us. But the atoms and molecules out there have electrons just like the ones on Earth and they jump between energy levels, emitting and absorbing photons – particles of light – as they do. These photons have characteristic wavelengths depending on the size of the jump.
The patterns of wavelengths then tell us which atoms and molecules exist out there. In fact, the patterns are shifted towards longer wavelengths depending on speed, which depends on distance, so this is also a way of working out how far away something is. For example, carbon monoxide occurs in star-forming regions of galaxies and has a distinct series of patterns that will allow Alma to measure the composition and distance of the birthplaces of stars.
Early dust and star formation are key targets for Alma. But new telescopes, like any other leap in scientific capability, open up new aspects of the universe to study. While you do not build them without some good targets in mind, much of the excitement is about the unknown. You do not know what you will find until you look.
