First light. Those two words have been echoing around my head for the past few weeks but reached a crescendo today with the release of the first images from the European Space Agency's Planck spacecraft.
In astronomy, first light is that special moment in the life of a new instrument or telescope when it does its first observation of the Universe. It is like a moment of birth.
For Planck, astronomical photons have been entering the telescope since it was released from its Sylda cocoon shortly after launch. But at that point the instruments weren't turned on and the whole spacecraft had to be checked and cooled down over several weeks. Even after they were switched on, the instruments had to be tuned and tweaked to make sure that they were working as well as possible in this new environment of space. It was only on around August 13th that the instruments were declared ready and the first astronomical observations could truly begin. That date marks first light for Planck.
In the case of Planck though, first light has a pertinent second meaning. Planck's main aim is to observe the radiation of the cosmic microwave background. Back when the Universe was a baby - around 380,000 years after the Big Bang - it was much more compact and much hotter. It was so hot that the entire Universe was like the surface of the Sun - a plasma - in which photons of light were continuously bouncing off of free electrons and protons. As the Universe expanded and cooled those electrons and protons were able to stick together to make neutral atoms (mostly hydrogen). At this point the photons of light were free to go on their merry way in whatever direction they happened to be going*. When we look at the cosmic microwave background (CMB) we are finally ending the journey of those first photons that started to stream freely through the Universe. By looking at the CMB we are looking at the earliest light you can see.
Being cautious, Planck dedicated its first two weeks to a sort-of dress rehearsal for the two full-sky surveys that would take place over the following 15 months. This First Light Survey was completed at 14:49 CET on 27 August and the images are now online. The image getting most of the attention is the one showing the survey overlaid on an optical image of the sky by Axel Mellinger. As well as illustrating the way that Planck sweeps out rings on the sky (it spins once per minute) it beautifully shows the contrast of what Planck will observe. It includes 'local' things like our Milky Way galaxy (the red band across the middle) as well as the fluctuations in the cosmic microwave background. It is pretty but there is something quite important about the zoomed in images comparing observations at 70 and 100 GHz. Take a look.
Map of the high-galactic-latitude sky at 70 and 100 GHz, detail from the Planck First Light Survey. CREDIT: ESA, LFI & HFI Consortia (Planck)
Over the coming 15 months or so the two instruments will work together to map the entire sky twice. At the same time, and for about a year after that, there will be the job of calibrating all the data, finding systematic errors, identifying all the 'foregrounds' (other astronomical things that have obscured our view of the cosmic microwave background) and calculating various properties of our Universe. It's a huge task but a tremendously exciting one and there's a great team of people working on it.
* Their 'merry way' would later be affected by heavy clusters of galaxies warping the space they were travelling through, huge patches of hot gas and, more recently, our messy Galactic neighbourhood.