The universe as seen by Planck. Click to expand and see it in all its glory. Credit: ESA and the Planck Collaboration.

Astronomers take pictures of stuff in the universe all the time, but it’s not every day that we get to take a picture of the universe. Today was one of those days, and this is the picture.

This is a map of the cosmic microwave background, or CMB–the fading glow of the Big Bang itself. By today, the expansion of the universe has cooled it down so much that it’s only 3 degrees above absolute zero, but it’s not quite uniform. The red spots are very, very slightly hotter than the rest, and the blue spots are a tiny bit cooler. Over time, those small fluctuations grew into the clusters of galaxies we all know and love.

Here’s how they did it. The European Space Agency’s Planck spacecraft has been watching the sky since 2009, sweeping over it again and again, taking many pictures of each part. Planck sees in the far infrared, terahertz, a microwave portions of the spectrum, where mostly only very cold things shine. To see things that cold, the camera also has to be that cold, so Planck used liquid helium to cool its camera to a fraction of a degree above absolute zero. After a year of watching, all the pictures were edited together to form this image:

Planck’s unfiltered view of the sky. Credit: ESA and the Planck Collaboration.

In this picture, the bright white stripe down the middle is our own Milky Way Galaxy, especially the dusty star-forming regions. The blue cloudy stuff is called infrared cirrus and is made of diffuse dust scattered above and below the plane of the galaxy. The grainy stuff in the background comes from other galaxies, and most importantly, the CMB. If you look carefully, you can see some similarities between the grainy background here and the CMB map in the first image.

This picture took one year to make, but it took nearly three years to filter out the light from our own Galaxy and from other galaxies to make the image at the top of the post. This was done by, among other things, comparing different wavelengths of light to figure out what each thing in the picture is made of and what its temperature is. Do it just right, and you can see what the universe looked like when it was just 370,000 years old.

Astronomers can learn a lot from pictures like this. For example, the universe is about 100 million years older than we thought, and it has a little less dark energy than we thought. See here for a good overview of the science or here for the original press release. We can learn so much from Planck because it has a much better telescope and camera than its predecessor, NASA’s WMAP. Here’s a comparison I made of the same part of the sky as seen by WMAP and by Planck:

Comparison of the same part of the sky at the same scale as seen by WMAP (left) and Planck (right). Credit: NASA/WMAP Team, ESA/Planck Collaboration, and own work.

Try to match up the red spots and convince yourself that you really are looking at the same part of the sky. In WMAP, you see a bunch of blobs that look that could just be noise, but when you look at the Planck image, you can see that the blobs are real, and they have little details inside them. That’s because the fluctuations in the temperature of the CMB are strongest when they’re about the size of those blobs–a few times the size of the full moon–but there are some that are larger and smaller.

So there it is: the best picture ever taken of the early universe, and it only took 4 years and 700 million euros to make. Now the cosmologists can get to the really hard part: interpreting it.