Scientists led by Anze Slosar, a physicist at the U.S. Department of Energy’s Brookhaven National Laboratory working with the Baryon Oscillation Spectroscopic Survey (BOSS), have completed the largest three-dimensional map of the distant Universe after making observations of light emitted from 14,000 quasars passing through clouds of interstellar gas. The map is the first glimpse into what the Universe looked like 11 billion years ago.

BOSS is part of the Sloan Digital Sky Survey (SDSS), an international survey that focuses on observing objects outside of our own Milky Way and mapping out the largest structures in the Universe. Its previous data releases have unmasked large regions of the sky, but this result is based data that has been collected from the deepest reaches of space.

Astronomers are very interested in understanding how matter binds together at the largest scales; instead of space dust coming together to form asteroids, they look at how galaxies drift together and form a network of filaments and clusters. These studies are normally conducted by looking at luminous objects, but at large distances, even incredibly bright sources such as quasars are difficult, if not impossible to detect using current instrumentation. Getting around this problem requires ingenuity, and as Slosar explains, this latest work takes an entirely different approach: “Usually we make our maps of the Universe by looking at galaxies, which emit light. But here, we are looking at intergalactic hydrogen gas, which blocks light. It’s like looking at the Moon through clouds – you can see the shapes of the clouds by the moonlight that they block.”

Just like the eye can notice the difference between light passing through different materials, astronomers use spectrometers mounted on telescopes to detect the passage of light through clouds of interstellar gas. These clouds absorb light at very specific wavelengths which depend on how distant the clouds are and leave a very well defined spectral signature known as the ‘Lyman alpha forest.’ By detecting these signatures in light it’s possible to accurately measure where the gas in the distant Universe is located. As the gas clouds move with galaxy clusters, finding these effectively maps out the clusters.

All of this information is very important in helping scientists understand the formation history of the Universe. “We can compare the Universe then to the Universe now, and learn how things have changed,” explains Andreu Font-Ribera, a graduate student at the Institute of Space Sciences in Barcelona and a member of Slosar’s team. Observing the Universe at such an early age is also a good test of dark energy, whose effects should be much easier to observe as the distribution of matter is much more uniform than it is today. Adds Slosar, “We now know we can use the Lyman-alpha forest to look at the dark energy. There is all this structure at the distant Universe that has never been seen before. Sometimes I feel like an adventuring cartographer from the Middle Ages!”