Here’s a pie (diagram) to wish everyone a Happy Thanksgiving!
This pie diagram is one of the most famous images from the original phase of SDSS, which mapped the distances to 1 million galaxies out to a distance of about 2 billion light years (z=0.15, or 615 Mpc in comoving radius).
The map shows a slice through the Universe with the Earth at the centre, and each of the 1 million galaxies in the SDSS Main Galaxy Sample as a point. The points are colour coded by local density to hi-light the cosmic web (with red points in the highest densities).
The black parts of the pie are where SDSS did not map galaxies, either because our Milky Way is blocking the view from Earth, or because those parts of the Universe are not visible from our telescope in New Mexico.
Even while the Universe is expanding, all the matter in it clumps due to gravity and the structures we see in this map are the result of that. The details of the growth of these structures over time depends on both the expansion history of the Universe and the total amount of matter in it. So by accurately mapping the locations of galaxies in this map, scientists in SDSS have been able to measure both of these things making an important contribution to our knowledge of how the Universe works.