Illustration by Soner n

By Caleb Scharf 2012-08-05T22:30:26Z

Astronomy, the most ancient of sciences, has always been about mapping.

Australian aborigines looked at the constellation we call Orion and saw a canoe carrying two banished brothers. The Finns saw a scythe. In India, it was obviously a deer. For the Babylonians, it was the heavenly shepherd, and for the Greeks, it was the hunter, a primordial giant.

Over the centuries, mapping the cosmos has been a gradual process of locating the brighter objects and then filling in the gaps. We have helped our eyes along by constructing telescopes, some gathering much more than just visible light to illuminate phenomena beyond our wildest imaginations.

Seeing the universe for what it is has required us to overcome many other blind spots, including the one that places ourselves at the center of the map. It took the insight and intellectual conviction of Galileo and Copernicus to challenge the orthodoxy that Earth was at the center of everything. Even then, the notion that our solar system was nonetheless located somewhere at the middle of the visible universe lasted into the first decades of the 20th century.

The discovery, by the astronomer Harlow Shapley in 1918, that our solar system was not even at the center of the Milky Way galaxy opened the floodgates for more revelations in the following decades. The Milky Way, it turned out, is merely one of many galaxies, all flying apart as the universe expands.

So what does our current map look like? It is both three- dimensional and four-dimensional, linked as it is to time. The farther away objects are, the longer their light has taken to reach us, all the way back through the universes 13.8-billion- year history. There are so many categories of objects and phenomena, and so much higgledy-piggledy data from several hundred years of telescopic astronomy, the best we can do to begin to grasp what this atlas looks like is to play out a thought experiment.

Let us pretend that a very large box has just been delivered to our doorstep, and we have hauled it inside. It contains an ominous-looking sack filled to bursting. An occasional wisp of gas escapes through the knotted top, and every so often a muffled thump or muted glow comes from within.

This sack contains what we could regard as a representative portion of the universe — a fair sample, a cosmologist would say. If you divided the total mass in the sack by its volume, you would obtain a good estimate of the average density of the universe as a whole. Equally, if you measured just how lumpy the arrangement of galaxies was within this volume, it would be a close match to the universal lumpiness of structure.

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