Reinventing Discovery - Michael Nielsen [49]
The Sloan Great Wall was discovered in 2003, when a team of eight scientists, led by J. Richard Gott III of Princeton university, decided to make a visual map of the entire known universe. This sounds grandiose, but they did it for the same reason we make maps of cities and countries: displaying our knowledge visually can make it easier to understand what we know. Imagine how difficult geography would be if we didn’t have maps, but instead had to rely entirely on verbal descriptions. Problems that are easy to solve visually, like figuring out how many continents there are, would all of a sudden become difficult research problems. One imagines early geographers holding research conferences on “Resolving the Number of Continental Land Masses,” perhaps with fiercrguments about questions such as whether Asia and North America are truly separate continents.
A big difficulty in making a map of the universe is knowing what’s out there. Modern telescopes let us see trillions of objects, but for the most part astronomers concentrate on looking at just a tiny fraction of those objects. This perhaps sounds surprising, but imagine you’re an astronomer: wouldn’t you prefer to spend your time observing something you already know is extremely interesting, such as the supermassive black hole in the core of the Milky Way galaxy, instead of some random star in some random galaxy? Most astronomers thus spend most of their time looking at objects already known to be interesting. It’s like the difference between exploring a city broadly to find interesting new places, versus the temptation to only revisit familiar haunts. To find interesting new objects in the sky, someone needs to strike out and explore the sky broadly.
This is where sky surveys come in. Instead of looking in exhaustive detail at known objects, the telescopes used in sky surveys systematically scan the whole sky, building up a broad picture of the universe. Sky surveys are the foundation of astronomy, often giving us the first clues about which objects to look at in more depth. One of the earliest sky surveys was the Almagest, written by the astronomer Ptolemy of Alexandria in the second century CE. Ptolemy didn’t have a telescope, but used his naked eye to compile all sorts of useful information about what he saw in the sky, ranging from a description of how the planets move to a detailed catalog of 1,022 stars. The Almagest remained the standard work of astronomy in Europe and the Middle East for the next 800 years.
As you’ve perhaps guessed, the modern-day Almagest is the Sloan Digital Sky Survey (SDSS), named for the Alfred P. Sloan Foundation, which provides much of the funding. The SDSS does its work using a superb telescope located just outside the tiny town of Sunspot, high in New Mexico’s Sacramento Mountains. The telescope captures light using a large mirror, 2.5 meters in diameter. The excellent location and large mirror mean the SDSS takes very good images, and can look all the way out to the edge of the known universe. The images aren’t quite as good as those from the world’s biggest telescopes, such as the enormous 10.4-meter Gran Telescopio Canarias, in the Canary Islands. But the SDSS telescope has a major advantage over most larger telescopes: it has a special wide-angle lens that lets it rapidly photograph large sections of the sky. In a single image it can capture an area eight times the size of the full moon. By contrast, the Gran Telescopio Canarias can only capture an area one sixteenth the size of the moon, making it unsuitable for the broad exploration required by