About the Image
Surveys of distant objects reveal that the Universe has a bubbly structure - sheets and filaments of galaxies form a twisting web punctuated by large voids. These voids can be as big as several hundred Mpc in size and are thought to fill about 95% of space. The largest void observed so far (as of August 2007) is 280 Mpc or about 1 billion light years across. For more information about Voids try this link.
One of the candidates for "largest known structure in the Universe" is the "Sloan Great Wall", discovered in 2003 by J. Richard Gott III and Mario Juric and their colleagues, using SDSS data. The wall is nearly 1.5 billion light years in length and is located approximately one billion light years from Earth. The Sloan Great Wall is almost 3 times longer than the "Great Wall" of galaxies discovered in Margaret Geller and John Huchra's 1989 survey (which is also sometimes called the CfA2 Great Wall). At about 500 million light-years long, 300 million light-years wide and 15 million light-years thick, it was was previously the "record holder" for largest known structure.
Here is an illustration by Gott and Juric that shows both the Sloan Great Wall and the CfA2 Great Wall.
It should be noted that the Sloan Great Wall is not a "structure" in the technical sense as the components of it are not gravitationally bound together.
In 2006, astronomers using the Suburu telescope discovered a 200 million light year long filiment of galaxies. Each discovery like this helps us to better understand the structure of the Universe!
How do We Calculate Distances of This Magnitude?
Hubble's Law can be used to find distances of astronomical objects out to the limits of the observable Universe. For more info, please see the section on calculating the distances to the Nearest Superclusters.
Why Are These Distances Important To Astronomers?
Are there structures that are on a still larger scale? It would take more extensive surveys to know. In only a few short years, we have gone from the 1998 Margaret Geller/John Huchra 150 Mpc survey to this Sloan Digital Sky Survey that goes out to over 600 Mpc. The Geller/Huchra survey mapped only about 1/10,000 of the total volume of the observable Universe. Margaret Geller pointed out that extending the results of this survery to the entire visible Universe is like using a map of Rhode Island to draw conclusions about the appearance of the entire surface of the Earth!