Expert: Jane Tang - 8/2/2006

Question
A recent piece of news reported that the Subaru Telescope has found it ▬a branching structure of galaxies measuring 200 million light▬years across▬ yet the Sloan Great Wall, discovered by the Sloan Digital Sky Survey in late 2003 or early 2004, spreads some one and a half billion light▬years, along the celestial equator, from the head of Hydra to the feet of Virgo.  The new object's greater density might make it more massive than the Wall, but not bigger, since it is not as far▬flung.  Why, then, is it described as the biggest thing ever found?  Is it because the combined length of the branching "filaments" is greater than the length of the SGW?(Incidentally, before the Sloan Great Wall, the biggest thing was the Great Wall (of galaxies) found by the CfA2 survey in 1989...the same year the Berlin Wall was brought down.  The SGW is three times as distant and about twice as long.)  

Answer
Hi there!
  Structure usually means that the components are gravitationally related with each other. For example, the solar system is a structure because the Earth is rotating around the Sun because of the gravity of the sun. The galaxies in that structure are gravitationally involved and might evolve to galaxy clusters later on. But the Sloan Great Wall is not a structure. That is an artificial view caused by the distance measurement that they used.

   Cheers,
       Jane