Week 8: The Hercules-Corona Borealis Great Wall

One of the largest objects ever observed is called the Hercules-Corona Borealis Great Wall. While the name alone is impressive, it pales in comparison to the object itself. The HCBGW is essentially a region that is more densely populated than average with galaxy clusters and other matter. This structure is approximately 10 billion light years across. Since this is so large, it's helpful to think about it in different terms: this structure is about 10% the size of the observable universe, and spans a third of our sky, despite being about 10 billion light years away from us. In cgs units, this object is hella big.

The HCBGW. The bright clumps indicate dark matter. Zooming in recommended.

The first indication of this object's existence was the detection of an unexpectedly large number of gamma ray bursts coming from the region. Gamma ray bursts generally indicate the presence of matter, and upon further analysis, scientists concluded that it was extremely likely that the bursts were coming from the same aggregation of matter. This whole process, as far as I can tell, is pretty routine for observational astronomers. What wasn't routine was how large the HCBWG turned out to be. 

I think the most interesting thing about the size of the HCBWG is the implication that it has towards our understandings of cosmology. The cosmological principle is the idea that the universe is uniform and the same in every direction, when viewed on a large enough scale. Basically, this idea is a more formal way of saying that no location in the universe is "special." Since we want to avoid inhomogeneities that don't disappear when you've zoomed out enough, there must be an upper limit on the size that any one astronomical object can attain. This theoretical limit was set on the order of hundreds of megaparsecs. 

The HCBWG, if it can truly be considered one object, however, is about 3000 megaparsecs, blowing this limit out of the water. Critics of the cosmological principle have cited the HCBWG and other almost-comparably-gigantic objects such as the Huge Large Quasar Group (not to be confused with any large huge quasar groups, apparently) as evidence against the hypothesis. While I have no beef with the cosmological principle, I can't help but hope that the Hercules-Corona Borealis Great Wall truly is one gargantuan object, because it would just be so cool.




Sources
http://listverse.com/2015/10/22/10-of-the-biggest-things-in-the-universe/
https://en.wikipedia.org/wiki/Hercules-Corona_Borealis_Great_Wall
https://upload.wikimedia.org/wikipedia/commons/f/fd/Hubble_image_of_MACS_J0717_with_mass_overlay.jpg
https://en.wikipedia.org/wiki/Cosmological_principle