Talk:Galaxy cluster

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Comments[edit]

Question: There is a line that says,

"With Chandra X-ray Observatory structures like cold front, shock front, minihalo have also been found in many galaxy clusters." Is the reference to "Cold Front" a misprint? The link leads to a page about weather fronts, which seems to have nothing to do with astrophysics. — Preceding unsigned comment added by 173.48.33.218 (talk) 03:09, 30 July 2012 (UTC)

WRONG FALSE INCORRECT BAD[edit]

The lead-in to this article states that clusters are the largest structures in the Universe. I got to this stub by starting with the Virgo Super cluster. Custers are usually components of super clusters which are components of the cosmological filaments which are the largest observed structures in the Universe. A cluster is smaller by many orders of magnitude. Remove of revise this terrible example of bad science information!71.31.152.220 (talk) 02:07, 29 August 2012 (UTC)

Good point. Supercluster sounds bigger. Anna Frodesiak (talk) 12:33, 23 October 2012 (UTC)
Galaxy filaments "...are the largest known cosmic structures in the universe...". Anna Frodesiak (talk) 12:39, 23 October 2012 (UTC)

Not the largest gravitationally bound structures?[edit]

This source says that (at least some) galaxy clusters are in fact gravitationally bound: http://www.reuters.com/article/2014/09/03/us-space-galaxies-idUSKBN0GY2C820140903

This source implies it: http://www.vox.com/2014/9/4/6105631/map-galaxy-supercluster-laniakea-milky-way

Therefore i suggest we change the text saying that galaxy clusters are the largest gravitationally bound structures. Any objections?

Fresheneesz (talk) 21:15, 5 September 2014 (UTC)

Those articles are not quite saying that superclusters are gravitationally bound. They do say that the mass within the Laniakea structure shows evidence of being gravitationally attracted. This is a somewhat subtle distinction that is probably not currently well explained on wikipedia. For an object to be gravitationally bound requires that gravity determine the dynamics of its structure. In the case of superclusters, Hubble expansion is significant. At some point in the future, objects in Laniakea will recede from one another due to the expansion of the universe, meaning it is not gravitationally bound (Nature News)
In academic literature, my understanding is that the consensus view is still that clusters are the largest gravitationally bound structures. The cited reference in the article says this indirectly, but this paper says so much more directly in its first sentence, and similar statements appear in other reviews of galaxy clusters. I have seen a few papers that claim evidence for bound superclusters, such as this paper by Pearson et al., which says that "the [Shapley supercluster] is the only confirmed bound supercluster in the Universe". However, Pearson et al. cite another paper that directly contradicts their claim that the Shapley supercluster is bound, saying "Including the cosmological constant, the SSC region is found to be gravitationally unbound."
For the time being, I think the article should be left as is, with the possible addition that there is conflicting evidence for some bound superclusters. I've had in mind to improve both this article and the supercluster article, but haven't had time to do it properly. James McBride (talk) 18:18, 6 September 2014 (UTC)
It would depend on what you mean by "bound". Superclusters have not yet had time to fully condense, while many clusters have condensed, and objects bound to it have started orbits. It has been said that clusters are the largest structures that will not be ripped apart by dark energy, if dark energy is like a cosmological constant instead of like quintessence, and if a "big rip" is not in our future. Or atleast, that is my understanding of the situation. -- 70.51.46.146 (talk) 06:57, 13 September 2014 (UTC)