What is the role of gravity in superclusters?

[ajgrebel]

Recently, I've taken an interest in Astronomy and decided to become better equated with the night sky. This of course led me to information on the formation of planets and stars as well as the different formations of galaxies. Eventually, I ran across superclusters which I'm assuming in laymen's terms are simply a large group of galaxies within a targeted area.

From what I have read; (I'll try to remember the source) it stated that superclusters were not bound by gravity. I'm assuming that I have misinterpreted what I read because I see no reason why it wouldn't be affected by gravity.

Hope some one can clarify this issue for me. Thanks in advance.

 

[cepheid]

Recently, I've taken an interest in Astronomy and decided to become better equated with the night sky. This of course led me to information on the formation of planets and stars as well as the different formations of galaxies. Eventually, I ran across superclusters which I'm assuming in laymen's terms are simply a large group of galaxies within a targeted area.

From what I have read; (I'll try to remember the source) it stated that superclusters were not bound by gravity. I'm assuming that I have misinterpreted what I read because I see no reason why it wouldn't be affected by gravity.

Hope some one can clarify this issue for me. Thanks in advance.

If you are going fast enough to reach escape velocity, then you're not gravitationally "bound" to the Earth. It doesn't mean that you don't feel its gravity. It just means that you have enough energy that your trajectory will take you away from Earth and out to "infinity", never to return. This is unlike an object with less than escape velocity, which will end up in a bound trajectory (a closed orbit) around Earth.

Galaxy clusters are the largest gravitationally-bound objects in the universe (the largest objects in which all of their individual members will remain a part of the cluster and none will eventually escape). On the other hand, galaxy superclusters are large enough that not all of their constituents will remain in the vicinity of each other forever. I suppose that makes galaxy superclusters somewhat transient over-dense features in the large scale structure of matter. One manifestation of this distinction between gravitationally-bound and unbound objects is as follows. On scales smaller than galaxy clusters, things are not participating in the universal expansion (i.e. although different galaxy clusters are all expanding away from each other, individual galaxies within a cluster don't "feel" the expansion and their velocities relative to each other are dominated by their mutual gravitational interactions. The cluster's self-gravity is dominant and keeps it together). In constrast, superclusters do increase in size along with/as a result of the expansion. Once again, the individual galaxy clusters and galaxy groups that make up that supercluster are all moving away from each other.

 

[phinds]

cepheid, I agree completely with what you have said with one exception that I may be wrong about and would appreciate clarification. You say the the galaxies in a bound cluster don't "feel" the expansion of the U, but it is my understanding that yes, they DO "feel" it, but overcome it, because the local gravitational forces are so much stronger. Is that not correct?

Thanks,

Paul

 

[cepheid]

cepheid, I agree completely with what you have said with one exception that I may be wrong about and would appreciate clarification. You say the the galaxies in a bound cluster don't "feel" the expansion of the U, but it is my understanding that yes, they DO "feel" it, but overcome it, because the local gravitational forces are so much stronger. Is that not correct?

Thanks,

Paul

Hmm, yeah well I guess I'm guilty of using an ill-defined term. I don't really know what it means to "feel" or "not feel" the expansion. During structure formation, when some overdense bit of matter undergoes collapse to form a bound object, that object has essentially "separated out" from the Hubble flow. It maintains the same physical size and does not expand because its constituents are held together by gravity.

I think the proper way to think about it is in terms of solving the Einstein field equations of general relativity (GR) in different situations. The solution that applies "globally" or "overall" is the Robertson-Walker metric (since the universe is essentially homogeneous and isotropic on the largest scales). That solution permits expansion as described by the Friedmann model, and so that's what spacetime does, overall. But locally, when you're looking at a specific deviation from that smoothness and homogeneity (such as a massive, bound object), the solution to the field equations is totally different, and as a result spacetime does something totally different, locally. But I'm not an expert on GR, so if somebody has a correction, feel free to jump in.