What is the hierarchy of structures in the Universe?

A computer simulation (more than 50 million light years across) of one possible scenario for the large-scale distribution of light sources in the universe. Credit: Andrew Pontzen and Fabio Governato.

What is the “hierarchy” of structures in the Universe? it seems that we have moon, then planet, then planetary system, then Galaxy, then maybe “several missing” in the hierarchy?, then universe.

How many “several missing” structures would there be? And would all of them just be considered clusters within clusters? Or is the “galaxy” structure the largest single independent organized type of structure in the universe and anything beyond that just one big disorganized mess?

Is there any organization beyond the galaxy structure, something containing galaxies, but again, not the entire universe and somewhat lesser than the entire universe? And if so, how many levels in the hierarchy are there before we get to “universe”?

There are definitely structures larger than galaxies in the Universe, but it’s not clear exactly how many types or levels. The most obvious and uncontroversial class of structures beyond the galaxy is the galaxy group or cluster. These structures consist of tens to thousands of individual galaxies that are bound by gravity to one other and orbit a common center. That center is not defined by some huge central object, as the Sun defines the center of our solar system, but instead by the center of all the mass in the cluster, including dark matter. In fact, the dark matter dominates the mass budget of galaxy clusters, outweighing the normal matter by about a five-to-one ratio.

Beyond the scale of galaxy clusters, most astronomers agree that there are “superclusters,” consisting of several galaxy clusters and groups bound to one another. Our own galaxy and the galaxy group it is part of (the Local Group – a very creative name) is generally accepted to be part of the Virgo Supercluster, which may in turn be part of an even larger supercluster (the Laniakea Supercluster), though this finding is new and not fully accepted.

The unresolved question is whether there are larger and larger structures, out to arbitrarily large scales, or whether things eventually approximate, as you say, “one big disorganized mess.” A fundamental principle of cosmology (the study of the origin and evolution of the Universe) is that on large enough scales the Universe is homogeneous (looks the same wherever you are) and isotropic (looks the same in whatever direction you look). But this is an axiom, not an observation. Our best evidence that the cosmological principle holds is the cosmic microwave background (CMB), the relic light left over from a few hundred thousand years after the beginning of the Universe. The intensity of the CMB is equal in all directions to within about one part in 100,000, implying that the density of the Universe was similarly uniform when the CMB was emitted. But as astronomers do galaxy surveys out to greater and greater distances, we always seem to find larger and larger structures – i.e., we have not yet conclusively detected the “homogeneity scale” in galaxy surveys – so it’s probably safest to say that the hierarchy of structures persists out to some scale between the farthest distance probed by current galaxy surveys (a few billion light years) and the distance to the CMB (essentially the size of the observable Universe).

Tom Crawford

University of Chicago