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Recently, I watched a YouTube video on Limits of Humanity, by famous Youtube channel Kurzgesagt. It talked about the local neighbourhood, and it is the only group of galaxies which are gravitationally bound to us.

Other galaxies and groups of galaxies are speeding away due to the expansion of our universe and are not gravitationally bound to us.

But I got confused because our supercluster of galaxies will obviously contain millions of these "local neighbourhoods", and if these individual groups are not gravitationally bound to us then how is our supercluster even stable? It should not have been formed in the first place since no groups are gravitationally bound to each other, but indeed it is a structure that we have identified, and is obviously formed by gravity only.

Am I misunderstanding something? Any help is massively appreciated!

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Structure formation

Structure in the Universe — galaxies, galaxy groups, clusters, and superclusters — forms from regions of the Universe which are denser than the average; dense enough the overcome the expansion of the Universe and collapse gravitationally.

Whereas it was previously thought that the largest structures collapsed first, later fragmenting to smaller structures (Eggen et al. 1962), we know now that structure has formed in a "bottom-up" manner, meaning that smaller structures form first, and progressively larger structures form subsequently.

Hierarchical build-up

The first structures to form were clumps with masses of roughly a million Solar masses (e.g. Mo et al. 2010), some 100 million years after the Big Bang. Galaxy formation peaked after 2–3 billion years, while most clusters of galaxies only formed several billion years later.

Virialization

When a structure forms, it doesn't of course do so instantly; it takes some time to "withstand" the expansion of the Universe and collapse, for the matter to whirl around, settle down gravitationally, and reach a final, dynamical equilibrium. When this has happened, we say that the structure has virialized (from the virial theorem).

Today, clusters are the largest structures in the Universe to have virialized. As you write, the Milky Way is part of the Local Group, which is part of the Virgo Cluster. But substructures do move around, and although we are gravitationally bound, we're in fact currently moving away from the center of the Virgo Cluster (Arp 1988).

The future

Superclusters are not yet virialized. They will be in the future (e.g. Araya-Melo et al. 2009), but due to the accelerated expansion of the Universe the galaxies will be so far apart that we might not be able to see most other galaxies (Nagamine & Loeb 2003).

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    $\begingroup$ Sorry to ask but what is the meaning 'virialized'? I am not an astrophysicist or astronomer, but just an enthusiaist. $\endgroup$
    – Kshitij Kumar
    Oct 3, 2021 at 9:12
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    $\begingroup$ @KshitijKumar I’m sorry, I’ll edit my answer to be a bit less jargon-ish, but I’m on my phone now so will be later :) But “virialized” means “to have settled down gravitationally”, so that it will not later be disassembled by expansion (or other forces). The word comes from “the virial theorem”, which is the mathematical description of the kinematics of bodies under the influence of gravity. $\endgroup$
    – pela
    Oct 3, 2021 at 10:07
  • $\begingroup$ thanks for commenting. Yes please edit, your answer although good is a bit heavy for people like me with no technical background. $\endgroup$
    – Kshitij Kumar
    Oct 3, 2021 at 10:39

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