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Has the Milky Way undergone an expansion of space time independent of the universe as a whole or did the gas that created the Milky Way heat up and expand? Whatever the answer, why the current diameter ?

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    $\begingroup$ Why is my car about the same length as an elephant? $\endgroup$
    – James K
    Sep 19, 2023 at 17:44
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    $\begingroup$ They aren't the same size. The part of the Universe that is observable today was ~86 million lightyears in diameter at recombination. The observable Universe at that time was much smaller, but still ~1.7 million lightyears, more than a factor of 10 larger than the Milky Way. $\endgroup$
    – pela
    Sep 19, 2023 at 19:37

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The visible Milky Way is about 100,000 light years in diameter (Goodwin et al. 1998) - so no coincidence there. Of course, this is a bit of a "how long is a piece of string problem" - the "edge" of the Milky Way is not sharp and some stars can be found, certainly as far as 300,00 light years from the centre (Kafle et al. 2014).

If you include the dark matter halo, then it is possibly ten times bigger again, so I can't see any numerical coincidence.

Also note that the Milky Way is just one galaxy. Other galaxies can be bigger or smaller, even in the local group of galaxies. Thus, any quoted similarity between the size of the Milky Way and the light travel time at the epoch of recombination must just be a coincidence.

Gravitationally bound objects like the Milky Way do not expand with the universe.

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The Baryonic portion of the Milky way (i.e not including dark matter) is 100,000 light years in diameter. The epoch of recombination, and the present galactic evolution's "finetuneness" is rather a coincidence. Big bang expanded rapidly due to dark energy i.e cosmological constant and inertia whereas the Milky way galaxy evolved slowly through mergers, formation of nebulae, primordial interstellar medium accumulating to form galaxies, so it is a mere coincidence. The present diameter is due to the amount of baryonic mass distribution (it has no galactic nucleus) it has swallowed, and that depends the rotational velocity which in turn concludes the diameter, other than things like the inertial speed of the gas when it started to accrete to form a proto galaxy.

Milky way has always been a condensed region ever since it started accretion, the cosmological constant was weaker at that time, so the Hubble velocity did not play a role in the galactic evolution. Nor did gas heat up, and cause it to expand, as it solely relies on mergers, and especially the tidally interactions by the local group which was more closer at that time, had affected the arm rotational speed a little bit, and it would not heat up, rather it would cool down. However in few galaxies the quasars may actually play a role to expand but this is not the case for the Milky Way.

Thanks, hope it helps you!

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