What is today’s equivalent of a world system? Buddhism mentions “world systems.” What’s the equivalent terminology for a world system in the parlance of contemporary astrophysics?
I don't think it refers to parallel universes as an alternate dimension of the sensous realm because the world systems shake when the Bodhisatta gains final attainment which would be strange if an eventuality caused effect over multiple worlds which are supposedly closed systems with laws of conservation, essentially closed systems of information. To me an eventuality causing a tremble over a multitude of such systems is philosophically like magical thinking as far as i can tell.

Also when the Buddha delivers a discourse such as the Brahmajala Sutta the ten thousandfold world system shakes.  

Furthermore given that afaik i've never seen a declaration of a thousandfold multitude of formless realms i think it is the thousandfold world system that undergoes expansion and contraction and the vastness of the universe of form explains the variety of sub worlds which could be planetary systems or galaxies.

Another point is that there is only one Buddha in a world system simultaneously (mn115).  Which is significant because if Buddhas can supposedly teleport and if one followed an intepretation of one Buddha per solar system (as some people do) then a Buddha could meet another and this would be contradictory.

I think there are the 31 planes and it is a single expanding &amp; contracting field of form that is inhabited by a multitude of galaxies with suns and habitable planets. I also think there is only one Maha Brahma per expanding system and one Buddha at a time.

Another point worth mentioning is that during a world system contraction, beings are said to be for the most part inhabiting formless realms.