Cosmic Clocks: Galaxies Behave Like Clocks, Rotating Once Every Billion Years
It’s not Swiss watch precision, but regardless of whether a galaxy is very big or very small, if you could sit on the extreme edge of its disk as it spins, it would take you about a billion years to go all the way round.
He said that by using simple maths, you can show all galaxies of the same size have the same average interior density.
“Discovering such regularity in galaxies really helps us to better understand the mechanics that make them tick — you won’t find a dense galaxy rotating quickly, while another with the same size but lower density is rotating more slowly.”
https://academic.oup.com/mnras/article- ... m=fulltext
Cosmic clocks: a tight radius–velocity relationship for H i-selected galaxies
if Galaxy A has a radius of 1,
Galaxy B has a radius of 2,
and Galaxy C has a radius of 1/2
and all do a revolution in 1 billion years
then is the speed of the outer edge
determined by a cosmic (time) factor
rather than the amount of "dark matter" within?