As the universe expands, why don't galaxies get stretched out? (Intermediate)
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There is enough matter in a galaxy, that the matter within the galaxy is not affected by the expansion of the universe. You can think of this as the gravity of the galaxy holding it together, but really it's more fundamental than that. The rate of the expansion of the universe depends on the amount of matter (and dark energy) in the universe. If you just consider a tiny fraction of the universe which just includes a galaxy and total the matter in that region, it's more than enough to have already stopped the expansion in that region.
This page was last updated on June 27, 2015
About the Author
Karen was a graduate student at Cornell from 2000-2005. She went on to work as a researcher in galaxy redshift surveys at Harvard University, and is now on the Faculty at the University of Portsmouth back in her home country of the UK. Her research lately has focused on using the morphology of galaxies to give clues to their formation and evolution. She is the Project Scientist for the Galaxy Zoo project.