What is the difference between homogeneity and isotropy? (Advanced)

This is a question more of semantics than cosmology. It is said that our universe is "homogeneous and isotropic" (on large enough scales). Doesn't the word 'homogeneous' imply or include 'isotropic' by default?

Neither word implies the other must be true; it is possible to have something that is homogeneous but not isotropic (eg. equally spaced stripes which definitely have a prefered direction) and it is also possible to have something isotropic but not homogeneous (eg. concentric rings which look very different depending on where you are, but are isotropic if you are in the middle).

This page was last updated June 27, 2015.

About the Author

Karen Masters

Karen Masters

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.

Twitter:  @KarenLMasters
Website:  http://icg.port.ac.uk/~mastersk/

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