# shear , viscosity and expansion of universe

what is the meaning of expansion ,shear and viscosity in context of universe? how can we conclude a result after getting a numerical value of above terms?

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The second question is better than the first here, in a sense. The problem is that "viscosity" is a term for fluids, shear applies to materials and expansion applies to anything with length. So what really is that first question? – Roy Simpson Mar 21 '11 at 11:14
@Roy, "shear" applies to fluids as well, without shear you cant measure viscosity. And materials can be liquid, solid or gaseous. – Georg Mar 21 '11 at 11:55
Indeed, so this makes the question extremely general, apparently about all of physics. – Roy Simpson Mar 21 '11 at 11:57
The Raychaudhuri equation (en.wikipedia.org/wiki/Raychaudhuri_equation) is often useful in modeling the expanding Universe, when we want to consider the possibility of inhomogeneous and/or anisotropic expansion. Shear and expansion are central there. – Ted Bunn Mar 21 '11 at 13:12
This theorem mentions "vorticity" not "viscosity" though - but it could be the question here. – Roy Simpson Mar 21 '11 at 16:48