According to the generally accepted theory of the Big Bang, the universe originated between 10,000 and 20,000 million ago years ago and has been expanding ever since.

The uncertain future of the universe: the expansion could be limited (closed universe), shrinking the universe upon itself, or it could be infinite (open universe), in which case the universe will expand forever. 

In the limiting case between these two possibilities (flat universe) nor cease the expansion. At large distances from the observational point of view, the universe is - homogeneous - isotropic. 

If $u$ is the mass density, call $(du / u)$ in $R$ a fractional fluctuations of the mass density, due to the existence of structures (galaxies, galaxy clusters, superclusters), within an area located randomly, radius R. Thus $(du / u)$ in $R$ decreases as a negative power of R. 

This suggests that we can model the universe as made of a soft background of average density, with superimposed fluctuations. A small scales these fluctuations are large but decrease as the scale grows. A sufficiently large scale (greater than about 100 Mpc), the universe can be treated as homogeneous and isotropic then a uniform density.

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What is meant when it is said that the universe is homogeneous and isotropic?
Despite my best effort to clean up a corroded spark plug and blowing the area with an airline before removing would it be bad news if dirt or grime falls into the engine in the process of removing it?
