# Does the distance to the cosmic horizon Lorentz-contract? Does the universe Lorentz-contract?

Our universe has a finite size. It is often called the "radius of the universe", or "distance of the cosmic horizon".

If we would fly with relativistic speed at the position of our Earth, would this finite size Lorentz-contract? Or would it stay the same?

In simple words: Does the universe Lorentz-contract?

Would be observe the same size for the universe whatever our speed with respect to the reference frame defined by the average mass in the universe?

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We only talk about the Lorentz contraction in situations that may be described by special relativity and where the object to be Lorentz-contracted has a uniform well-defined speed $v$. Both of these conditions are violated in the case of the "whole Universe". We need to use general relativity to describe cosmology – changes of the size or speed of the whole Universe and/or many distant galaxies simultaneously – so what happens with them should be described by the full concepts of general relativity and "Lorentz contraction" isn't something that may be applied to the whole Universe. Moreover, different galaxies have a different $v$ so one can't attribute them a common Lorentz factor, either.