To investigate the cosmic no hair conjecture, we analyze numerically one-dimensional plane symmetrical inhomogeneities due to gravitational waves in vacuum spacetimes with a positive cosmological constant. Assuming periodic gravitational pulse waves initially, we study the time evolution of those waves and the nature of their collisions. As measures of inhomogeneity on each hypersurface, we use the three-dimensional Riemann invariant I=Rijkl(3)Rijkl(3) and the electric and magnetic parts of the Weyl tensor. We find a temporal growth of the curvature in the waves' collision region, but the overall expansion of the universe later overcomes this effect. No singularity appears and the result is a "no hair" de Sitter spacetime. The waves we study have amplitudes in the range 0.020Λ≤I12≤125.0Λ and widths in the range 0.080lH≤l≤2.5lH where lH=(Λ3)-12, the horizon scale of de Sitter spacetime. This supports the cosmic no hair conjecture.
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