Astronomy and Astrophysics – Astrophysics
Scientific paper
2003-06-30
Phys.Rev.D68:123518,2003
Astronomy and Astrophysics
Astrophysics
11 pages, 1 figure, minor changes, references added. Accepted for publication in Phys.Rev.D
Scientific paper
10.1103/PhysRevD.68.123518
In this letter we review the separate universe approach for cosmological perturbations and point out that it is essentially the lowest order approximation to a gradient expansion. Using this approach, one can study the nonlinear evolution of inhomogeneous spacetimes and find the conditions under which the long wavlength curvature perturbation can vary with time. When there is one degree of freedom or a well-defined equation of state the nonlinear long wavelength curvature perturbation remains constant. With more degrees of freedom it can vary and this variation is determined by the non-adiabatic pressure perturbation, exactly as in linear theory. We identify combinations of spatial vectors characterizing the curvature perturbation which are invariant under a change of time hypersurfaces.
Rigopoulos Gerasimos I.
Shellard Paul. E. S.
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