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Slides of 5 lectures at XV Brazilian School on Cosmology and ...

Slides of 5 lectures at XV Brazilian School on Cosmology and ...

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Coarse-graining, averaging, backreacti<strong>on</strong>Coarse-graining: Replace the the microphysics <str<strong>on</strong>g>of</str<strong>on</strong>g> agiven spacetime regi<strong>on</strong> by some collective degrees <str<strong>on</strong>g>of</str<strong>on</strong>g>freedom sufficient to describe physics <strong>on</strong> scales largerthan the coarse-graining scale. BOTTOM UP.Averaging: C<strong>on</strong>sider overall macroscopic dynamics <strong>and</strong>evoluti<strong>on</strong>, without direct c<strong>on</strong>sider<str<strong>on</strong>g>at</str<strong>on</strong>g>i<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> the details <str<strong>on</strong>g>of</str<strong>on</strong>g>the course-graining procedure. TOP DOWN.Often assumes the existence <str<strong>on</strong>g>of</str<strong>on</strong>g> a particular average,e.g., FLRW background, without showing th<str<strong>on</strong>g>at</str<strong>on</strong>g> suchan average exists.Backreacti<strong>on</strong>: C<strong>on</strong>sider the effects <str<strong>on</strong>g>of</str<strong>on</strong>g> departuresfrom the average, perturb<str<strong>on</strong>g>at</str<strong>on</strong>g>ive or n<strong>on</strong>perturb<str<strong>on</strong>g>at</str<strong>on</strong>g>ive, <strong>on</strong>the average evoluti<strong>on</strong>.15th <str<strong>on</strong>g>Brazilian</str<strong>on</strong>g> <str<strong>on</strong>g>School</str<strong>on</strong>g> <strong>on</strong> <strong>Cosmology</strong> <strong>and</strong> Gravit<str<strong>on</strong>g>at</str<strong>on</strong>g>i<strong>on</strong>, August 2012 – p.7/143

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