The flatness problem is the fact that the observed density of matter is unexpectedly differentfrom the critical density predicted by the inflationary epoch.

Question

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The flatness problem is about how the observed density of the universe is very close to the critical density, which makes the universe appear flat. This is unexpected without the fine-tuning introduced by the inflationary epoch, which explains why the universe appears flat and not due to a discrepancy between observed and critical density.

The Flatness Problem Is the Fact That the Observed Density