Definitions for "Flatness problem"
One of two conceptual problems with the Standard Big Bang model, which is that there is no natural way to explain why the density of the universe is so close to the critical density.
The unexpected result that the Universe is not expanding so slowly that it will clearly collapse back on itself in a Big Crunch, nor expanding so quickly that it will clearly keep expanding forever. Instead, measurements show that the Universe is treading a fine line between the two—the Universe is referred to as "flat". Astrophysicists would not expect this to be the case unless there were some cause for the Universe to tread such a fine line. Finding this cause is the "flatness problem".
One of the three problems associated with the standard models of cosmology. The problem has to do with the basic question, why the universe is close to being spatially flat.