What is flat {\Lambda}CDM, and may we choose it?

Understand

The Universe is neither homogeneous nor isotropic, but it is close enough that we can reasonably approximate it as such on suitably large scales.

• The inflationary-$\Lambda$-Cold Dark Matter ($\Lambda$CDM) concordance cosmology builds on these assumptions to describe the origin and evolution of fluctuations.
• With standard assumptions about stress-energy sources, this system is specified by just seven phenomenological parameters, whose precise relations to underlying fundamental theories are complicated and may depend on details of those fields.
• Nevertheless, it is common practice to set the parameter that characterizes the spatial curvature, $\Omega_K$, exactly to zero.