= Solution
{c}
The actual large-scale expansion is well described by the <Lambda-CDM model>: <radiation in cosmology>, ordinary baryonic <matter>, <cold dark matter> and late-time <dark energy> coexist. A component becomes dominant through continuous changes in their density ratios, rather than every previous component disappearing at an instantaneous phase boundary. Matter <pressure> is small in its nonrelativistic regime, but <radiation> persists, and massive <neutrinos> interpolate between relativistic and nonrelativistic behavior.
There is also a late accelerated era, absent from a model ending permanently in S3. It is consistent with nearly constant <vacuum energy>, whose negative <pressure> eventually dominates because <pressureless matter> dilutes as $R^{-3}$. The <scale factor> then ceases to follow the pure $t^{2/3}$ <Einstein-de Sitter universe> law.
An early phase of <cosmic inflation> is a well-motivated model rather than a directly observed exact $w=-1$ phase. Successful <cosmic inflation> must end and transfer its <energy> to matter and <radiation> through <reheating>; realistic models generally have slowly varying $H$ and $w$ close to, rather than exactly equal to, $-1$. Extrapolating a strictly constant $H$ to arbitrarily small $R$ is not a measurement of the universe's past duration. Finally, <spatial homogeneity> and <cosmological isotropy> apply statistically on large scales: galaxies, <gravitational waves> and other perturbations are absent from the exact <FLRW metric>, and small spatial <curvature> remains an observational possibility.
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