= Coasting cosmic-string universe
{title2=$a(t)=B(t-t_B)$}
An isotropic <cosmic string network> with $\rho=C/a^2$ has <equation-of-state parameter> $w=-1/3$. The <Friedmann equation> gives $\dot a^2=8\pi GC/3-K$. On the expanding branch with positive right side, $a=B(t-t_B)$, the <comoving Hubble radius> is constant, and the <cosmological density parameter> is constant. There is no growing <Flatness problem>, but flatness is not an attractor. If this exact coasting solution extends all the way to $t_B$, then $\int_{t_B}^t dt'/a(t')$ diverges and there is no finite <particle horizon>. A finite starting time changes that global conclusion.
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