Can someone provide an equation that would enable me to plot the red "pear shaped" line shown in this diagram? http://www.astro.ucla.edu/~wright/omega0.gif(adsbygoogle = window.adsbygoogle || []).push({});

The red line represents the path of a photon as the universe expands in comoving coordinates. Can the path be derived from the Friedmann-Robertson-Walker (FWR) metric? I do not really need the derivation. Just a simple equation that I can plot on a graph, like the parametric equation for a circle, x^2+t^2=1

Thanks :)

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# Worldline of photon in comoving coordinates.

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