Azimuthal geodesics in closed FLRW cosmologies

Modern cosmology is closely linked to our understanding of radial null geodesics as these model the propagation of light signals through an expanding universe. Azimuthal geodesics, on the other hand, are perhaps best known for their relevance within closed cosmological models. Such models typically have a finite lifetime: the universe expands up to a maximum size after which it recollapses during the so-called big crunch. An azimuthal geodesic starting at the beginning of the universe will travel a finite angular distance during the expansion and recollapse. It is well-known that this angle is $2\pi$ for a matter-dominated universe and $\pi$ for a radiation-dominated solution. Here we derive the simple formula $$\Delta \varphi = \frac{2\pi}{1+3w}$$ for an arbitrary linear equation of state parameter $w$. To the best of our knowledge this result has not been reported elsewhere and fills a small gap in the literature.