What does this equation mean?

Cosmology removes the Killing vector altogether rather than merely complicating it. The Friedmann-Lemaître-Robertson-Walker spacetime describing the large-scale universe has no timelike Killing vector in comoving coordinates: its metric depends explicitly on time through the scale factor, and no coordinate change removes that dependence globally. There is consequently no analog of the ADM or Bondi construction supplying a global conserved energy for, say, the universe’s total photon population. A single photon’s cosmological redshift is the precise observational face of that absence: its frequency, and with it its energy, decreases as the universe expands, and this is exactly what an absent time-translation symmetry predicts, not a violation of $\nabla_\mu$ $T^{\mu\nu}$=0 , which continues to hold for the photon fluid throughout, and not evidence that anything is unaccounted for. Francis, Barnes, James, and Lewis addressed this exact confusion directly, showing that objections to treating cosmological redshift as a consequence of “expanding space” typically smuggle in physical properties for that expansion which general relativity does not assign it [ 8 ] .