 Signal propagation is determined by the equation "ds" = 0.In the isotropic model near the singularity "t" = 0 the interval element is "ds"2 = "dt"2 — 2"t" formula_161, where formula_161 is a time-independent spatial differential form.Substituting "t" = η2/2 yields 
The "distance" Δformula_163 reached by the signal is
Since η, like "t", runs through values starting from 0, up to the "moment" η signals can propagate only at the distance Δformula_163 ≤ η which fixes the farthest distance to the horizon.