An Absorption Percolation Threshold, Not a Unique Reionization Epoch
A sharp rise in intergalactic medium opacity at high redshift, observed directly in the absorption spectra of distant quasars, is treated by the standard model as decisive evidence for one universal reionization epoch, a single global event completing at approximately one specific redshift across the entire observable universe. This piece keeps the underlying observations, the real, robust Lyman-alpha forest data, entirely intact, and proposes a different mechanism for the sharp transition itself.
What a Percolation Threshold Actually Is
The mechanism proposed here is an absorption percolation threshold: when the coverage of light-absorbing material along a given line of sight crosses a critical density, the transmitted flux collapses sharply, not because a single coordinated global event occurred at that moment, but because percolation thresholds generically produce sharp, sudden-looking transitions from gradual, continuously varying underlying conditions. This is the same basic mathematical behaviour found throughout physics wherever a connected network forms from randomly distributed components. Below the threshold, most possible paths through the material remain open. Cross the threshold, and the network of absorbers becomes sufficiently connected that transmission collapses abruptly, even though the underlying absorber density was changing smoothly and continuously the whole time.
The Simulation, Worked Through Directly
A smooth absorber gradient, with no imposed epoch boundary written into the model anywhere, produces transmitted flux declining from 1.000 at low redshift to 0.042 at high redshift. Both the 20% transmission threshold and the 10% transmission threshold get crossed within the same narrow transition window, centred near redshift 6.35, reproducing the sharp-looking opacity rise actually observed, with no single global reionization event coded into the simulation to produce it. The sharpness the standard model treats as proof of a coordinated global transition emerges here directly from percolation mathematics applied to a smoothly varying absorber population.
A Direct Test: Nudge the Density, Watch the Onset Shift
This mechanism makes a specific, checkable prediction that a genuinely fixed global epoch boundary would not make. Modest absorber density changes, tested at 0.78, 1.00, and 1.25 times the baseline density used in the simulation, shift the apparent onset redshift by approximately 0.9 in z. That sensitivity to modest density variation is the signature of a percolation threshold, not a fixed global epoch boundary: a genuine single coordinated event, tied to one specific cosmic condition reached everywhere simultaneously, would not be expected to shift its apparent redshift so readily in response to modest local density adjustments.
Larger Surveys Should Show This Directly
This account produces a further, directly observable prediction: larger surveys will show that the apparent onset of strong Lyman-alpha absorption depends on environment and sightline, not one universal transition redshift, with opacity scatter and proximity-zone behaviour around individual quasars showing stronger environmental dependence than the standard single-epoch model predicts. Under the standard picture, every sightline through the intergalactic medium should show reionization completing at essentially the same redshift, since it's treated as one coordinated global event. Under the percolation account, different sightlines pass through regions of somewhat different absorber density, and should therefore show somewhat different apparent onset redshifts, a specific, testable point of disagreement between the two accounts as observational surveys continue to grow.
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