Equation 1 · Part 3 · From Origins to Frontier: A History of Precision Cosmology
Symbol D_A
What this part means
the angular diameter distance to that same epoch.
Its job in the formula
occurs below the fraction bar. The quantity above the bar is divided by this expression; zero is excluded as a denominator.
Full expression→Symbol D_A→Article meaning
Where the article explains it
where is the physical size of the sound horizon at recombination — how far a pressure wave in the primordial plasma could travel before the plasma became neutral and transparent — and is the angular diameter distance to that same epoch.
The passage around this formula
…on it: . where is the physical size of the sound horizon at recombination — how far a pressure wave in the primordial plasma could travel before the plasma became neutral and transparent — and is the angular diameter distance to that same epoch. Planck measures directly from the angular spacing of peaks in the CMB power spectrum to extraordinary precision; converting that angle into a present-day expansion rate requires assuming a cosmological model (matter content,…
Learn the underlying idea
A subscript is a label attached below a symbol. It often selects a time step, component, category, or member of a sequence.
Open the illustrated subscripts: which member of a family? guide →
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Sources cited in the surrounding passage
These citations provide research context; check each source for the exact claim it supports.