Equation 7 · Part 1 · 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
is a part of this expression. Its role is fixed by the surrounding article and by the operations shown in the formula.
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
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 article section
- [5] Nine-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Cosmological Parameter Results ↗
- [6] Planck 2018 results. VI. Cosmological parameters ↗
These citations provide research context; check each source for the exact claim it supports.