← All parts of this equation

Equation 1 · Part 2 · From Origins to Frontier: A History of Precision Cosmology

Symbol r_*

θ∗=r∗DA(z∗)\theta_* = \frac{r_*}{D_A(z_*)}
r∗r_*

What this part means

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 —.

Its job in the formula

r∗r_* occurs above the fraction bar. The numerator is divided by the entire denominator below it.

Where the article explains it

where r∗r_* 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 DA(z∗)D_A(z_*) is the angular diameter distance to that same epoch.

The passage around this formula

…per second per megaparsec [ 6 ] . The acoustic scale itself follows from a genuine physical model worth writing down explicitly, because the whole CMB-based measurement chain depends on it: θ∗=r∗DA(z∗)\theta_* = \frac{r_*}{D_A(z_*)}. where r∗r_* 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 DA(z∗)D_A(z_*) is the angular diameter distance to that same epoch. Planck measures θ∗\theta_* directly from the angular…

Read this part in the article →

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 →

See this notation across published equations →

Sources cited in the surrounding passage

These citations provide research context; check each source for the exact claim it supports.