Acoustic scale
Definition: The angle subtended by the sound horizon at last scattering.
Why it matters here: It is required to answer Why is the microwave background a distance probe? and to distinguish direct observation from cosmological inference.
Relationships: Read it with Sound horizon at recombination, the model \(theta_* = r_s(z_*)/D_M(z_*)\), and the evidence statement The official Planck Legacy Archive binned TT/TE products were requested during production but could not be retrieved (TT extraction failure; TE HTTP 401). Only the published parameter summary is used; the plotted peak sequence is explicitly theoretical.
Boundary or common confusion: The plotted schematic is not Planck data, and theta_* is not simply the angular size of a rigid object with no recombination physics.
Sound horizon at recombination
Definition: The comoving acoustic propagation distance evaluated near photon decoupling.
Why it matters here: It is required to answer Why is the microwave background a distance probe? and to distinguish direct observation from cosmological inference.
Relationships: Read it with Last scattering, the model \(theta_* = r_s(z_*)/D_M(z_*)\), and the evidence statement The official Planck Legacy Archive binned TT/TE products were requested during production but could not be retrieved (TT extraction failure; TE HTTP 401). Only the published parameter summary is used; the plotted peak sequence is explicitly theoretical.
Boundary or common confusion: The plotted schematic is not Planck data, and theta_* is not simply the angular size of a rigid object with no recombination physics.
Last scattering
Definition: The epoch when the photon mean free path became large enough for the CMB to free-stream.
Why it matters here: It is required to answer Why is the microwave background a distance probe? and to distinguish direct observation from cosmological inference.
Relationships: Read it with Multipole, the model \(theta_* = r_s(z_*)/D_M(z_*)\), and the evidence statement The official Planck Legacy Archive binned TT/TE products were requested during production but could not be retrieved (TT extraction failure; TE HTTP 401). Only the published parameter summary is used; the plotted peak sequence is explicitly theoretical.
Boundary or common confusion: The plotted schematic is not Planck data, and theta_* is not simply the angular size of a rigid object with no recombination physics.
Multipole
Definition: An angular Fourier index whose scale is approximately inverse to angle on the sky.
Why it matters here: It is required to answer Why is the microwave background a distance probe? and to distinguish direct observation from cosmological inference.
Relationships: Read it with Recombination model, the model \(theta_* = r_s(z_*)/D_M(z_*)\), and the evidence statement The official Planck Legacy Archive binned TT/TE products were requested during production but could not be retrieved (TT extraction failure; TE HTTP 401). Only the published parameter summary is used; the plotted peak sequence is explicitly theoretical.
Boundary or common confusion: The plotted schematic is not Planck data, and theta_* is not simply the angular size of a rigid object with no recombination physics.
Recombination model
Definition: The ionization and thermal history needed to predict visibility and acoustic features.
Why it matters here: It is required to answer Why is the microwave background a distance probe? and to distinguish direct observation from cosmological inference.
Relationships: Read it with Acoustic scale, the model \(theta_* = r_s(z_*)/D_M(z_*)\), and the evidence statement The official Planck Legacy Archive binned TT/TE products were requested during production but could not be retrieved (TT extraction failure; TE HTTP 401). Only the published parameter summary is used; the plotted peak sequence is explicitly theoretical.
Boundary or common confusion: The plotted schematic is not Planck data, and theta_* is not simply the angular size of a rigid object with no recombination physics.