Redshift
Definition: The fractional change between observed and emitted wavelength after identifying the same spectral feature.
Why it matters here: It is required to answer What does a measured redshift actually tell us? and to distinguish direct observation from cosmological inference.
Relationships: Read it with Scale factor, the model \(1+z = a(t_0)/a(t_e)\), and the evidence statement Spectroscopic line identification supplies wavelength ratios; the Pantheon+ excerpt provides heliocentric/CMB-frame redshift fields for supernovae.
Boundary or common confusion: At low z, peculiar velocities can be a substantial fraction of cz. Redshift alone is not a distance measurement.
Scale factor
Definition: The time-dependent relative size assigned to spatial separations between comoving observers in an FLRW model.
Why it matters here: It is required to answer What does a measured redshift actually tell us? and to distinguish direct observation from cosmological inference.
Relationships: Read it with Cosmological redshift, the model \(1+z = a(t_0)/a(t_e)\), and the evidence statement Spectroscopic line identification supplies wavelength ratios; the Pantheon+ excerpt provides heliocentric/CMB-frame redshift fields for supernovae.
Boundary or common confusion: At low z, peculiar velocities can be a substantial fraction of cz. Redshift alone is not a distance measurement.
Cosmological redshift
Definition: Wavelength stretching associated with the change in scale factor between emission and observation.
Why it matters here: It is required to answer What does a measured redshift actually tell us? and to distinguish direct observation from cosmological inference.
Relationships: Read it with Peculiar velocity, the model \(1+z = a(t_0)/a(t_e)\), and the evidence statement Spectroscopic line identification supplies wavelength ratios; the Pantheon+ excerpt provides heliocentric/CMB-frame redshift fields for supernovae.
Boundary or common confusion: At low z, peculiar velocities can be a substantial fraction of cz. Redshift alone is not a distance measurement.
Peculiar velocity
Definition: Motion relative to the local comoving expansion flow.
Why it matters here: It is required to answer What does a measured redshift actually tell us? and to distinguish direct observation from cosmological inference.
Relationships: Read it with CMB frame, the model \(1+z = a(t_0)/a(t_e)\), and the evidence statement Spectroscopic line identification supplies wavelength ratios; the Pantheon+ excerpt provides heliocentric/CMB-frame redshift fields for supernovae.
Boundary or common confusion: At low z, peculiar velocities can be a substantial fraction of cz. Redshift alone is not a distance measurement.
CMB frame
Definition: A redshift reference frame corrected for the observer's motion relative to the microwave-background dipole.
Why it matters here: It is required to answer What does a measured redshift actually tell us? and to distinguish direct observation from cosmological inference.
Relationships: Read it with Redshift, the model \(1+z = a(t_0)/a(t_e)\), and the evidence statement Spectroscopic line identification supplies wavelength ratios; the Pantheon+ excerpt provides heliocentric/CMB-frame redshift fields for supernovae.
Boundary or common confusion: At low z, peculiar velocities can be a substantial fraction of cz. Redshift alone is not a distance measurement.