Peculiar velocity

Peculiar velocity is the part of a celestial body's motion measured relative to the universe's smooth expansion. It distinguishes local motion caused by gravity from the recession associated with cosmic expansion.

Hubble–Lemaître law: The relation between a galaxy's distance and its recession velocity in an expanding universe. It describes the smooth recession component from which peculiar motion is distinguished.
Peculiar velocity survey: An observational program that estimates galaxies' motions relative to the Hubble expansion. Such surveys map how matter flows through the nearby universe.
Edwin Hubble: The American astronomer whose observations established a relation between galaxy distance and recession speed. The expansion law associated with his work created the baseline for identifying peculiar motion.
Hubble flow: The systematic recession of distant objects associated with the expansion of space. Peculiar velocity is the deviation from this smooth background flow.
Cosmological redshift: The wavelength stretching of light as the universe expands during its journey to an observer. Its signal combines with Doppler shifts caused by peculiar motion.
Tully–Fisher relation: An empirical relation between a spiral galaxy's rotation speed and intrinsic luminosity. It provides galaxy distances needed to infer peculiar velocities from observed redshifts.
Georges Lemaître: The Belgian physicist and priest who derived an expanding-universe model from general relativity. His theoretical account clarified that cosmic recession is distinct from local velocity.
Proper motion: The angular change in a star's position across the sky over time. It is a projected observational motion, whereas peculiar velocity is a three-dimensional velocity relative to expansion.
Doppler effect: The change in observed wave frequency caused by relative motion between a source and an observer. Peculiar velocities produce Doppler shifts that can be separated from expansion redshift.
Fundamental plane: An empirical relation among the size, surface brightness, and velocity dispersion of elliptical galaxies. It supplies distance estimates that help separate local motion from expansion.

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