Peculiar Velocities in Cosmology
Summary
Peculiar velocities denote the motions of galaxies or matter beyond the uniform expansion of the Universe. While the Hubble–Lemaître law describes a smooth recession of galaxies with distance, real structures experience additional gravitational pulls from neighbouring overdensities or underdensities. These departures from pure expansion encode vital information on the distribution of dark matter, the growth of cosmic structure and the underlying theory of gravity. By mapping peculiar motions on both small and large scales, cosmologists can infer the amplitude and scale-dependence of density fluctuations, test models of modified gravity and refine estimates of fundamental parameters such as the Hubble constant and the matter density parameter. In the local Universe, measurements of bulk flows and dipole variations reveal the influence of prominent superclusters and voids, while on larger scales pairwise velocity statistics trace the transition to linear perturbation theory. Together, these observations underpin efforts to reconcile discrepancies between early-Universe and late-Universe probes and to chart the dynamical history of cosmic structure formation.
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Peculiar Velocities in Cosmology publication trend
The graph below shows the total number of articles in peculiar velocities in cosmology across all publications each year (not limited to Nature Index journals).
Technical terms
Peculiar velocity: The component of an object’s velocity arising from gravitational interactions, in excess of the uniform cosmic expansion.
Bulk flow: The coherent motion of galaxies within a given volume, reflecting the net gravitational pull of large-scale structures.
Pairwise velocity: The relative velocity between pairs of galaxies, averaged over separations, used to probe density fluctuations and cosmological parameters.
Growth rate (fσ₈): A dimensionless measure of the rate at which cosmic structures grow, combining the linear growth rate f and the normalisation σ₈ of the matter power spectrum.
Hubble constant (H₀): The present-day expansion rate of the Universe, expressed in units of velocity per distance (e.g. km s⁻¹ Mpc⁻¹).
References
- Measuring the Hubble Constant through the Galaxy Pairwise Peculiar Velocity. The Astrophysical Journal Letters (2024).
- Bulk Flow Motion Detection in the Local Universe with Pantheon+ Type Ia Supernovae. The Astrophysical Journal (2024).
- Gravity in the local Universe: Density and velocity fields using CosmicFlows-4. Astronomy & Astrophysics (2023).
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