Cosmological Anisotropy and Peculiar Motion
Summary
The standard cosmological model rests on the assumption that, on the largest scales, the Universe is homogeneous and isotropic. Cosmological anisotropy and peculiar motion probe departures from this ideal, testing whether structures or flows impart measurable directional signals in the cosmic microwave background (CMB) or in the distribution of galaxies and quasars. Peculiar motion describes the velocity of an object or group of objects relative to the smooth Hubble expansion, and bulk flows trace collective motions induced by large-scale density inhomogeneities. Observationally, these effects appear as dipole anisotropies in number counts, brightness or redshift‐parameter distributions. Precise mapping of anisotropy and peculiar velocities offers a window on the matter distribution beyond galaxy surveys, constrains models of dark energy and structure formation, and assesses the fidelity of the cosmological principle. Discrepancies or confirmations of kinematic dipoles linked to the CMB frame carry direct implications for the geometry, dynamics and growth of structure in the Universe.
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Cosmological Anisotropy and Peculiar Motion publication trend
The graph below shows the total number of articles in cosmological anisotropy and peculiar motion across all publications each year (not limited to Nature Index journals).
Technical terms
Cosmological principle: The postulate that the Universe is homogeneous and isotropic on sufficiently large scales.
Peculiar motion: The velocity of an object relative to the uniform expansion of space, arising from gravitational attraction by inhomogeneities.
Dipole anisotropy: A first‐order angular variation in observed quantities (counts, brightness, temperature) characterised by a single preferred direction.
Cosmic microwave background dipole: The temperature anisotropy pattern in the CMB dominated by the kinematic effect of the observer’s motion with respect to the CMB rest frame.
Luminosity function: A statistical description of the number density of sources as a function of intrinsic brightness, crucial for projecting three‐dimensional distributions onto the sky.
References
- Testing the cosmological principle with the Pantheon+ sample and the region-fitting method. Astronomy & Astrophysics (2024).
- Testing the cosmological principle with CatWISE quasars: a bayesian analysis of the number-count dipole. Monthly Notices of the Royal Astronomical Society (2023).
- The Universe is Brighter in the Direction of Our Motion: Galaxy Counts and Fluxes are Consistent with the CMB Dipole. The Astrophysical Journal Letters (2022).
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