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peculiar velocities [Q&A]


Rakshit (Vidyodaya Public School) and Tilak (SDM College) ask, if Hubble’s Law says all galaxies are moving away from each other, why is it that the Andromeda Galaxy is coming towards us?

Hubble’s Law describes the behavior of space in the Universe. It holds in an average sense, and local effects can cause deviations from it. For instance, objects may be close enough to another body that the resulting gravitational acceleration produces velocities that are comparable or larger than the Hubble flow. These deviations are called Peculiar Velocities. Such is the case with the Milky Way and Andromeda. The Hubble flow induces recession velocities of 72 km/s for every million parsecs. At nearer distances, the Hubble flow is negated by the gravity of the Milky Way for stars in the galaxy (at 10 kpc, the Hubble recession velocity is <1 km/s, compare with the orbital velocity of the Sun of 250 km/s), by the Sun for the planets in our system (the size of the solar system is <10-10 Mpc, leading to a Hubble flow of <1 mm/s at the edge of the solar system; for the Sun-Earth system, it is <1 micron/s, compare with the Earth’s orbital velocity of 30 km/s), etc.

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