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In theory, the expansion of the universe should be a pretty smooth process; if one galaxy is twice as far from us as another, it should appear to recede from us twice as rapidly. In reality, many factors conspire to make this only approximately correct. One reason this happens has to do with the nearby cluster of galaxies in Virgo. Its gravitational pull modifies the velocity of the Milky Way as it travels through the cosmos (and the velocity of some other galaxies, as well). If you compute that effect and subtract it out, then you get galaxy velocities that better match the Big Bang theory. At least one source, the Principal Galaxy Catalog (PGC), often lists such velocities corrected for this Virgo infall.

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