退行方向上的运动速度称为退行速度。退行速度是经由测量被观测到的星系发射的电磁辐射红移得到的，因此星系的距离是用哈勃定律估计的值。
Hubble(99年)公布了遥远的星系退行速度正比于它们到我们的距离的划时代观测事实。
Hubble (99) published the epochal observational fact that the receding speed of galaxies is proportional to their distances away from us.
他们得出的结论均为：远距离的超新星的退行速度要比只有物质（暗物质或非暗物质）作为引力作用源的预期速度要低。
Both papers reached the same conclusion: Distant supernovae are receding at a slower rate than you'd expect if matter (dark and nondark) was the only source of gravitational action.
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