Hubble's law describes the observation that the vast majority of galaxies in the universe are moving away from us, and the speed at which they are receding is proportional to their distance from us. This phenomenon is often explained using the concept of the expansion of the universe.
According to our current understanding, the universe has been expanding since the Big Bang, which is the initial hot and dense state from which the universe originated. As space itself expands, it carries galaxies and other structures along with it. Picture an inflating balloon with dots representing galaxies on its surface—the dots move away from each other as the balloon expands.
Hubble's law, formulated by astronomer Edwin Hubble in the 1920s, mathematically expresses the relationship between the recessional velocity (speed at which a galaxy is moving away) and the distance of a galaxy from us. The law states that the recessional velocity of a galaxy is proportional to its distance:
v = H₀d,
where v is the recessional velocity, d is the distance, and H₀ is the Hubble constant, which represents the rate of expansion of the universe.
The Hubble constant determines the slope of the relationship between distance and recessional velocity. Its current value is estimated to be around 70 kilometers per second per megaparsec (km/s/Mpc). This means that for every 3.26 million light-years (1 megaparsec) farther away a galaxy is from us, it appears to recede an additional 70 km/s faster.
It's important to note that Hubble's law describes a relationship between the observed recessional velocity and the distance due to the expansion of space itself, not the motion of galaxies through space. In this expansion, there is no physical limit that restricts recessional velocities to be slower than the speed of light. The recessional velocities can indeed exceed the speed of light because it is the expansion of space itself that is causing the apparent motion.
However, it is crucial to understand that Hubble's law applies to the large-scale structure of the universe. On smaller scales, gravity dominates and can cause galaxies and other structures to move differently. The expansion of the universe doesn't play a significant role in the local interactions of galaxies within clusters or other gravitationally bound systems.
In summary, the apparent motion of galaxies moving away from us at speeds greater than the speed of light, as described by Hubble's law, is a consequence of the expansion of space itself. It does not violate the laws of physics, as it is the metric expansion of the universe that leads to this observation.