Hubble's law describes the observation that, on average, galaxies in the universe appear to be moving away from each other. It is based on the empirical finding that the more distant a galaxy is from us, the faster it appears to be receding. This observation is commonly interpreted as evidence for the expansion of the universe.
The expansion of the universe is a fundamental concept in cosmology. It suggests that the space between galaxies is stretching, causing them to move away from one another. It is important to note that it is the space itself that is expanding, rather than galaxies moving through a preexisting space.
The reason why distant galaxies appear to be moving away from us faster than the speed of light is a consequence of the expansion of space itself. The expansion of the universe is not limited by the speed of light because it is a property of the fabric of space. While objects within the universe cannot move through space faster than the speed of light, the expansion of space itself is not constrained by this limitation.
It is crucial to understand that the apparent recession velocity of galaxies is not due to the galaxies themselves moving away at tremendous speeds but rather the result of the expansion of space between them and us. This expansion stretches the wavelength of light as it travels through space, causing a redshift that is observed as a velocity proportional to the galaxy's distance from us, as described by Hubble's law.
It's worth noting that the expansion of the universe does not violate the principle of special relativity, which states that no object with mass can reach or exceed the speed of light within space. The expansion is a property of spacetime itself and is not subject to the same limitations as objects within spacetime.