Why is the hubble law considered one of the most important discoveries in th ehistory of astronomy?

Answer 1

Hubble's law is a statement in physical cosmology which states that the redshift in light coming from distant galaxies is proportional to their distance. The law was first formulated by Edwin Hubble and Milton Humason in 1929 after nearly a decade of observations. It is considered the first observational basis for the expanding space paradigm and today serves as one of the most often cited pieces of evidence in support of the Big Bang.

Answer 2

It prompted Lemaitre to come up with the Primeval Atom hypothesis, from which modern cosmology has evolved.

The theory that grew out of the hypothesis (later named "Big Bang" by its opponents) explains what we see in the universe by supposing that the universe has been expanding from an unbounded state of temperature/density/energy at some point in the past.

It also provides a "quick" way to determine the relative size of the part of the universe that we can see, and to measure the distance (and "look-back time) to the furthest galaxies and even to the Cosmological Microwave Background radiation.

If one galaxy appears to recede from us at twice the speed of another galaxy, then the "fast" galaxy is twice as far as the slower one. And we see it twice as "far" in the past.

Even is the scaling is wrong (50 km/s per Megaparsec or 70 or 100?), the ratio remains the same: one galaxy is still twice as far as the other one. This allows us to compare the two galaxies (luminosity, linear size, stage of evolution) and the comparisons still remain OK even if our scaling of the universe is uncertain.