What is the meaning of Hubble’s Law, which says that all the galaxies in the universe are moving away from Earth at velocities that are proportional to their distances from Earth?

Hubble’s Law leads us directly to conclude that the universe is expanding and that it had a beginning – the Big Bang!

To understand this, consider a simpler, down-to-Earth example. Imagine that we were on the top of the Seattle Space Needle (on a clear day in Seattle) using a telescope to look at traffic leaving the city for a long weekend. Also imagine that we discover that every vehicle on the road happens to be moving away from us at a velocity proportional to their distance (Hubble’s Law v = H d). Thus if we see a minivan 75 miles away going 25 mph, we know H = 1/3. And if we see a sports car 150 miles away, it must be going 50 mph per Hubble’s Law. If none of the vehicles changed their velocities, then where were they 3 hours earlier? Since the minivan covers 75 miles in 3 hours (25 mph times 3 hours), and it is 75 miles away now, it must have been at the base of the Space Needle 3 hours ago. Similarly, the sports car covers 150 miles in 3 hours, so it too was at the Space Needle 3 hours ago. In fact, if every vehicle on the road is obeying Hubble’s Law, then every one of them must have been in the same place 3 hours ago (probably when everyone got off work). This must be true because we can rearrange Hubble’s Law to read d – v/H = 0.

What’s all that got to do with the universe? If all the galaxies are moving away from us at velocities proportional to their distances (Hubble’s Law), as we observe that they do indeed, then all the galaxies must have been in one place at one time long ago – we call that moment the Big Bang.

Hubble’s Law leads us directly to conclude that the universe is expanding and that it had a beginning – the Big Bang!

To understand this, consider a simpler, down-to-Earth example. Imagine that we were on the top of the Seattle Space Needle (on a clear day in Seattle) using a telescope to look at traffic leaving the city for a long weekend. Also imagine that we discover that every vehicle on the road happens to be moving away from us at a velocity proportional to their distance (Hubble’s Law v = H d). Thus if we see a minivan 75 miles away going 25 mph, we know H = 1/3. And if we see a sports car 150 miles away, it must be going 50 mph per Hubble’s Law. If none of the vehicles changed their velocities, then where were they 3 hours earlier? Since the minivan covers 75 miles in 3 hours (25 mph times 3 hours), and it is 75 miles away now, it must have been at the base of the Space Needle 3 hours ago. Similarly, the sports car covers 150 miles in 3 hours, so it too was at the Space Needle 3 hours ago. In fact, if every vehicle on the road is obeying Hubble’s Law, then every one of them must have been in the same place 3 hours ago (probably when everyone got off work). This must be true because we can rearrange Hubble’s Law to read d – v/H = 0.

What’s all that got to do with the universe? If all the galaxies are moving away from us at velocities proportional to their distances (Hubble’s Law), as we observe that they do indeed, then all the galaxies must have been in one place at one time long ago – we call that moment the Big Bang.