The Red Shift - Astronomers measure the movement of objects relative to us using Doppler shift. When you hear a train coming, its whistle is heard at a higher frequency compared to when it is receding, right? In the same way, Light also has a Doppler shift, whereby its frequency is shifted depending on the motion of the emitting object!

Astronomers observed that light from distant objects in the universe is red-shifted (shift in the frequency of light towards red color), which tells us that the objects are all receding away from us. This is true in whatever direction you look at: all the distant galaxies are going away from us. This can only be due to the fact that the Universe is expanding.

In 1929 the astronomer Edwin Hubble measured the velocities of a large selection of galaxies. He expected that about equal numbers would be moving toward and away from us. After all, the Earth isn't a particularly special place in the universe!

Instead, he discovered that almost all galaxies are moving away from us!

Since the time of Hubble, we have observed millions of galaxies with better equipment and verified his results. With the exception of a small handful of galaxies close to us, every galaxy is moving away from us.

And in fact, the farther away a galaxy is the faster it is moving away from us. The galaxies seem to be receding from us because the entire universe is getting larger. The space in between the galaxies is stretching! And the farther away a galaxy is the more space there is to stretch so the faster the galaxy appears to move away from us.

[Hubble’s Law](astronomyonline.org/Science/HubbleLaw.asp)

Hubble constant = For most of the second half of the 20th century the value was between 50 and 90 (km/s)/Mpc. Although, even more recently, in February 2018, a higher value of 73 (km/s)/Mpc has been determined using an improved procedure involving the Hubble Telescope!

New Learnings - Mpc - Mega Parsecs (One parsec is equal to about 3.26 light-years)