16// Hubble's law

Hubble's law explains that galaxies move away from us proportionally to their distance. It's also the first evidence for the Big Bang theory. 

If you imagine a balloon with three dots on it (A, B, C), and dot B on the balloon will be twice as far as dot C from dot A, if you inflate the balloon, you'll see that dot B moved from dot A twice as fast than point C. This is what Hubble's law is about, but instead of dots it's galaxies, and the balloon is space.

Hubble's law is expressed by the equation:

 v = H₀D

What does this mean? 

We can imagine it again as the balloon and the dots. H0 is Hubble's constant and it's the value for the rate of the expansion of the universe. We will imagine this as the balloon's surface expanding. Hubble's constant is measured in (km/s)/Mpc. (I will explain what a parsec is later)

D is the distance between us (dot A) and another object (dot B). It includes the expansion of the universe (the balloon).

v is the recessional velocity, typically measured by redshift. We will imagine this as the speed at which dot B was moving away from dot A. 

If we put the equation into words, it would be: recessional velocity (eg. of a galaxy) equals the Hubble's constant times proportional distance (eg. distance of a galaxy from us).

What is a parsec?

A parsec is the biggest unit of length we have. A parsec is a distance where an AU is 1 arc-second or 1/3600 of a degree. Here's a picture of that so you can visualize it (or a video): 

A megaparsec (Mpc) is this distance times 1 000 000.

this is everything I have to say, you can correct me or add anything in the comments!!

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