According to observations made of the motion of distant nebulae (clouds of gas in outer space) the Universe is expanding. How can we infer the Universe’s expansion from the motion of nebulae?
In 1929 Edward Hubble released a paper titled ‘A relation between distance and radial velocity among extra galactic nebulae’. In other words, Hubble wrote about how the speeds (directly away from us) of large clouds of gas outside our galaxy (a cluster of dark matter, gas, dust and stars which are all bound by their gravitational pull on one another) change according to the distance such gas clouds are from us.
This graph was published in Hubble’s paper, and demonstrates a fairly linear proportionality between distance and velocity (nebulae that are further away from us move faster from us).
Such findings come from observing how bright the nebulae are (for distances) and velocity presumably from how the observed energies of light from the nebulae differ from the expected energies (not detailed in the paper). So, celestial bodies (which are particularly large, for instance galaxies) move away from one another at accelerating rates. There is little reason to say that gravity is responsible for them moving apart, because gravity is a force which only attracts large bodies (it doesn’t repel them).
Now imagine each dust cloud was a raisin in a fruity dough. Put the dough in the oven and the dough expands (the raisins get further apart). It’s key to note that the motion of the dough causes the raisins to move apart. One can argue the raisins are not moving on their own volition, instead they are moved because of the dough moving. The same can be said of space itself; space is stretching at an accelerating rate and the nebulae raisins are testament to that. How delicious is the Universe! Taken to its logical conclusion, the evidence suggests space must have began where at a single point all matter was condensed so much so that the point had infinite density — the origin of our Universe. This can be dealt with another time.
Key to today’s learnings is the fact that space is being stretched at an accelerating rate.
http://www.pnas.org/content/15/3/168.full (Hubble’s paper).