It was 1929. Edwin Hubble was observing a dozen of galaxies. He recorded the data and found out that all the galaxies he observed are moving away from us. In fact they are moving away from everything*! It was a shocking discovery. It broke prevailing philosophical prejudices that that universe is fixed, static, and unchanging. It showed that that our universe is not in steady state.
BREAD AND RESIN MODEL OF UNIVERSE:
This was not I saw on Discovery Channel and this is not I like. But since it was similar to what I saw and this was the only relevant image I found so I inserted it. Now, this is a brown bread (or a balloon as I saw on TV). This balloon represents the space of the universe and the resins (coins as I saw it on TV) represents the galaxies. In the first clip, the universe is small and distance between two pairs of galaxy is 5cm and 10 cm. In next clip, when universe expands the distance between the these pairs, becomes 10 cm and 20 cm. Same way when our cosmos expands it moves each and every galaxy away from each other**.
MISCONCEPTION REGARDING UNIVERSE AND RELATIVITY:
Another thing to notice in figure is that, more the distance of the 2 galaxies, at faster rate will they move away. Research shows that galaxies 100 million light years (mly) away move about 5.5 million miles per hour, those which are 200 million light years apart move at 11 miles per hour and similarly those 300 mly move at thrice speed of 16.5 miles per hour. So a light traveling from galaxies which are at infinite distances are actually traveling more than 299 792 458 m/s so that they can cover the extra distance constantly generated by expanding space and the receding galaxy. You must be thinking that this contradicts special relativity. But in fact it does not. Special relativity tells us that an object cannot travel with velocity greater than that of light through the space, not with the space.
This topic will be continued. It will include some more topics. To learn more wait for my next post.
* Hubble made this observation by studying red shift in the light spectrum. Here is a link to know about red shifts : Redshift
** Problem in this model is that we take only those galaxies which are on the surface, and not the one which are inside the surface or outside. But this model gives a clear and easy pictures. Rigorous mathematics can be used to show that same thing is valid for 3D perspective, but that does not gives us a clear picture. Therefore physicist stick on this model as it is easy to visualize.