How do we know for certain that space is expanding? Let\'s say that in the year

Question

How do we know for certain that space is expanding?

Let's say that in the year 1950, we observe that galaxy 1 is 5 billion light years away from us and galaxy 2 is 10 billion light years away from us, putting both galaxies at a distance of 5 billion light years from each other. Then in 2013, we observe that they are now 7 billion light years away from each other so we conclude space is expanding.

We see that galaxies are move away from each other but how does that prove that space is expanding? Could this illusion of expanding space simply be due to something larger in mass pulling the farther galaxy 2 away from galaxy 1? This larger mass could be accelerating the velocity of galaxy 2 faster then it's accelerating the velocity of galaxy 1.

Explanation / Answer

The expansion of the universe is not based on the observation of one or two galaxies moving away from us. Basically all stars and galaxies we have observed seem to follow the pattern known as Hubble's law, i.e. the further away they are the faster they are moving from us. Sure, for the observation of a single galaxy, there could be some unknown local effect, but it's not very likely that there would be such an explanation for every single galaxy. There are also additional proof of an expanding universe.

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