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Observations show that galaxies are moving away from one another on the macroscopic scale.

Now, scientists interpret this by saying this happens not because galaxies are really moving away from each other in a static background but because more and more space is created between galaxies.

Now how scientists can distinguish between the 2 scenarios? what made them believe in the 2nd and discard the 1st?

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Some scientists interpret the expansion as space being created, but it's not a universally accepted interpretation by any means. Here's a related question, by the way. – David Z Feb 19 '12 at 19:39
@DavidZaslavsky This is the interpretation by Nima Arkani-Hamed, he stated explicitly that it is the creation of space. Those who adopt one interpretation over another must have strong reasons to discard the other one. – Revo Feb 19 '12 at 21:16
Yes, and if you're interested in looking into that, I'd suggest starting by finding out Nima Arkani-Hamed's strong reason for believing in the creation of space as opposed to motion through space. In any case, just because one prominent scientist accepts one interpretation doesn't mean that scientists in general do. – David Z Feb 19 '12 at 21:34

According to the rules of general relativity, there is no way to measure a difference between the two things. They are different interpretations of the same physical scenario.

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Yes, there's a nice intuitive description of this issue here – twistor59 Feb 19 '12 at 20:35

Objects that are further away accelerate faster than objects that are closer. Quite an odd anomaly for objects moving on a static background. Furthermore, everything in the universe is moving away from everything else dependent on your reference frame. Say I travel 1 light year into space, now I take an observational survey of all of the "fixed" points of light in the space around me. I soon see some of them receding away dependent on my distance from them so it changes with respect to my observations on Earth (Hubble's law). Also, metric expansion only occurs globally, or between galaxies that aren't locally bound. For instance, there is no metric expansion between the Earth and our local cluster of galaxies.

Beyond this, there is the theoretical background which comes from the interpretation of Einstein's Field Equations. Friedmann derived a set of equations which described the non-static universe in Einstein's Field Equations as a change in metric over time.

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It's an anomaly, but if you start with the Roberrtson-Walker spacetime and make the coordinate substitution $R=ra$, replacing $r$ with $R$, it's exactly what you'll see--a static 3-geometry with a ``frame dragging'' effect. – Jerry Schirmer Feb 20 '12 at 13:33

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