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How do we know that the universe is expanding?

How do we know that the universe is expanding, and it’s not just the case that objects are moving around in a fixed volume? Given that the universe is expanding, where is the edge, and what is it expanding into?
Kat Parkes from Shropshire (Age 25-34)
Hector Yapp from Staffordshire (Age:5-14)

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One Response

  1. Edwin Hubble’s observations in the 1920s showed that

    (a) our Universe contained many galaxies beyond our own Milky Way
    (b) most of the those Galaxies appeared to be moving away from us with a speed proportional to their distance.

    If the galaxies were moving around in a fixed volume, then why were they moving away from us? What was so special about our location?…

    Hubble’s discovery made sense if, instead of the galaxies moving through space, it was space itself which was expanding between them. exactly as predicted within the theoretical framework, General Relativity, which Einstein had developed about a decade earlier.

    A common metaphor for this expansion is to think of dots stuck onto the surface of a balloon. This metaphor is useful since, from *any* dot’s point of view it will look as if it is the centre of the expansion and the rest of the dots are moving away from it. The expanding balloon picture is also useful in that the surface of the balloon has no edge, just as there need be no edge to the Universe. However we can, of course, see the space into which the balloon is expanding; in the case of the Universe, on the other hand, there doesn’t have to be any exterior “superspace” into which it is expanding.

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