Once the Big Bang theory is understood, it is tempting to ask "what happens next?". This is a fundamental question: the Universe is currently seen to be expanding. Will it carry on doing so forever, or will the force of gravity, acting between all the billions of galaxies in the Universe, slow it down? Perhaps the expansion will stop altogether? Maybe it will begin a collapse back to the singularity that existed before? Let us re-state the question: Will the Universe expand forever, slow down to a stop, or start collapsing again?
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