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The Big Bang Theory is a much more general and less specific description of our theory about the origin of the Universe than the $\Lambda{\rm CDM}$ model (by the way, I don't think that the hyphen is written in that acronym). The Big Bang Theory says that the Universe was expanding and the distances between two places where galaxies sit today used to be ...


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A fine tuning problem is only a problem if we require that the considered model is a good or complete model of how we think the universe behaves. In this case, it appears that we require the density of the universe to be as close as 1 part in $10^{64}$ to the exact density that will make it a flat universe that will expand forever, asymptotically coming to ...


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One of ideas associated with string theory is the ekpyrotic universe. This starts with brane cosmology i.e. the idea that our universe is a four dimensional brane floating around in the ten dimensional string theory spacetime. There will be many such brane worlds and the ekpyrotic idea is that a collision between two branes would appear just like the Big ...


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The answer is that we don't know. Why? Because the theory of gravity which we have and use, GR, has a singularity. Things which should be finite in a physical theory, like the density, become infinite. And theories with a singularity are simply wrong, they need a modification, and this modification is necessary not only at the singularity itself, but already ...



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