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The Big Bang model does not give a unique description of our present Universe, but ratherleaves quantities such as the present expansion rate, or the present composition of the Uni-verse, to be fixed by observation.It is a standard practice to specify cosmological models via a few parameters, which onethen tries to determine observationally to decide which version of the model best describesour Universe. In this chapter and the next I’ll discuss the most commonly-considered pa-rameters, including ones we have already seen and new ones.6.1The expansion rate H 0The Hubble constant H 0 tells us the present expansion rate of the Universe, making it avital cosmological parameter as measurements o...

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