Big Rip - dark energy increasing, an ultimate expansion of the universe to the extent that the particles and space and time of which it consists are torn apart.
Constant Dark Energy - as space expands, the dark energy density remains constant, rather than decreasing or increasing. As a result, after the Universe has expanded for long enough, dark energy comes to dominate the energy budget of the Universe.
Big Crunch - Dark energy decreasing, gravity takes over - a contraction of the universe to a state of extremely high density and temperature (a hypothetical opposite of the Big Bang).
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Soviet mathematician Alexander Friedmann's model gave rise to three different types of models for the evolution of the Universe.
First, the Universe would expand for a given amount of time, and if the expansion rate is less than the density of the Universe (leading to gravitational attraction), it would ultimately lead to the collapse of the Universe at a later stage.
Secondly, the Universe would expand, and at some time, if the expansion rate and the density of the Universe became equal, it would expand slowly and stop, leading to a somewhat static Universe.
Thirdly, the Universe would continue to expand forever, if the density of the Universe is less than the critical amount required to balance the expansion rate of the Universe.
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