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theoretical models in cosmology

In cosmology: Einstein’s model
…and isotropic universe had a closed spatial geometry. As described above, the total volume of a three-dimensional space with uniform positive curvature would be finite but possess no edges or boundaries (to be consistent with the first assumption).
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In cosmology: Friedmann-Lemaître models
…both positively curved spaces (“closed” universes) as well as negatively curved spaces (“open” universes). The difference between the approaches of Friedmann and Lemaître is that the former set the cosmological constant equal to zero, whereas the latter retained the possibility that it might have a nonzero value. To simplify…
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In cosmology: Friedmann-Lemaître models
…geometry of space in Friedmann’s closed models is similar to that of Einstein’s original model; however, there is a curvature to time as well as one to space. Unlike Einstein’s model, where time runs eternally at each spatial point on an uninterrupted horizontal line that extends infinitely into the past…
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In cosmology: Friedmann-Lemaître models
For example, in a closed Friedmann model, there could be galaxies that started, when R was small, very close to the Milky Way system on the opposite side of the universe. Now, 10¹⁰ years later, they are still on the opposite side of the universe but at a distance…
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In cosmology: Bound and unbound universes and the closure density
…large timescales in the Friedmann closed and open models and the Einstein–de Sitter model allow a different classification scheme than one based on the global structure of space-time. The alternative way of looking at things is in terms of gravitationally bound and unbound systems: closed models where galaxies initially separate…
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In cosmology: Bound and unbound universes and the closure density
…the universe is bound (closed) and, though currently expanding, will end in a crush of unimaginable proportion. If it is less, the universe is unbound (open) and will expand forever. The result is intuitively plausible since the smaller the mass density, the smaller the role for gravitation, so the…
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