The discussion centers on the implications of the Cosmic Microwave Background Radiation (CMBR) and observations of Type Ia supernovae for the geometry of the universe, specifically contrasting flat and open universe models. It explores theoretical aspects of cosmic expansion, including the role of dark energy and cosmological constants.
Participants express differing views on the implications of CMBR and Type Ia supernova observations for the universe's geometry. There is no consensus on whether the universe is flat or open, and the role of dark energy remains a point of contention.
The discussion includes assumptions about the presence of dark energy and the implications of different cosmological models, which are not fully resolved. The relationship between the geometry of the universe and the behavior of cosmic expansion is complex and not definitively established in this exchange.
Willfrid Somogyi said:But in a flat universe is expansion not supposed to slow exponentially, stopping after an infinite time?
Willfrid Somogyi said:the universe's expansion to be accelerating, which would point to an open universe.
PeterDonis said:Only if there is no cosmological constant/dark energy.
No, it points to there being a nonzero cosmological constant/dark energy. An open universe with zero cosmological constant would still have slowing expansion; but it would never slow to a complete stop, even after an infinite time. The only way to get accelerating expansion is to have a nonzero cosmological constant/dark energy; and such models are possible with closed, flat, or open spatial geometry.