How much matter is "missing" for the universe to end in a big crunch?
Present best estimates are that the universe contains only about 10 to 20 percent of the mass density required to close the universe.
Actually those who want Omega=1 are trying to avoid special conditions by positing something physical that happened to force the universe into the special state of Omega=1, no matter how it began. This is an important reason for the popularity of the inflationary model.
The inflationary model attempts to explain why the universe appears so homogeneous and isotropic and so flat. In the standard model these conditions have to be built into the initial conditions. In the inflationary addition to the standard model these conditions arise as a result of the inflationary epoch.
Given inflation at the beginning of this universe describes a period of rapid accelerated universal expansion, which slowed, why is universal expansion accelerating again?
It has not yet been completely established that the expansion is accelerating currently (although that is one interpretation of some recent data). Considerable further work and observational data will be required to establish whether or not the universe is truly accelerating (Chapter 12). Nor, for that matter, has it been completely established that inflation occurred (Chapter 15). However, in either case the source of the acceleration is what is referred to in the Einstein field equations as the cosmological constant term (Chapter 11). Several interpretations can be given to the cosmological constant - in inflation it is a vacuum energy density. Any present cosmological acceleration could also be caused by a vacuum energy density which has a value different from that during inflation. Or it could be caused by something else that acts like a negative energy density.
In the chaotic inflation model other fluctuations could have expanded in to other universes.
Only by inference from what we can learn from stuff inside our past light cone.
Copyright © 1998 John F. Hawley