Rake said:
And this made me wonder how it is possible that the cosmic acceleration will eventually revert back to a deceleration if the attractive gravitational effects of matter will weaken as space expands whereas the repulsive gravitational effect of the cosmological constant will stay the same?
As I said, it depends on your model of DE.
Before the discovery of 'fainter than expected' Type Ia Super Novae at about z = 1, which is normally interpreted as cosmic acceleration, it was believed that there were three possible destinies for the Universe linked to the shape of space: a spherically spatially curved, high density, Universe would eventually recollapse; a hyperbolically spatially-curved low density Universe would expand forever; and the flat spatially uncurved, critical density Universe would continue to slow down but would never recollapse.
While it is now generally thought (the mainstream model) that the universe is flat, because of the WMAP data, the discovery of cosmic acceleration and dark energy has severed the original link between the universe's geometry and destiny.
There are several suggestions as to the nature of the dark energy, and therefore the nature of cosmic acceleration, and so at present the destiny of the Universe in the mainstream model is unknown.
The possibilities in that mainstream model are wide open.
If the dark energy
remains constant, as is consistent with the data presented today, and is in a form which mimics the cosmological constant, then the expansion will continue to accelerate and in a hundred billion years or so we will only be able to see a few hundred galaxies, compared to the hundreds of billions we can see today.
On the other hand, it is also possible that dark energy will
decrease with time so that eventually the normal gravitational fields overwhelm it and lead to a deceleration and recollapse, the 'Big Crunch'. (A nice thought!)
Finally, the most radical possibility has been called the 'Big Rip', where the dark energy
increases with time and within a 100 billion years or so rips apart every galaxy, star and atom in the Universe. (Another nice thought!)
However, there are several caveats to remember.
1. Cosmic acceleration is based on the observation of fainter than predicted distant Type Ia S/N, and there could be other explanations for their lack of apparent magnitude.
2. The mainstream model depends on there being a period of intense inflation in the earliest moments of the BB, this itself depends on there being some fundamental particle, for example the Higgs Boson or Inflaton, which cannot be found even after many decades of intense investigation.
3. The mainstream model depends on there being a non-baryonic Dark Matter particle which cannot be identified and discovered in a laboratory also after much intense investigation.
4. Although the 'raw' version of GR has been well tested in solar system and laboratory experiments, the cosmological constant version, as required by Dark Energy, cannot be so verified. Attempts to identify it with the QM Zero Point Energy field fail by a factor of about 10
120!
5. Inflation was invoked to solve some major coincidences in the horizon, density and smoothness problems of GR cosmology. However we now find that it not only necessarily invokes DM and DE, so far undiscovered by laboratory physics, but does so in such a way that they
at least at present are roughly equal in density. Is this not also rather a coincidence? A theory that explains one set of coincidences by introducing another one is somewhat unsatisfactory.
Garth
