Chapter 13 Questions
Question What is the fate of the universe? Could it be determined if the Hubble constant were known for sure?
Answer

If we knew the Hubble constant for certain, we would still need to know additional parameters to know the fate of the universe such as age, mass density, geometry, and whether there is a nonzero Lambda force.

Question How far away are we from knowing whether the universe is flat, spherical, or open?
Answer

We will have a pretty certain answer right now from the WMAP data. The universe seems to be flat. We have made a lot of progress on this question in recent times and telescopes and detectors are getting better and better, so we can expect that the remaining experimental uncertainty in this answer will rapidly decrease.

Question What is the fate of the universe? Do we have a notion as to its end?
Answer Taking the concordance model as the description of the universe we know that the universe will be increasingly dominated by the Lambda force.  If this acts like a usual cosmological constant the expansion rate will continue to accelerate. Gravitationally bound entities like galaxy clusters will be increasingly isolated. The stars in them will die out leading eventually to a "heat death" sort of universe. However, it is possible that the Lambda force isn't a truly "constant" force. If the term gradually decreases in strength then, unless it becomes negative, the universe will still end up expanding forever. If the Lambda force increases in strength there is the possibility of the "Big Rip." This highly speculative model holds that eventually the Lambda force would be larger than any other force, including the strong interaction that holds matter together at the subatomic level. In this picture all matter would be torn apart. The universe would expand forever without any matter in it at all. As a philosophical or practical matter, do you think this would make any difference from any other heat death fate?
question The density of the universe determines its fate. When calculating density do cosmologies take into account the total energy as well as mass? Is mass-energy conserved in the universe?
Answer Yes, you must take into account the total mass-energy density. One can compute an Omega term for radiation. While this is important in the early universe, at the present time the energy contribution from the microwave background is the only significant energy component and it is small compared to the matter density. Because the photons in the CBR lose energy due to redshift, mass-energy doesn't seem to be conserved in the overall expansion of the universe.
Question What are the recent observations involving gamma ray bursters? Any new conclusions?
Answer

The major controversy over gamma-ray bursters used to be whether they are of cosmological, i.e. extragalactic, origin, or whether they come from within the Galaxy. Obviously, this has a bearing on the identity of what is producing the bursts. The distribution of bursters proved to be quite isotropic, which suggests an extragalactic source, since the Galaxy is quite anisotropic, except possibly for its dark halo. However, some galactic sources can be remarkably isotropic, so that does not completely settle the question. Observations have suggested evidence of cosmological time dilation in gamma ray bursts, implying that they are occurring at high redshifts.

Now there are optical identifications of gamma ray burst sources. These sources were then studied with the HST and the Keck Telescope. It is more and more evident that gamma ray bursts are associated with high redshift galaxies. Further evidence suggests that gamma ray bursts originate with something called a hypernova, which is an especially energetic supernova explosion of a very massive star.
Question How much matter is "missing" for the universe to end in a big crunch?
Answer

Present best estimates are that the universe contains only about 10 to 20 percent of the mass density required to close the universe. In the absence of a Lambda term you would need 3 to 4 times more mass. With the Lambda force you would need even more so that the overall geometry is spherical and the Lambda value is less than the Lambda critical value of the Einstein static model.
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Copyright © 2005 John F. Hawley