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Throughout its history, a number of criticisms have been offered against the Big Bang theory. Some of them are today mainly of historical interest, and have been avoided either through modifications to the theory or as the result of better observations. Others issues, such as the cuspy halo problem and the dwarf galaxy problem of cold dark matter, are considered to be non-fatal as they can be addressed through relatively minor adjustments to the theory. Finally, there are proponents of non-standard cosmologies who believe that there was no Big Bang at all.
Critics cite the abundance of continual ad hoc modifications and refinements to big bang in order for it to comply with observable reality as prima facie evidence that it is a weak theory.
One unanswered question is how the Big Bang might have occurred. The difficulty of answering this question lies with the absence of a theory of quantum gravity. As one goes back in time, the temperature and the pressures increase to the point where the physical laws governing the behavior of matter are unknown. It is hoped that as we understand these laws that we will be better able to answer the question of what happened "before" the Big Bang.
The magnetic monopole problem was an objection that was raised in the late-1970s. Grand unification theories predicted point defects in space which would manifest themselves as magnetic monopoles, and the density of these monopoles was much higher than what could be accounted for. This problem is resolvable by the addition of cosmic inflation, which critics dismiss as yet another ad hoc addition, constituting further evidence that big bang is a weak theory.
The horizon problem results from the premise that information cannot travel faster than light, and hence two regions of space which are expanding at faster than the speed of light relative to each other cannot communicate. This means that there is no mechanism to ensure that they have the same temperature. In the 1970s, no anisotropies had been observed which contradicted non-inflationary theories of the Big Bang. This problem was partially resolved by cosmic inflation which reduced the horizon problem by arguing that the early universe suddenly underwent a period of massive expansion before which regions that would later not be in contact with each other could equalize their temperatures.
However, cosmic inflation predicted that the anisotropies in the Big Bang would be reduced but not eliminated. Even with inflation, there would be regions of space that could not be in thermal contact. In the early 1990s, there was some excitement and nervousness, as satellite detectors such as COBE at first failed to detect any anisotropy, and various inflationary scenarios began to be invalidated. Had another few years passed without any detections of anisotropy, the Big Bang would have been very badly hurt, but this was not the case, and the expected anisotropies were detected.
The horizon problem is still of major interest because it allows one to deduce large amounts of information from the CMB. Different expansion rates will result in different amounts of lumpiness in the CMB as a result of material falling past the horizons at different times, and this provides much data about the conditions within the universe at the time the CMB was formed.
One major issue that had the potential of challenging the Big Bang occurred in the mid-1990s. Computer simulations of globular clusters suggested that they were about 15 billion years old, which conflicted with some values of the Hubble constant suggesting that the universe was 10 billion years old. This issue was resolved in the late 1990s with other new computer simulations which included the effects of mass loss due to stellar winds indicated a much younger age for globular clusters.
During the mid-1990s, measurements of the amount of primordial helium abundance suggested the possibility that the helium abundance of the first stars would have been less than 20%. If this were the case, it would have posed major problems for the Big Bang theory, as it is very difficult to get low amounts of helium from the Big Bang. This potential problem was resolved in the late-1990s by better measurements of helium abundances.
As mentioned earlier, there are also issues with the baryon density and the observation of heavy elements with quasars. These are widely considered to be less serious challenges to the Big Bang, however, they have the potential to undermine the theory if explanations advanced for them prove inadequate.
For example, the consensus is that in order to explain heavy elements in quasars, a large burst of massive star formation is needed, and as of 2004, much current research is aimed at trying to find these stars. If these population III stars are found, this will strengthen the Big Bang theory.
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