This is a completely fair question, but actually a difficult one to answer. The 'problem' is that there are a great many contingencies that have to come together to generate a severe contradiction. That's how good Big Bang theory seems to be. It is like asking for 10 things that contradict quantum mechanics. Here is my list of the 'weak' points of Big Bang cosmology:
1...Big Bang theory is based on general relativity. General relativity predicts how gravity and space-time operate. If it is found that GR is incomplete as a classical theory, then Big Bang could be doomed because all current cosmological solutions or 'models' are based on solving Einstein's original, general relativistic formula for gravity. So far, 5 tests have been performed and compared with GR predictions and it has been found that Einstein's original version remains the simplest version of GR. In the next few years, a completely new test of GR will happen which will search for 'gravito- magnetism'. If it is not found, GR is in severe trouble, and so will be Big Bang theory.
2...A basic feature of Big Bang theory is that the universe has a specific age given its expansion rate. As you know, the ages estimated for the oldest known stars are between 12 and 17 billion years, however if the Hubble Space Telescope estimates for the local expansion rate are correct, an age for the universe closer to 12 billion years is predicted. There are only two known ways to save this disagreement, either the galaxies used to measure 'Hubble's constant' are too local to measure the true, slower expansion rate, or there is a new cosmological 'anti-gravity' force present produced by the so-called cosmological constant. If it should come to pass that the expansion speed for the most distant galaxies holds fast to 60 - 65 kilometers/sec/megaparsecs AND the ages of stars holds to 12-17 billion years AND observations can eliminate the cosmological constant as being negligible THEN Big Bang theory is in severe trouble since you cannot have stars older than the Big Bang.
3...If some new processes is discovered that can REDUCE the cosmological abundance of deuterium, then one could have the situation that Big Bang theory predicts MORE deuterium than can be accommodated by the expansion rate and baryonic density of the universe. This would be a severe contradiction with Big Bang nucleosynthesis calculations which already give excellent agreement between deuterium and plausible expansion rates today.
4...If a new explanation for the cosmological redshift is discovered, the existing redshift which leads to plausible solutions from GR, would be over estimating the cosmological effects which are already consistent with other types of observations.
5...If a new mechanism is found for producing a smooth cosmic background radiation, then the uniformity of the measured cosmic background would not be explained by Big Bang theory, or plausible extensions of it.
6...If a new, long-range force is discovered, then Big Bang theory will have been incompletely describing the dynamics of the expansion process. This could work either in favor of Big Bang, or contradict it depending on the strength of this 'fifth force'.
7...If dark matter is found to be baryonic, and exceeds the amount of baryonic matter upon which the Big Bang nucleosynthesis calculations are based, then there is something severely wrong with Big Bang theory predictions during the first 10 minutes after the Big Bang...a time period that is recognized as a 'piece of cake' for physics because of its simplicity.
8...If galaxies are found to have formed earlier than the first million years after the Big Bang, this would conflict with Big Bang theory and the Inflationary extension of it because the predicted strength of the seed fluctuations at galactic scale would be stronger than the theory allows.
9...If something like Inflationary Big Bang theory is not vindicated, then there will be no simple solution to the problem of how the cosmic background radiation can be so smooth and at the same temperature at scales larger than a degree, since at earlier times in the expansion, regions of the sky farther apart than this would not have been in communication to coordinate their temperatures so exactly.
10...If 'dark matter' is discovered and found to have undergone a complex history during the nucleosynthesis epoch, there might be the opportunity for this dynamically important component to have affected the Big Bang abundance calculations, which currently give acceptable ratios for helium and deuterium abundances. Any dark matter perturbation could make these predictions in error.
So, this is my list. It is not comprehensive. I want to warn you that these are weaknesses, but currently NONE have been established from the data at hand to be actual problems for Big Bang theory. This makes Big Bang theory a falsifiable theory of the highest caliber, and still makes it the leading candidate for THE theory of how our universe works. Thus far, Big Bang theory or versions of it, have survived a number of important tests. Whether it continues with this track record for the next few decades, we will have to await the outcomes of some important, on-going and planned experiments.
Copyright 1997 Dr. Sten Odenwald
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