The concept of the 'tooth fairy' in theoretical physics is a metaphorical cautionary tale about the dangers of over-speculation and the importance of empirical evidence. It emphasizes the need for new ideas to be grounded in existing data and to make novel, testable predictions. The author uses the idea of the tooth fairy to illustrate the principle of 'Occam's razor' in physics, which suggests that the simplest explanation is often the best. This principle is used to evaluate new theories and ideas, which are compared against existing data and theories. The author argues that a single 'tooth fairy' is acceptable, but invoking multiple 'tooth fairies' (i.e., introducing multiple new theories or ideas that are not supported by existing data) can lead to an unscientific fantasy. The author provides several examples of how this principle is applied in cosmology, including the debate over dark matter versus modified gravity, and the evaluation of new theories like cosmic inflation and tired light. The author concludes that while it is acceptable to explore new ideas and theories, they must be grounded in existing data and make novel, testable predictions. Only then can they be considered worthy of serious consideration and further exploration.
