Cosmic microwave background
Alternative theories

The standard cosmology that includes the Big Bang "enjoys considerable popularity among the practicing cosmologists" However there are challenges to the standard big bang framework for explaining CMB data. In particular standard cosmology requires fine-tuning of some free parameters, with different values supported by different experimental data. As an example of the fine-tuning issue, standard cosmology cannot predict the present temperature of the relic radiation, T 0 {\displaystyle T_{0}}. This value of T 0 {\displaystyle T_{0}} is one of the best results of experimental cosmology and the steady state model can predict it. However, alternative models have their own set of problems and they have only made post-facto explanations of existing observations. Nevertheless these alternatives have played an important historic role in providing ideas for and challenges to the standard explanation.
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