A cosmology without a horizon problem---and therefore without inflation

What is it about?

The horizon problem in cosmology arises only when the early cosmic expansion is decelerated. If the Universe expanded steadily, there is no horizon problem today and, therefore, no need to introduce a poorly understood inflationary scenario. After 30 years of development, there is still no self-consistent picture of how/why inflation would have occurred. This is a good indication that the Universe may have done without it.

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Why is it important?

Without inflation, the standard model of cosmology suffers from a serious internal inconsistency. But no complete theory of inflation has yet emerged. In addition, we still do not have any direct observational confirmation that it actually occurred. It is starting to look more likely that inflation never happened, which begs the question of how the Universe may have survived without it. This paper demonstrates that the horizon problem is not common to all models. It only arises for models in which the early expansion was decelerated. So the solution to the horizon problem may simply involve the kind of early expansion, rather than some anomalous exponentiated growth due to a poorly understood mechanism.