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INFLATION, one of the most important ideas in big bang cosmology, may have hit another stumbling block.

The best explanation for how the newborn universe went through a period of ultrafast expansion has been under increasing scrutiny recently, after researchers found irregularities in the cosmic microwave background – the radiation left over from the big bang – that appeared to contradict some of the predictions of the simplest version of the model (Â鶹´«Ã½, 7 June, p 30).

Now this model faces yet another challenge, this time linked to hypothetical particles called axions, which are thought to have precipitated a fundamental…