麻豆传媒

THESE days, cosmologists think of the big bang not as a single moment, but a longer period in which the nascent cosmos, then a ludicrously hot and dense soup, underwent a series of transformations before the universe we see today eventually emerged through a slower process of cooling and expansion.

But our understanding of this 鈥渉ot big bang鈥� remains sketchy at best. That means cosmologists have some freedom to generate new ideas about what really happened in the very early universe.

One intriguing possibility is that dark matter, the mysterious stuff thought to hold galaxies together, was formed in a separate event to normal matter. This 鈥渟econd big bang鈥� might initially appear far-fetched. But as we explore in 鈥淎 second big bang? The radical idea rewriting dark matter鈥檚 origins鈥�, there are good reasons to consider it.

First, our failure to identify what dark matter is, despite decades of attempts to detect it, suggests we might need to rethink its origins. The scenarios being explored for a subsequent genesis throw up several new and exotic kinds of dark matter particle, including extremely massive ones called 鈥渄arkzillas鈥� that would have evaded our detectors.

Second, the idea of an alternative moment of creation for dark matter is in line with the realisation among cosmologists that the early universe was characterised by a series of phase transitions 鈥� moments in which a physical system undergoes a profound transformation.

But most importantly, perhaps, we may finally have a way to test this and other proposals for phase transitions in the early universe. In July, the North American Nanohertz Observatory for Gravitational Waves announced the detection of a background gravitational wave signal that could be coming from the first few weeks of the universe鈥檚 existence.

If this observation holds up, it will open a window on the formative moments of the universe for the first time. Whether or not we see evidence of a dark matter big bang, we can expect a new understanding of how the cosmos we see came to be.