Cosmic dark matter and energy balance – for now. Coincidence?

 作者:池桄戥     |      日期:2019-03-15 06:20:12
Mark Chaves/EyeEm/Getty By Gilead Amit INVISIBLE dark matter and dark energy make up around 95 per cent of the universe between them. What’s more, their densities are roughly equal – a state of affairs so unlikely that cosmologists have taken to calling it the cosmic coincidence problem. Is this a genuine conundrum? At first blush, it seems contrived. Dark matter, which gravitates like normal matter, accounts for about 27 per cent of the universe. Meanwhile some 68 per cent is dark energy, the stuff that is causing the expansion of the cosmos to speed up. Not quite so equal after all, then. But the values are still close enough to be perplexing – and according to our standard cosmological model, the similarity is relatively new. The very early universe was dominated by dark matter. “At that time, dark matter density was 95 orders of magnitude larger than the density of dark energy,” says Nicolao Fornengo at the University of Turin, Italy. But dark matter’s density has been dropping as the universe expands, while the density of dark energy is widely assumed to remain constant over time, making it steadily more dominant. A few billion years ago, dark energy became denser than dark matter – causing the universe’s expansion to begin racing away (see “The universe is flat as a pancake. Coincidence?“). Still, it seems we live in a special time where neither entity is able to dominate the other. According to Andrew Pontzen at University College London,