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.
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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 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, this golden age started around 3 billion years ago and will last for perhaps another 6 billion. “But if you fast forward 10 billion years or so into the future,” he says, “the matter we are made of will seem like a wisp in an overwhelmingly dark-energy-dominated universe. And that will last for the rest of time.”
Perhaps the current equilibrium is most easily explained by appealing to some form of the anthropic principle (see “Cosmic cop-out: Certain ideas can make all the implausibility vanish“): our existence depends on having a planet, star and ultimately a galaxy to live in, and their existence at this time depends on the balance of dark matter and dark energy.
Or perhaps it is no coincidence. Fornengo, for one, wonders whether dark matter and dark energy are in fact two sides of the same coin. “Like particles interact, maybe dark energy and dark matter interact,” he says. If dark energy were gradually to decay into dark matter, that would put a brake on the plummeting dark matter density, ensuring the cosmic broth never gets too dilute.
Such interactions require new cosmological models, not least because they allow the density of dark energy to change over time. “If you add these to the game, you change dramatically the evolution of the universe,” says Fornengo, generating fluctuations in its background temperature and giving rise to structures visible at the largest scales.
Data from the next generation of telescopes, peering back to the early universe, should give us a chance to see which of these models is the best fit. But at present, says Pontzen, there’s nothing to suggest any deviation from the standard picture – so this remains an inexplicable coincidence.
This article appeared in print under the headline “Matter and energy are in balance”
