For now, though, there is a dearth of data to shore up or shoot down diverse conjectures, leaving a vast playground for theorists to mess around in. The most credible ideas involve a fundamental constant of physics—perhaps the cosmological constant itself—varying in time or space. So far, it appears that dark energy’s effects came into prominence only in the past 6 billion years or so. A time-varying cosmological constant would thus be an easy fix.
That is the idea behind a theory that a handful of physicists have cleverly called quintessence. But such adjustments all run into a whack-a-mole problem: fix the darknesses and your modified theory fails to account for some other phenomenon.
Banishing the darkness is a worthy goal. Indeed, the ultimate fate of the universe is at stake. The amount and nature of dark energy, and whether its properties vary with the age or size of the universe, will dictate which of three futures happens. In one, the universe expands calmly for ever. More dramatically, the expansion may speed up as the effects of dark energy overwhelm everything else. The universe then thins out of existence in a “Big Rip”. Or matter may eventually win out over dark energy—its gravity slowing and reversing the expansion, and drawing everything together again in a “Big Crunch”.
In an era when so much science seems understood, all this uncertainty might appear to be an embarrassment. Not so. Any physicist would tell you it is an opportunity, a frontier. After all, from his point of view, there remains 95% of the universe—a quintessential part, it might be said—yet to be discovered.
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