Our universe isn't just expanding: it's doing so faster and faster. It's like raisin bread dough in the oven: the raisins move apart, and the farther they've already separated, the faster they continue to recede. The strange force pushing the cosmos apart is called dark energy. Most scientists think it's constant, like a permanent spring built into the very fabric of space.
But there's another idea: dark energy might have been different in the past. The article examines models where its equation of state (pressure divided by density) crosses the value −1. Ordinary dark energy always pushes with a force equal to minus its density; here, the pressure can become either stronger or weaker than this threshold. This behavior is called 'quintom'—from the Latin word for 'fifth,' hinting at a fifth element beyond ordinary matter, dark matter, radiation, and constant dark energy.
The authors of the review remind us of a no-go theorem: in the simplest models, dynamic dark energy cannot cross −1. But adding multiple fields or unusual interactions bypasses this prohibition. Such scenarios are interesting not only for explaining acceleration. They could show up in the rotation of the polarization of the cosmic microwave background — ancient light that has reached us from the universe's hot beginning. If we measure this rotation with high precision, we'll get a clue about the nature of dark energy.
So the spring pushing the raisins in our cosmic dough might not always have been equally elastic. And how much it has changed determines whether the dough will tear apart or someday clump back together.
The name 'quintessence' for dynamic dark energy is borrowed from alchemy, where it referred to a fifth element, more subtle than fire, air, earth, and water.
In Isaac Asimov's novel 'The Last Question,' the fate of the universe depends on whether humanity can reverse its expansion — much like in quintom energy models with a crossing of −1.