Researchers have set to work trying to test the conjecture and explore its implications. Wrase has already written two papers, including one that may lead to a refinement of the conjecture, and both mostly while on vacation with his family. He recalls thinking, “This is so exciting. I have to work and study that further.”

The conjectured formula—posed in the June 25 paper by Vafa, Georges Obied, Hirosi Ooguri, and Lev Spodyneiko, and further explored in a second paper released two days later by Vafa, Obied, Prateek Agrawal, and Paul Steinhardt—says, simply, that as the universe expands, the density of energy in the vacuum of empty space must decrease faster than a certain rate. The rule appears to be true in all simple string-theory-based models of universes. But it violates two widespread beliefs about the actual universe: It deems impossible both the accepted picture of the universe’s present-day expansion and the leading model of its explosive birth.

Since 1998, telescope observations have indicated that the cosmos is expanding ever so slightly faster all the time, implying that the vacuum of empty space must be infused with a dose of gravitationally repulsive “dark energy.”

In addition, it looks like the amount of dark energy infused in empty space stays constant over time (as best as anyone can tell).

But the new conjecture asserts that the vacuum energy of the universe must be decreasing.

Vafa and colleagues contend that universes with stable, constant, positive amounts of vacuum energy, known as “de Sitter universes,” aren’t possible. String theorists have struggled mightily since dark energy’s 1998 discovery to construct convincing stringy models of stable de Sitter universes. But if Vafa is right, such efforts are bound to sink in logical inconsistency; de Sitter universes lie not in the landscape, but in the “swampland.” “The things that look consistent but ultimately are not consistent, I call them swampland,” he explained recently. “They almost look like landscape; you can be fooled by them. You think you should be able to construct them, but you cannot.”

According to this “de Sitter swampland conjecture,” in all possible, logical universes, the vacuum energy must either be dropping, its value like a ball rolling down a hill, or it must have obtained a stable negative value. (So-called anti–de Sitter universes, with stable, negative doses of vacuum energy, are easily constructed in string theory.)

The conjecture, if true, would mean the density of dark energy in our universe cannot be constant, but must instead take a form called “quintessence”—an energy source that will gradually diminish over tens of billions of years. Several telescope experiments are underway now to more precisely probe whether the universe is expanding with a constant rate of acceleration, which would mean that as new space is created, a proportionate amount of new dark energy arises with it, or whether the cosmic acceleration is gradually changing, as in quintessence models. A discovery of quintessence would revolutionize fundamental physics and cosmology, including rewriting the cosmos’ history and future. Instead of tearing apart in a Big Rip, a quintessent universe would gradually decelerate, and in most models would eventually stop expanding and contract in either a Big Crunch or Big Bounce.