Let’s say you have a baby. Maybe you really do, maybe you don’t. But Dan Scolnic, a cosmologist at the University of Chicago, does have one, and perhaps that's why a hypothetical baby helps him explain the universe. If you take this baby to the doctor, that doctor will weigh and measure the baby, plot those points on a growth chart, and predict how big they’ll be later.

“We kind of have this same situation now with measuring the universe,” says Scolnic, who'll begin a professorship at Duke next month. Scientists have a great picture of what the universe was like as a baby. They also have one of what it looks like all grown up, today. And as with the doctor’s growth chart, a curve—following physics as we know it—should connect the two cleanly.

“You should be able to put in that universe’s baby picture, trace our standard cosmology, and see our universe today—if everything has gone right,” says Scolnic. But that’s not what is happening. “Something,” says Scolnic, “is not going right.”

Cosmologists aren’t sure what that something is, exactly. Maybe they're wrong in their measurement or analysis of the baby universe. Or of its present state.

Those are the boring options, though. “The other,” says Scolnic, “is that our standard model of cosmology isn’t correct.” In other words, the way humans think about the early years, maturation, and fate of the universe might be wrong somehow.

Over the past few years, scientists like Scolnic have investigated those first two hypothetical misunderstandings. They've whittled down their error bars, hardened their methods, re-analyzed the results of competitors and colleagues, and gathered sharper and bigger data. Nevertheless, the discrepancy persists.

Scolnic calls this moment “the era of tension cosmology.” Others just call it a crisis.

To those who don’t study the origin and evolution of the universe, that sounds like a bad thing. To cosmologists, it’s the opposite. To be wrong is to learn that the universe is more interesting than they thought. “We’re right on the cusp of this being the coolest thing ever,” says Scolnic.

One number has led us to said cusp. That number is called the Hubble constant, and it is the rate at which the universe is expanding today (not today like “Tuesday” but today like “in this cosmic moment”). The Hubble constant is an elusive beast, even for cosmology—a kind of white stag among white stags.

Astronomers have devised a few ways to estimate its value, and it is the conflict between their outcomes that’s causing the trouble. One method begins with the universe’s baby picture—a map of the so-called “cosmic microwave background,” or the remnant radiation from the Big Bang. From that picture, astronomers plug what they (think they) know about dark energy, dark matter, regular matter, and gravity into a model. Out pops a present state of the universe, and a prediction of the Hubble constant. Most recently, astronomers did this using cosmic microwave background data from the Planck telescope, a space-based observatory that was decommissioned in 2013.

Another method uses the “cosmic distance ladder.” Astronomers figure out how far away objects are, and how fast they’re moving even farther away, starting from here(ish) and building outward. They calculate the distances to nearby stars, and from them to more distant stars in other galaxies, and from them to supernovae in still-farther-off galaxies. They measure their movement away from us, providing another estimate of the Hubble constant.

Scolnic was part of a big team called SH0ES that used the ladder method. Its Hubble estimate disagrees with Planck's. Behold: the crisis.