To calculate the Hubble constant, astronomers need to know how far away things are. In the nearby cosmos, scientists measure distances using stars called Cepheid variables that periodically change in brightness. There’s a well-known relationship between how fast one of these stars swings from brightest to faintest and how much energy it radiates. That relation, which was discovered in the early 20th century, allows astronomers to calculate the star’s intrinsic brightness, and by comparing that to how bright it appears, they can calculate its distance.

Using these variable stars, scientists can measure the distances to galaxies up to about 100 million light-years from us. But to see a bit farther away, and a bit further back in time, they use a brighter mile marker — a specific type of stellar explosion called a type Ia supernova. Astronomers can also calculate the intrinsic brightness of these “standard candles,” which allows them to measure distances to galaxies billions of light-years away.

Over the past two decades, these observations have helped astronomers pin a value on how fast the nearby universe is expanding: roughly 73 kilometers per second per megaparsec, which means that as you look further away, for each megaparsec (or 3.26 million light-years) of distance, space is flying away 73 kilometers per second faster.

But that value clashes with one derived from another ruler embedded in the infant universe.

In the very beginning, the universe was searing plasma, a soup of fundamental particles and energy. “It was a hot mess,” said Vivian Poulin-Détolle, a cosmologist at the University of Montpellier.

A fraction of a second into cosmic history, some occurrence, perhaps a period of extreme acceleration known as inflation, sent jolts — pressure waves — through the murky plasma.

Then, as the universe cooled, light that was trapped in the elemental plasma fog finally broke free. That light — the cosmic microwave background, or CMB — reveals those early pressure waves, just as the surface of a frozen lake holds onto the overlapping crests of waves frozen in time, Poulin-Détolle said.

Cosmologists have measured the most common wavelength of those frozen pressure waves and used it to calculate a value for the Hubble constant of 67.6 km/s/Mpc, with an uncertainty of less than 1%.

The peculiarly discordant values — roughly 67 versus 73 — have ignited a fiery debate in cosmology that is still unresolved.

Astronomers are turning to independent cosmic mile markers. For the past six years, Wendy Freedman of the University of Chicago (who has worked on the Hubble constant for a quarter century) has focused on a type of old, red star that typically lives in the outer portions of galaxies. Out there, fewer overlapping bright stars and less dust can lead to clearer measurements. Using those stars, Freedman and her colleagues have measured an expansion rate of around 70 km/s/Mpc — “which is actually in pretty good agreement with the Cepheids,” she said. “But it’s also in pretty good agreement with the microwave background.”