There are crises and crises, and unless you're an avid science buff, you may not realize that cosmologists — the few thousand researchers whose lives are dedicated to understanding the history of the universe from birth to death — are currently faced with a huge predicament that's only getting worse. It's to do with the expansion of the universe, which we've known about for a hundred years. 

Contrary to what Isaac Newton and Albert Einstein (originally) thought, the universe isn't static, it's expanding. In the 1920s, Edwin Hubble, working with what was then the largest telescope in the world on California's Mount Wilson, established the Hubble law: The farther away a galaxy is, the faster it's moving away from us. And that's true for all observers — we're not special. This suggests that a movie of the universe run backwards would show everything eventually contracting to an infinitely dense point, confusingly dubbed the Big Bang.

Until 25 years ago, the consensus was that the expansion rate, the Hubble constant, Ho (the "zero" subscript refers to the expansion rate now, which is constant throughout the universe), had to be slowing down, since all the matter in the universe is gravitationally attracted to all the other matter. However, in 1998, two groups working independently — one in Chile, the other in California — published their groundbreaking discovery that the expansion is speeding up. (Instruments capable of analyzing deep space objects to the required accuracy only became available in the '90s, as film cameras were supplanted by more sensitive digital ones.) A repulsive force, dark energy, as in, "We haven't a clue what it is," must permeate the universe, causing the expansion to accelerate rather than to slow down under the influence of gravity. And dark energy isn't trivial: It constitutes about 70 percent of the combined mass and energy in the universe. 

The "crisis" stems from the conflicting results given by the two main ways in which the rate of the cosmic expansion is measured. One depends on a model derived from our map of the very early universe, the cosmic microwave background or CMB. Some 370,000 years after the singularity, the formerly opaque universe became transparent with the formation of neutral hydrogen atoms. The CMB is the starting point from which astrophysicists have modeled the history of the universe, from 370,000 years after the Big Bang to the present time, 13.8 billion years on. From this so-called "lambda-CDM" model (which has been spectacularly successful in solving many outstanding cosmic problems), cosmologists have derived a current expansion rate, or Hubble constant, of about 67 kilometers/second/megaparsec — the red shading in the diagram above.

The second way to calculate the Hubble constant is to directly measure the velocities of galaxies and galaxy clusters speeding away from us plotted against their distances, the latter derived from a well-established "Distance Ladder." These ongoing observations are homing in at around 74 kilometers/second/megaparsec, indicated by blue shading in the diagram.

To you and me, the difference between 67 and 74 is meh. Big deal. But to those whose careers depend on it (not to mention potential Nobel prizes) it's huge. Either our best cosmic model, including lambda-CDM and/or Einstein's relativity, is wrong — which could lead to a whole new understanding of physics; or the way we're making observations of the universe is completely cock-eyed. Hopefully in the next few years, data from the Webb Space Telescope, the European Gaia spacecraft and perhaps gravity wave observatories, will resolve the 67-74 gap. Until then, researchers are up against a crisis of cosmic proportions. Literally.

Barry Evans (he/him, [email protected]) thinks this is a terrific time to be following this conundrum.