Constant

IN THE 1980S, when people in East and West Berlin still lived in two very different universes, politically speaking, Checkpoint Charlie was an intimidating and heavily guarded crossing point between communist oppression and liberal democracy. Today, it is one of the most popular tourist attractions in the capital of united Germany. But 29 years after the Berlin Wall opened in 1989, another insuperable barrier, this time scientific in nature, manifested itself just 600 metres eastward of Checkpoint Charlie, in the Auditorium Friedrichstrasse. On a drizzly Saturday in November 2018, this unadorned Soviet-style building served as the intellectual battleground for a cosmological Cold War.

Some 130 scientists flocked to a one-day symposium here to discuss an unnerving crisis in our understanding of the universe. It was a diverse bunch from all over the world: astrophysicists and cosmologists, observers and theorists, young postdocs and eminent professors. Some of them had spent more time in the air on their way there than they would in the lecture room. Their mutual worry: The universe appears to be expanding too fast, and no one knows why. At the end of the meeting, Brian Schmidt (Australian National University, ANU), co-recipient of the 2011 Nobel Prize in Physics, said: “I’m even more puzzled after today.”

Here’s what astronomers and physicists alike scratch their heads about: Detailed observations of the cosmic microwave background (CMB, the cooled-down ‘afterglow’ of the Big Bang) yield a very precise value for the current expansion rate of the universe, with an error margin of just 1%. However, measurements of objects in the ‘local’ universe arrive at