Dark energy flips its sign, but the Hubble tension refuses to budge

For nearly a century, astronomers have known that the universe is expanding.

In the late 1990s, two independent teams, the Supernova Cosmology Project, led by Saul Perlmutter, and the High-Z Supernova Search Team, led by Brian Schmidt and Adam Riess, discovered something strange: The expansion is speeding up.

The finding earned them the 2011 Nobel Prize in Physics.

The leading explanation for this acceleration is "dark energy," a mysterious force usually modeled as a constant called Lambda, pushing space apart.

Combined with cold dark matter, this gives us the LCDM model, the standard picture of the cosmos for the past 25 years.