The Universe Is Racing Apart. We May Finally Know Why

Get a special 35% discount on an annual digital subscription to The Economist at https://www.economist.com/PBS We've known that the universe is expanding since 1929, and that its expansion is accelerating since 1998. The culprit behind the acceleration is unknown, so we live with a stand-in term "dark energy". Our modern cosmological model assumes that dark energy has a constant density--always the same amount of the outward-shoving stuff per volume. But there's recent evidence to the contrary--which may be why our primary efforts to measure the expansion rate of the universe disagree with each other. Sign Up on Patreon to get access to the Space Time Discord! https://www.patreon.com/pbsspacetime Check out the Space Time Merch Store https://www.pbsspacetime.com/shop Sign up for the mailing list to get episode notifications and hear special announcements! https://mailchi.mp/1a6eb8f2717d/spacetime Search the Entire Space Time Library Here: https://search.pbsspacetime.com/ Hosted by Matt O'Dowd Written by Matt O'Dowd Post Production by Leonardo Scholzer Directed by Andrew Kornhaber Associate Producer: Bahar Gholipour Executive Producer: Andrew Kornhaber Executive in Charge for PBS: Maribel Lopez Director of Programming for PBS: Gabrielle Ewing Assistant Director of Programming for PBS: Mike Martin Spacetime is a production of Kornhaber Brown for PBS Digital Studios. This program is produced by Kornhaber Brown, which is solely responsible for its content. © 2026 PBS. All rights reserved. End Credits Music by J.R.S. Schattenberg: https://www.youtube.com/user/MultiDroideka Space Time Was Made Possible In Part By: Big Bang Alexander Tamas David Paryente Juan Benet Kenneth See Mark Rosenthal Morgan Hough Peter Barrett Santiago Tj Steyn Vinnie Falco Supernova Ethan Cohen Glenn Sugden Grace Biaelcki Mark Heising Stephen Wilcox Tristan Lucian Claudius Aurelius Tyacke Hypernova Alex Kern Ben Delo Cal Stephens chuck zegar David Giltinan Dean Galvin Donal Botkin Gr