Perhaps the strongest evidence we've ever acquired in support of the Big Bang has been the discovery of the leftover radiation from its early, hot, dense state: today's cosmic microwave background, or CMB. While there were many competing ideas for our cosmic origins, only the Big Bang predicted a uniform, omnidirectional bath of blackbody radiation: exactly what the CMB is.

But it turns out the CMB encodes much more information than just our cosmic origins; it allows us to map the very early Universe from when it was just 380,000 years old, and gives us vital information about what has happened to light from that time over its 13.8 billion year journey to our eyes. It encodes information about our cosmic expansion history, about dark matter and dark energy, about intervening galaxy clusters, and about the material here in our own galaxy, along with much more. It is, arguably, the richest source of information from any one single observable in our entire Universe.

Here to guide us through what CMB scientists are working on here in 2025, including what we've learned and what we're still trying to find out, I'm so pleased to welcome Dr. Patricio Gallardo to the show. We've got more than an hour and a half of quality science to go through, and by the end, I bet you'll be more excited about the upcoming Simons Observatory, designed to measure the CMB to higher precision than ever before, than you knew you should be. Enjoy!

(This image shows the Large Aperture Telescope's colossal, 6-meter primary and secondary mirrors at the Simons Observatory in February of 2025. The telescope has already seen first light, and will soon begin delivering new CMB science as never before. Credit: M. Devlin/Simons Observatory)