Universe EXPANDING FASTER?! | NOVA | PBS

When astronomers measured distant supernovas in the 1990s, the numbers implied the universe's expansion was speeding up instead of slowing down, a result that upended cosmology overnight. This short walks through why that data was so hard to believe and how it reshaped what scientists think the universe is made of.

Why it's free: Free on NOVA PBS's own channel; GBH produces NOVA for PBS.