The classical Big Bang correctly predicted the cosmic microwave background and the hydrogen-helium ratio, but by the 1970s it had three glaring problems. The horizon problem: opposite sides of the sky share the same 2.7 Kelvin temperature despite never having been in causal contact. The magnetic monopole problem: grand unified theories predicted heavy stable monopoles that have never been found. And the flatness problem: the universe's density had to be tuned to one part in 10 to the 60th to avoid a big crunch or a runaway expansion that never formed galaxies. This episode explains why physicists treat that level of coincidence as a sign of a missing mechanism.

The hosts then describe the fix proposed by Alan Guth, Alexei Starobinsky and Andrei Linde: a fraction of a second of exponential expansion driven by a false vacuum with repulsive gr ...