Cosmic inflation is a faster-than-light expansion of the universe that spawned many others.
Inflation was invented to explain a couple of features of the universe that are really hard to explain without it. The first is that Einstein鈥檚 general theory of relativity famously makes mass bend space and time 鈥 so you鈥檇 expect a universe like ours, containing mass, to be overall curved in some way, either in on itself like a ball (鈥減ositive鈥 curvature) or out on itself like a saddle (鈥渘egative鈥 curvature).
In fact, it鈥檚 darn nearly flat. Not only that, but even bits of it far off in different directions as observed from Earth have roughly the same temperature, although in an expanding universe there wouldn鈥檛 have been time for heat to pass between them to even things out. That seems a naked assault on the laws of thermodynamics.
Cosmic inflation solves these problems at a stroke. In its earliest instants, the universe expanded faster than light (light鈥檚 speed limit only applies to things within the universe). That ironed out wrinkles in its early chaotic self and meant that even now far-flung parts were once in close contact, so could swap heat.
Inflation is now a built-in piece of our standard story of cosmic evolution. But it鈥檚 still controversial. In 2014, researchers claimed to have seen ripples from inflation imprinted on the cosmic microwave background. But this proved mistaken, and it鈥檚 not clear what would have made the early universe inflate anyway. Worse, inflation is very difficult to stop, creating a multiverse of causally disconnected universes that eternally bud off from one another.
One way out might be to weaken the constant speed of light. If the speed of light was faster in the early universe, that would also explain the temperature problem. Perhaps light is still slowing now, just at a rate that is imperceptible even to our most sensitive detectors.
