Cosmic Inflation
Let’s talk about one of the controversial topic of astrophysics, that is Inflation/Cosmic inflation. Cosmic inflation is a faster-than-light expansion of the universe that gave birth to a slew of new universes. Inflation was created to explain a few aspects of the universe that would be difficult to explain otherwise. The first is that matter, according to Einstein's general theory of relativity, bends space and time, so you'd expect a universe like ours, which has mass, to be overall curved in some way, either inward like a ball ("positive") or outward like a saddle ("negative"). In reality, it's almost completely flat. Furthermore, even bits of it far away in different directions as seen from Earth have roughly the same temperature, despite the fact that in an expanding universe, there wouldn't have been enough time for heat to pass between them to even things out. That appears to be a direct challenge to the rules of thermodynamics.
Cosmic inflation solves all of these issues at once. The universe grew faster than light in its early moments (light's speed restriction only applies to things within the cosmos). That smoothed out the wrinkles in its early chaotic state and ensured that even now, far-flung areas could exchange heat because they were formerly in close proximity. Inflation is now a part of our basic cosmic evolution narrative. However, it is still a contentious issue. In 2014, researchers claimed to have discovered ripples in the cosmic microwave background caused by inflation. But this turned out to be incorrect, and it's unclear what caused the early universe to expand in the first place. 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.