In the beginning, our cosmos experienced exponential space expansion, known as ‘Inflation’. The cosmos stretched as gravity quadrupled its size at least 100 times, from a billionth of a photon to a grapefruit.
Space-time extended several times the speed of light for this to happen. No physical rules were violated since cosmic speed limit only refers to space-time changes, not their movement. However, cosmic expansion is making distant galaxies appear to recede at multiple times the speed of light.
Cherenkov radiation slows light from a vacuum into a denser media like water or glass, like a sonic boom. High-energy particles traveling in the same direction move faster than light.
They generate a shock wave that blocks light. Cherenkov radiation is common at water-covered nuclear reactors. The radiation casts a ghostly blue glow around them.
‘Tachyon’ refers to all particles that move faster than light. The hypothetical idea was named in 1976. A particle that traveled faster than light would break all known physics rules.
If such a particle existed, it could go backwards in time and never slow down below the speed of light. Though intriguing, these tachyons have never been detected.