How Can the Universe Expand Faster Than the Speed of Light? | Space
Universe expanding faster than the speed of light.
Think of the universe as a checkerboard. That board (called the metric) is expanding.
Parts of the metric are going away from one another faster than the speed of light.
Nothing can travel across the metric faster than the speed of light in a vacuum without being beyond our ability to detect it. So, the metric, itself, may exceed the speed of light.
Cherenkov radiation - Wikipedia
Light slows in a medium (this is why a spectrum separates light colors, because higher frequency blue light is not slowed as much through the medium).
Particles (such as neutrinos) can ram into other matter (rarely happens, but happens), and that gives off particles that exceed the speed of light in a vacuum, and gives off electromagnetic radiation (perhaps visible light or gamma radiation). This is called Cherenkov radiation.
IceCube – IceCube Neutrino Observatory
The IceCube observatory is a mile under antarctic ice (to block ordinary subatomic particles, since only neutrinos can penetrate to that depth). Dr. Fred Reines, Nobel Laureate for first detecting neutrinos, told me that his neutrino detectors had small gas filled tubes (very expensive). Cheaper, IceCube puts scintillation (light flash) detectors around a mile of antarctic clear ice. It
observes neutrinos indirectly when neutinos crash into regular matter, and give off Cherenkov radiation (light) as those particles exceed the speed of light in the medium.