travelling forward in time

[joe-90]

Couple of questions. Where does the light in the Universe come from? If light passes through glass it slows down, yes? What mechanism allows it to speed up again afterwards? If it doesn't speed up, will the Universe be black one day?

 

[Space cat]

Where does the light in the Universe come from?

The light in the universe comes from a number of sources. Stars are pretty obvious. The fusion reactions that make them go release energy which eventually finds its way out as electromagnetic radiation - of which visible light is a tiny fraction - and fast moving particles.

Anything that releases energy can generate 'light'. A hot kettle emits invisible infra-red. A hot planet like venus puts out considerably more. Even a dead star can be a light source. Neutron stars typically emit X-rays, not from any internal process but from external material smashing into the surface.

If light passes through glass it slows down, yes? What mechanism allows it to speed up again afterwards?

It slows down because the permittivity of glass is higher than that of free space. The permittivity and permeability of a medium determine the speed at which an E/M wave travels through it. (Look up Maxwell's equations if you want to know how this works.) When light emerges from the other side of the glass, it reverts to its original speed.

will the Universe be black one day?

Possibly. It depends on whether there is enough mass in the universe to halt its expansion. The second law of thermodynamics dictates that everything will eventually come to thermal equilibrium. If the contraction reverses, the universe will get very hot again as it collapses back to --- something. If not then temperature will fall towards absolute zero. That's pretty black!