denser early Cosmos had high index of refraction ?

[Widdekind]

A light-year is defined as "the distance that light would travel through space, if there was nothing to slow it down". Does that imply, that, in the early universe, when the ambient matter density was much higher, that the index of refraction n > 1, and light traveled (slightly) slowly ??

[Klaynos]

A light year is the distance that light travels in a vacuum in a year. The speed of light, c, is taken to be when n=1, in a vacuum. It is effectively the speed of a photon, which does not change.

[Widdekind]

v = c / n, yes? And, n depends on the density, and ionization state, of the gas.

[imatfaal]

But as Klaynos said the light year is the distance travelled in a vacuum - this is very closely approximated in space but not the same.

 

The confusion is that a lightyear is not how far light travels in outerspace in a year - but how far light travels in a vacuum in a year. To all intents it is the same thing - but as soon as space gains a measurable density and refractive index then there is a difference; but this does not change the speed of light in vacuum which is constant, nor the lightyear, but it does change how much distance light will travel in space in a certain time.

[BJC]

A light-year is defined as "the distance that light would travel through space, if there was nothing to slow it down". Does that imply, that, in the early universe, when the ambient matter density was much higher, that the index of refraction n > 1, and light traveled (slightly) slowly ??

 

When the temperature of the early universe was > 3,000 degree Kelvin, the universe existed in a plasma form and was opaque to EM radiation (i.e. no light escaped). Not exactly the same as refraction more that a light ray could not form - a photon would be absorbed by other particles before there was any chance of EM radiation forming and escaping. Similar to conditions inside a star.