Because light can only travel so fast, all of the light we see in the sky was emitted at a previous moment in time.  So if for example we see a supernova or some other great stellar event, by the time we see it, it maybe long over.  That made me kind of curious, what is the most ancient light we can see from earth?  

The universe is supposedly ~13+ billion years old, but we are probably not at the very edge of the known universe so all the light we see is probably less than 13 billion years old.  So what is the oldest light we can see? and as an optional follow-up question how do we know the age of that light? 

I guess the light itself may not actually be literally 'old', but its probably obvious what I'm asking here, put another way: what's the longest distance that now earth visible light emitted has traveled to reach the earth?  Though that reformation of the question gets kind of tangled with lensing effects.

The oldest light in the universe is the cosmic microwave background. Roughly 380,000 years after the Big Bang, protons and electrons "recombined"1 into hydrogen atoms. Before this, any photons scattered off the free electrons in the plasma filling space, and the universe was essentially opaque to light. Once recombination occurred, however, photons were able to "decouple" from the electrons and move through space unimpeded. This relic radiation is still observable today; it has been redshifted and cooled.

We can detect light from very distant objects, and we have. It makes more sense to talk about distance in terms of redshift; the larger the redshift, the farther away an object is. There are a number of extremely high-redshift objects, some of which have had their measurements confirmed, and others of which have not. Candidates include


MACS0647-JD (redshift $z=10.7^{+0.6}_{-0.4}$), a very distant galaxy.
UDFj-39546284 (redshift from $z=10$ to $z=12$), another galaxy or protogalaxy (although its distance is uncertain, and its exact nature is disputed).
GN-z11 (redshift $z=11.09^{+0.08}_{-0.12}$), a very luminous distant galaxy.


All of these objects would have formed some hundreds of millions of years after the Big Bang, however, so the light we see from them is much "younger" than that of the cosmic microwave background.



1 I've never liked the usage in this context, as this was the first time they combined; the "re" is kind of misleading.
