First, I don't need a reference, because this fact is easily provable from well known facts (like the expansion of the universe, and the uniformity and isotropy of space.) You can follow the argument, which is quite simple, in Steven Weinberg's The First Three Minutes which, while dated, is still quite solid factually.
It is very easy for a layman to follow.
Second, Google "Observable Universe" "Horizon of Observable Universe" or "Cosmological Horizon" and you will have access to thousands of unique references, and even pictures.
Finally, if all that is unconvincing, just have a look here. UCLA is a reputable institution of higher education.
Okay, the UCLA link is useful, but I’m actually referring to an object that has achieved a velocity greater than c. The mathematics are interesting, but have any observations been made (if possible) to validate the models?
A lot of interesting theories and mathematics and these subjects fascinate me to no end!