[Antoninus]

By the middle of the 6th century, the Emperor Justinian had spread his Byzantine Empire around the rim of the Mediterranean and throughout Europe, laying the groundwork for what he hoped would be a long-lived dynasty.

If this sentence could be projected back through time and read to Justinian himself, his response would have been: "Wha?"

Justinian was a Roman Emperor.

[Physicist]

Justinian was a Roman Emperor.

Only by conquest. He assumed the Purple in the Eastern (read: Byzantine) Empire and went on to conquer Rome, which by that time had been in Ostrogoth hands for some 60 years.

I doubt that the Eastern emperors normally thought of themselves as "Roman emperors", since Rome was in the Western empire. I'm not sure how the word "Byzantine" came into vogue to describe the East.