In my opinion, "just" in this sentence means "exactly, precisely". The theory of an expanding universe is a controversial one. Not all scientists agree on the matter. (http://www.sci-news.com/astronomy/science-universe-not-expanding-01940.html) (http://astronomynow.com/2015/04/13/universe-expanding-more-slowly-than-previously-thought/) (http://news.discovery.com/space/astronomy/is-the-universe-expanding-or-just-getting-heavier-130822.htm)

Here ' just' is an adverb and means 'exactly ,precisely ' Exactly how long the universe will continue to expand ...... You can use it or leave it out ! And the question is still valid ,because in spite of finding out about the accelerating expansion of the universe , there is still no 100 % guarantee of its infinite expansion ,it depends on other factors ,that will result in expanding forever or else collapsing back on itself in a so called ' big crunch " . Not certain yet !

Just how long the universe will continue to expand is a question of intense interest to astronomers.?? Just has no explicit meaning, it just emphasizes the "how". How long the universe will continue to expand is a question of great interest to astronomers and physicists alike. On a side note did you mean to say this in the past tense? Since we found out that the expansion of the universe is accelerating, I don't think this is even a topic of discussion anymore.