[dmorgan81]

That would be argued in cross-examination. A witness can be shown to be not a good witness. Perjury is very specific to knowingly lying while testifying under oath. We really don't want to expand it to areas of ignorance or disagreement; that way would stop people from testifying entirely.

[bluGill]

An expert is someone who claims to know though, and thus if they say something that contradicts established facts they are lying under oath.

[singleshot_]

This is not even near the truth. An expert (under Daubert) is someone who convinces the court they can say something relevant and reliable based on a technique that passes a test concerning:

Whether the technique or theory in question can be, and has been tested; Whether it has been subjected to publication and peer review; Its known or potential error rate; The existence and maintenance of standards controlling its operation; and Whether it has attracted widespread acceptance within a relevant scientific community.

The expert does not “know.” The expert is the only witness who can give an opinion, more or less. Because the opinion is backed up by something, the court considers it useful.

The technique they use is what’s important, not whether their opinion contradicts a fact. I think you will find in many expert trials, two experts get the same facts and come to two completely contradictory opinions, neither of which is perjury.