Somebody noticed that \abstract changes the linewidth for the whole document (here).
The problem was the syntax: with correct syntax as below this is avoided.
\begin{abstract}
This is an abstract. This is an abstract.
\end{abstract}
while this keeps the linewith for the rest of the document:
\abstract
This is the abstract
But what if I want to intentionally keep the linewidth the same for the rest of the document, but use correct syntax? Is there a way of knowing the linewidth used in the abstract and setting it manually? This is interesting, if one uses the fancyhdrpackage, and one wants the headers be wider than the text.

changepagepackage provides an environmentadjustwidthwhich uses different margins than the rest of the document. I guess you can enclose the whole document in that environment. – JLDiaz Mar 16 '13 at 13:45