There seemed to be some growth in criticisms in string theory after evidence of dark energy was discovered in 1998 and the 2003 work increased the number of known solutions.
The attempts to make the theory fit physical reality were growing a bit more strained, in the eyes of some, and a discontent that had always existed under the surface began to seep out of the back rooms at physics conferences and onto the front pages of major science magazines.
While innovative new variants — such as the Randall-Sundrum models and the incorporation of a positive cosmological constant — were rightly recognized as brilliant, some people believed that physicists had to come up with contrived explanations to keep the theory viable.
The growth in criticism became glaringly obvious to the general public in 2006 with the publication of two books criticizing — or outright attacking — string theory. The books were Lee Smolin’s The Trouble with Physics: The Rise of String Theory, the Fall of a Science, and What Comes Next and Peter Woit’s Not Even Wrong: The Failure of String Theory and the Search for Unity in Physical Law.
These books, along with the media fervor that accompanies any potential clash of ideas, has put string theory on the public relations defensive even while many (possibly most) string theorists dismiss the Smolin and Woit claims as failed attempts to discredit string theory for their own aggrandizement.
The truth is likely somewhere in between. The criticisms have a bit more merit than string theorists would give them, but are not quite as destructive as Woit, at least, would tend to have readers believe. (Smolin is a bit more sympathetic toward string theory, despite his book’s subtitle.)
None of the critics propose abandoning string theory entirely; they merely would like to see more scientists pursuing other areas of inquiry.