  Dark energy appears to be the dominant component of the physical Universe,
yet there is no persuasive theoretical explanation for its existence or
magnitude. The acceleration of the Universe is, along with dark matter, the
observed phenomenon that most directly demonstrates that our theories of
fundamental particles and gravity are either incorrect or incomplete. Most
experts believe that nothing short of a revolution in our understanding of
fundamental physics will be required to achieve a full understanding of the
cosmic acceleration. For these reasons, the nature of dark energy ranks among
the very most compelling of all outstanding problems in physical science. These
circumstances demand an ambitious observational program to determine the dark
energy properties as well as possible.
