Scalar particles, as well as being an enigma from the point of view of elementary
particle physics are also an enigma from the point of view of general relativity.
They can mix with the determinant of the metric tensor. In cosmology, scalar
particles can contribute to dark matter. A cosmological classical background due to
a scalar field can cause maximally positive or negative pressure. Such a negative
pressure is most probably responsible for the present observed acceleration of the
expansion of the universe.
Research supported by Boğaziçi University
Scientific Research Project 06B301