Cosmology with a Continuous Tower of Scalar Fields
  We study the cosmological evolution for a universe in the presence of a
continuous tower of massive scalar fields which can drive the current phase of
accelerated expansion of the universe and, in addition, can contribute as a
dark matter component. The tower consists of a continuous set of massive scalar
fields with a gaussian mass distribution. We show that, in a certain region of
the parameter space, the {\it heavy} modes of the tower (those with masses much
larger than the Hubble expansion rate) dominate at early times and make the
tower behave like the usual single scalar field whose coherent oscillations
around the minimum of the potential give a matter-like contribution. On the
other hand, at late times, the {\it light} modes (those with masses much
smaller than the Hubble expansion rate) overcome the energy density of the
tower and they behave like a perfect fluid with equation of state ranging from
0 to -1, depending on the spectral index of the initial spectrum. This is a
distinctive feature of the tower with respect to the case of quintessence
fields, since a massive scalar field can only give acceleration with equation
of state close to -1. Such unique property is the result of a synergy effect
between the different mass modes. Interestingly, we find that, for some choices
of the spectral index, the tower tracks the matter component at high redshifts
(or it can even play the role of the dark matter) and eventually becomes the
dominant component of the universe and give rise to an accelerated expansion.
