Does Quartessence Ease Tensions?
  Tensions between cosmic microwave background observations and the growth of
the large-scale structure inferred from late-time probes pose a serious
challenge to the concordance $\Lambda$CDM cosmological model. State-of-the-art
data from the Planck satellite predicts a higher rate of structure growth than
what preferred by low-redshift observables. Such tension has hitherto eluded
conclusive explanations in terms of straightforward modifications to
$\Lambda$CDM, e.g. the inclusion of massive neutrinos or a dynamical dark
energy component. Here, we investigate models of 'quartessence' -- a single
dark component mimicking both dark matter and dark energy -- whose
non-vanishing sound speed inhibits structure growth at late times on scales
smaller than its corresponding Jeans' length. In principle, this could
reconcile high- and low-redshift observations. We put this hypothesis to test
against temperature and polarisation spectra from the latest Planck release,
SDSS DR12 measurements of baryon acoustic oscillations and redshift-space
distortions, and cosmic shear correlation functions from KiDS. This the first
time that any specific model of quartessence is applied to actual data. We show
that, if we naively apply $\Lambda$CDM nonlinear prescription to quartessence,
the combined data sets allow for tight constraints on the model parameters.
Apparently, quartessence alleviates the tension between the total matter
fraction and late-time structure clustering, although in fact the tension is
transferred from the latter to the quartessence sound speed parameter. However,
we found that this strongly depends upon information from nonlinear scales.
Indeed, if we relax this assumption, quartessence models appear still viable.
For this reason, we argue that the nonlinear behaviour of quartessence deserves
further investigation and may lead to a deeper understanding of the physics of
the dark Universe.
