The current concordance model of cosmology is dominated by two mysterious
ingredients: dark matter and dark energy. In this paper, we explore the
possibility that, in fact, there exist two dark-energy components: the
cosmological constant Λ, with equation-of-state parameter
wΛ=−1, and a `missing matter' component X with wX=−2/3, which we
introduce here to allow the evolution of the universal scale factor as a
function of conformal time to exhibit a symmetry that relates the big bang to
the future conformal singularity, such as in Penrose's conformal cyclic
cosmology. Using recent cosmological observations, we constrain the present-day
energy density of missing matter to be ΩX,0=−0.034±0.075. This is
consistent with the standard ΛCDM model, but constraints on the energy
densities of all the components are considerably broadened by the introduction
of missing matter; significant relative probability exists even for
ΩX,0∼0.1, and so the presence of a missing matter component
cannot be ruled out. As a result, a Bayesian model selection analysis only
slightly disfavours its introduction by 1.1 log-units of evidence. Foregoing
our symmetry requirement on the conformal time evolution of the universe, we
extend our analysis by allowing wX to be a free parameter. For this more
generic `double dark energy' model, we find wX=−1.01±0.16 and
ΩX,0=−0.10±0.56, which is again consistent with the standard
ΛCDM model, although once more the posterior distributions are
sufficiently broad that the existence of a second dark-energy component cannot
be ruled out. The model including the second dark energy component also has an
equivalent Bayesian evidence to ΛCDM, within the estimation error, and
is indistinguishable according to the Jeffreys guideline.Comment: Revised version emphasising a different version of the underlying
symmetry, as published in JCA