Abstract:
We use data onmassive galaxy clusters (Mcluster>
8 × 1014h
−1M within a comoving radius of Rcluster =
1.5h
−1 Mpc) in the redshift range 0.05 z 0.83 to place
constraints, simultaneously, on the nonrelativistic matter
density parameter Ωm, on the amplitude of mass fluctuations
σ8, on the index n of the power-law spectrum of the
density perturbations, and on the Hubble constant H0, as
well as on the equation-of-state parameters (w0,wa) of a
smooth dark energy component.
For the first time, we properly take into account the dependence
on redshift and cosmology of the quantities related
to cluster physics: the critical density contrast, the growth
factor, the mass conversion factor, the virial overdensity, the
virial radius and, most importantly, the cluster number count
derived from the observational temperature data.
We show that, contrary to previous analyses, cluster data
alone prefer low values of the amplitude of mass fluctuations,
σ8 ≤ 0.69 (1σ C.L.), and large amounts of nonrelativistic
matter, Ωm ≥ 0.38 (1σ C.L.), in slight tension with
the ΛCDM concordance cosmological model, though the
results are compatible with ΛCDM at 2σ. In addition, we
derive a σ8 normalization relation, σ8Ω
1/3
m = 0.49 ± 0.06
(2σ C.L.).
Combining cluster data with σ8-independent baryon
acoustic oscillation observations, cosmic microwave background
data, Hubble constant measurements, Hubble parameter
determination from passively evolving red galaxies, and magnitude–redshift data of type Ia supernovae, we find
Ωm = 0.28+0.03
−0.02 and σ8 = 0.73+0.03
−0.03, the former in agreement
and the latter being slightly lower than the corresponding
values in the concordance cosmological model. We also
find H0 = 69.1+1.3
−1.5 km/s/Mpc, the fit to the data being almost
independent on n in the adopted range [0.90, 1.05].
Concerning the dark energy equation-of-state parameters,
we show that the present data on massive clusters
weakly constrain (w0,wa) around the values corresponding
to a cosmological constant, i.e. (w0,wa) = (−1, 0). The
global analysis gives w0 =−1.14+0.14
−0.16 and wa = 0.85+0.42
−0.60
(1σ C.L. errors). Very similar results are found in the case of
time-evolving dark energy with a constant equation-of-state
parameter w = const (the XCDM parametrization). Finally,
we show that the impact of bounds on (w0,wa) is to favor
top-down phantom models of evolving dark energy.