Constraining the time evolution of dark energy, curvature and neutrino properties with cosmic chronometers

TL;DR

The paper uses cosmic chronometer measurements of the Hubble parameter H(z) to constrain the time evolution of dark energy, spatial curvature, and neutrino properties in a largely model-independent framework. By combining late-Universe CC data with Planck15 CMB, BAO, SNe, and local H0 measurements, the authors fit both CPL and Chebyshev representations of the dark energy equation of state w(z) and perform MCMC parameter estimation. They find that dark energy is consistent with a cosmological constant (with deviations at most about 40% over 0<z<2); the curvature is tightly constrained to be near flat; and neutrino properties are tightly bounded (Neff around 3.17 and sum of neutrino masses below 0.27 eV at 95% CL). The results illustrate the power of CC data to break degeneracies and test non-expansion-related parameters, providing robust constraints that challenge many dynamical dark energy models and highlighting the value of CC in future cosmological surveys.

Abstract

We use the latest compilation of observational H(z) measurements obtained with cosmic chronometers in the redshift range $0<z<2$ to place constraints on cosmological parameters. We consider the sample alone and in combination with other state-of-the art cosmological probes: CMB data from the latest Planck 2015 release, the most recent estimate of the Hubble constant $H_{0}$, a compilation of recent BAO data, and the latest SNe sample. Since cosmic chronometers are independent of the assumed cosmological model, we are able to provide constraints on the parameters that govern the expansion history of the Universe in a way that can be used to test cosmological models. We show that the H(z) measurements obtained with cosmic chronometer from the BOSS survey provide enough constraining power in combination with CMB data to constrain the time evolution of dark energy, yielding constraints competitive with those obtained using SNe and/or BAO. From late-Universe probes alone we find that $w_0=-0.9\pm0.18$ and $w_a=-0.5\pm1.7$, and when combining also CMB data we obtain $w_0=-0.98\pm0.11$and $w_a=-0.30\pm0.4$. These new constraints imply that nearly all quintessence models are disfavoured, only phantom models or a pure cosmological constant being allowed. For the curvature we find $Ω_k=0.003\pm0.003$, including CMB data. Cosmic chronometers data are important also to constrain neutrino properties by breaking or reducing degeneracies with other parameters. We find that $N_{eff}=3.17\pm0.15$, thus excluding the possibility of an extra (sterile) neutrino at more than $5σ$, and put competitive limits on the sum of neutrino masses, $Σm_ν< 0.27$ eV at 95% confidence level. Finally, we constrain the redshift evolution of dark energy, and find w(z) consistent with the $Λ$CDM model at the 40% level over the entire redshift range $0<z<2$. [abridged]

Figures (9)

• Figure 1: Late-time expansion history dataset used in this analysis. The blue lines do not represent a fit to the data, but show the fiducial Planck $\Lambda$CDM cosmology ($H_{0}=67.8$ km/s/Mpc, $\Omega_{m}=0.308$). Lower panels show the residuals of the data with respect to the fiducial Planck cosmology. CC data have been taken from Refs. Simon2005Stern2010Moresco2012aZhang2014Moresco2015Moresco2016, SNe data from Ref. Betoule2014, and BAO data from Refs. Beutler2011Ross2015Anderson2014.
• Figure 2: Constraints on a flat $w_{0}w_{a}$CDM cosmology from cosmic chronometers (in green, upper-left panel), SNe (in blue, upper-right panel), BAO data (in red, lower-left panel), and from the combination of the various probes (in grey, lower-right panel) obtained with a MCMC approach. In each panel the contour plots are shown at 68% and 95% confidence level, and the posterior distribution of H$_{0}$, $\Omega_{m}$, $w_{0}$ and $w_{a}$, with the 68% and 95% confidence level limits.
• Figure 3: Constraints for the Chebyshev expansion fit obtained from the combination of cosmic chronometer, SNe and BAO data. In each panel the contour plots are shown at 68% and 95% confidence level, and the posterior distribution of H$_{0}$, $\Omega_{m}$, $\omega_{0}$, $\omega_{1}$ and $\omega_{2}$, with the 68% and 95% confidence level limits. For these constraints we assumed a Gaussian prior on $H0=73\pm2.4$Riess2011Humphreys2013Cuesta2015.
• Figure 4: Constraints on $\Omega_{\rm m}$, $w_0$ and $w_a$ for a flat $w_{0}w_{a}$CDM model obtained with different combinations of data sets. Cosmic chronometers and BAO have a similar constraining power in the $w_{0}$-$w_{a}$ plane, as shown in Fig. \ref{['fig:fw0waCDM']}, and the combination of CMB+BAO and CMB+CC provides comparable results. We used $H_{0}73\pm2.4$ km/s/Mpc from Ref. Riess2011Humphreys2013Cuesta2015.
• Figure 5: Reconstruction of the time evolution of dark energy EoS obtained from different probes. Red contours the constraints obtained from late-Universe probes with a CPL parameterisation (see Eq. \ref{['eq:Hztheor']} and Tab. \ref{['tab:fw0waCDM']}), green contours show the constraints obtained from late-Universe probes with Chebyshev decomposition up to the first order (see Eq. \ref{['eq:Cheb1']} and Tab. \ref{['tab:Cheb']}), and blue contours the constraints obtained adding also CMB information with the CPL parameterisation (see Eq. \ref{['tab:fw0waCDM_Pl']}). The darker regions show in each case the 68% confidence level contours, while the light ones the 95% confidence level contours. These constraints assume $H_{0}=73\pm2.4$ km/s/Mpc, taken from Ref. Riess2011Humphreys2013Cuesta2015.