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Register a new user$H_0$ tension without CMB data: beyond the $Lambda$CDM
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We investigate the $H_0$ tension in a range of extended model frameworks beyond the standard $Lambda$CDM without the data from cosmic microwave background (CMB). Specifically, we adopt the data from baryon acoustic oscillation, big bang nucleosynthesis and type Ia supernovae as indirect measurements of $H_0$ to study the tension. We show that the estimated value of $H_0$ from indirect measurements is overall lower than that from direct local ones regardless of the data sets and a range of extended models to be analyzed, which indicates that, although the significance of the tension varies depending on models, the $H_0$ tension persists in a broad framework beyond the standard $Lambda$CDM model even without CMB data.
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Cosmological constraints are usually derived under the assumption of a $6$ parameters $Lambda$-CDM theoretical framework or simple one-parameter extensions. In this paper we present, for the first time, cosmological constraints in a significantly extended scenario, varying up to $12$ cosmological parameters simultaneously, including the sum of neutrino masses, the neutrino effective number, the dark energy equation of state, the gravitational waves background and the running of the spectral index of primordial perturbations. Using the latest Planck 2015 data release (with polarization) we found no significant indication for extensions to the standard $Lambda$-CDM scenario, with the notable exception of the angular power spectrum lensing amplitude, $A_{rm lens}$ that is larger than the expected value at more than two standard deviations even when combining the Planck data with BAO and supernovae type Ia external datasets. In our extended cosmological framework, we find that a combined Planck+BAO analysis constrains the value of the r.m.s. density fluctuation parameter to $sigma_8=0.781_{-0.063}^{+0.065}$ at $95 %$ c.l., helping to relieve the possible tensions with the CFHTlenS cosmic shear survey. We also find a lower value for the reionization optical depth $tau=0.058_{-0.043}^{+0.040}$ at $95$ % c.l. respect to the one derived under the assumption of $Lambda$-CDM. The scalar spectral index $n_S$ is now compatible with a Harrison-Zeldovich spectrum to within $2.5$ standard deviations. Combining the Planck dataset with the HST prior on the Hubble constant provides a value for the equation of state $w < -1$ at more than two standard deviations while the neutrino effective number is fully compatible with the expectations of the standard three neutrino framework.
We do not solve tensions with concordance cosmology; we do obtain $H_0approx 74,$km/s/Mpc from CMB+BAO+SN data in our model, but that is not the point. Discrepancies in Hubble constant values obtained by various astrophysical probes should not be viewed in isolation. While one can resolve at least some of the differences through either an early time transition or late time transition in the expansion rate, these introduce other changes. We advocate a holistic approach, using a wide variety of cosmic data, rather than focusing on one number, $H_0$. Vacuum metamorphosis, a late time transition physically motivated by quantum gravitational effects and with the same number of parameters as lcdm, can successfully give a high $H_0$ value from cosmic microwave background data but fails when combined with multiple distance probes. We also explore the influence of spatial curvature, and of a conjoined analysis of cosmic expansion and growth.
In this article we compare a variety of well known dynamical dark energy models using the cosmic microwave background measurements from the 2018 Planck legacy and 2015 Planck data releases, the baryon acoustic oscillations measurements and the local measurements of $H_0$ obtained by the SH0ES (Supernovae, $H_0$, for the Equation of State of Dark energy) collaboration analysing the Hubble Space Telescope data. We discuss the alleviation of $H_0$ tension, that is obtained at the price of a phantom-like dark energy equation of state. We perform a Bayesian evidence analysis to quantify the improvement of the fit, finding that all the dark energy models considered in this work are preferred against the $Lambda$CDM scenario. Finally, among all the possibilities analyzed, the CPL model is the best one in fitting the data and solving the $H_0$ tension at the same time. However, unfortunately, this dynamical dark energy solution is not supported by the baryon acoustic oscillations (BAO) data, and the tension is restored when BAO data are included for all the models.
Flavour oscillations experiments are suggesting the existence of a sterile, $4$th neutrinos generation with a mass of an eV order. This would mean an additional relativistic degree of freedom in the cosmic inventory, in contradiction with recent results from the Planck satellite, that have confirmed the standard value $N_{eff} approx 3$ for the effective number of relativistic species. On the other hand, the Planck best-fit for the Hubble-Lema^itre parameter is in tension with the local value determined with the Hubble Space Telescope, and adjusting $N_{eff}$ is a possible way to overcome such a tension. In this paper we perform a joint analysis of three complementary cosmological distance rulers, namely the CMB acoustic scale measured by Planck, the BAO scale model-independently determined by Verde {it et al.}, and luminosity distances measured with JLA and Pantheon SNe Ia surveys. Two Gaussian priors were imposed to the analysis, the local expansion rate measured by Riess {it et al.}, and the baryon density parameter fixed from primordial nucleosynthesis by Cooke {it et al.}. For the sake of generality, two different models are used in the tests, the standard $Lambda$CDM model and a generalised Chaplygin gas. The best-fit gives $N_{eff} approx 4$ in both models, with a Chaplygin gas parameter slightly negative, $alpha approx -0.04$. The standard value $N_{eff} approx 3$ is ruled out with $approx 3sigma$.
We show that the $H_0$ tension can be resolved by making recombination earlier, keeping the fit to cosmic microwave background (CMB) data almost intact. We provide a suite of general necessary conditions to give a good fit to CMB data while realizing a high value of $H_0$ suggested by local measurements. As a concrete example for a successful scenario with early recombination, we demonstrate that a model with time-varying $m_e$ can indeed satisfy all the conditions. We further show that such a model can also be well fitted to low-$z$ distance measurements of baryon acoustic oscillation (BAO) and type-Ia supernovae (SNeIa) with a simple extension of the model. Time-varying $m_e$ in the framework of $Omega_kLambda$CDM is found to be a sufficient and excellent example as a solution to the $H_0$ tension, yielding $H_0=72.3_{-2.8} ^{+2.7},$km/sec/Mpc from the combination of CMB, BAO and SNeIa data even without incorporating any direct local $H_0$ measurements. Apart from the $H_0$ tension, this model is also favored from the viewpoint of the CMB lensing anomaly.
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