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Eleonora Di Valentino
Publication date
2021
fields
Physics
and research's language is
English
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With the entrance of cosmology in its new era of high precision experiments, low- and high-redshift observations set off tensions in the measurements of both the present-day expansion rate ($H_0$) and the clustering of matter ($S_8$). We provide a simultaneous explanation of these tensions using the Parker-Raval Vacuum Metamorphosis (VM) model with the neutrino sector extended beyond the three massless Standard Model flavours and the curvature of the universe considered as a model parameter. To estimate the effect on cosmological observables we implement various extensions of the VM model in the standard texttt{CosmoMC} pipeline and establish which regions of parameter space are empirically viable to resolve the $H_0$ and $S_8$ tensions. We find that the likelihood analyses of the physically motivated VM model, which has the same number of free parameters as in the spatially-flat $Lambda$CDM model, always gives $H_0$ in agreement with the local measurements (even when BAO or Pantheon data are included) at the price of much larger $chi^2$ than $Lambda$CDM. The inclusion of massive neutrinos and extra relativistic species quantified through two well known parameters $sum m_{ u}$ and $N_{rm eff}$, does not modify this result, and in some cases improves the goodness of the fit. In particular, for the original VM+$sum m_ u$+$N_{rm eff}$ and the Planck+BAO+Pantheon dataset combination, we find evidence for $sum m_{ u}=0.80^{+0.18}_{-0.22}~{rm eV}$ at more than $3sigma$, no indication for extra neutrino species, $H_0=71.0pm1.2$~km/s/Mpc in agreement with local measurements, and $S_8=0.755pm0.032$ that solves the tension with the weak lensing measurements. [Abridged]
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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.
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 $approx 3sigma$.
The cosmological term, $Lambda$, was introduced $104$ years ago by Einstein in his gravitational field equations. Whether $Lambda$ is a rigid quantity or a dynamical variable in cosmology has been a matter of debate for many years, especially after the introduction of the general notion of dark energy (DE). $Lambda$ is associated to the vacuum energy density, $rho_{rm vac}$, and one may expect that it evolves slowly with the cosmological expansion. Herein we present a devoted study testing this possibility using the promising class of running vacuum models (RVMs). We use a large string $SNIa+BAO+H(z)+LSS+CMB$ of modern cosmological data, in which for the first time the CMB part involves the full Planck 2018 likelihood for these models. We test the dependence of the results on the threshold redshift $z_*$ at which the vacuum dynamics is activated in the recent past and find positive signals up to $sim4.0sigma$ for $z_*simeq 1$. The RVMs prove very competitive against the standard $Lambda$CDM model and give a handle for solving the $sigma_8$ tension and alleviating the $H_0$ one.
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.
On the assumption that quasars (QSO) and gamma-ray bursts (GRB) represent standardisable candles, we provide evidence that the Hubble constant $H_0$ adopts larger values in hemispheres aligned with the CMB dipole direction. The observation is consistent with similar trends in strong lensing time delay, Type Ia supernovae (SN) and with well documented discrepancies in the cosmic dipole. Therefore, not only do strong lensing time delay, Type Ia SN, QSOs and GRBs seem to trace a consistent anisotropic Universe, but variations in $H_0$ across the sky suggest that Hubble tension is a symptom of a deeper cosmological malaise.
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