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Register a new user$H_0$ Ex Machina: Vacuum Metamorphosis and Beyond $H_0$
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Added by
Eleonora Di Valentino
Publication date
2020
fields
Physics
and research's language is
English
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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.
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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]
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 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.
Phantom dark energy can produce amplified cosmic acceleration at late times, thus increasing the value of $H_0$ favored by CMB data and releasing the tension with local measurements of $H_0$. We show that the best fit value of $H_0$ in the context of the CMB power spectrum is degenerate with a constant equation of state parameter $w$, in accordance with the approximate effective linear equation $H_0 + 30.93; w - 36.47 = 0$ ($H_0$ in $km ; sec^{-1} ; Mpc^{-1}$). This equation is derived by assuming that both $Omega_{0 rm m}h^2$ and $d_A=int_0^{z_{rec}}frac{dz}{H(z)}$ remain constant (for invariant CMB spectrum) and equal to their best fit Planck/$Lambda$CDM values as $H_0$, $Omega_{0 rm m}$ and $w$ vary. For $w=-1$, this linear degeneracy equation leads to the best fit $H_0=67.4 ; km ; sec^{-1} ; Mpc^{-1}$ as expected. For $w=-1.22$ the corresponding predicted CMB best fit Hubble constant is $H_0=74 ; km ; sec^{-1} ; Mpc^{-1}$ which is identical with the value obtained by local distance ladder measurements while the best fit matter density parameter is predicted to decrease since $Omega_{0 rm m}h^2$ is fixed. We verify the above $H_0-w$ degeneracy equation by fitting a $w$CDM model with fixed values of $w$ to the Planck TT spectrum showing also that the quality of fit ($chi^2$) is similar to that of $Lambda$CDM. However, when including SnIa, BAO or growth data the quality of fit becomes worse than $Lambda$CDM when $w< -1$. Finally, we generalize the $H_0-w(z)$ degeneracy equation for $w(z)=w_0+w_1; z/(1+z)$ and identify analytically the full $w_0-w_1$ parameter region that leads to a best fit $H_0=74; km ; sec^{-1} ; Mpc^{-1}$ in the context of the Planck CMB spectrum. This exploitation of $H_0-w(z)$ degeneracy can lead to immediate identification of all parameter values of a given $w(z)$ parametrization that can potentially resolve the $H_0$ tension.
In this letter we propose a reduction of the $H_0$ tension puzzle by means of a theory of minimally modified gravity which is dubbed VCDM. After confronting the theory with the experiments, we find that the data allow for a low-redshift transition in the expansion history of the universe at either $zsimeq 0.3 $ or $z simeq 1.8,$, corresponding to one of the two local minima of the total $chi^2$. From the bestfit values the total fitness parameter is improved by $Delta chi^2 simeq 12$, for the data set considered. We then infer the local Hubble expansion rate today within this theory by means of low redshift Pantheon data. The resulting local Hubble expansion rate today is $H^{rm{loc}}_0=73.6pm1.4$. We find the tension is reduced within the VCDM theory.
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