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Register a new userCosmological cluster tension
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Blanchard Alain Prof.
Publication date
2018
fields
Physics
and research's language is
English
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The abundance of clusters is a classical cosmological probe sensitive to both the geometrical aspects and the growth rate of structures. The abundance of clusters of galaxies measured by Planck has been found to be in tension with the prediction of the LCDM models normalized to Planck CMB fluctuations power spectra. The same tension appears with X-ray cluster local abundance. Massive neutrinos and modified gravity are two possible solutions to fix this tension. Alternatively, others options include a bias in the selection procedure or in the mass calibration of clusters. We present a study, based on our recent work, updating the present situation on this topic and discuss the likelihood of the various options.
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The exquisite measurements of the cosmic microwave background (CMB) fluctuations by Planck allows us to tightly constrain the amplitude of matter fluctuations at redshift $sim 1100$ in the $Lambda$-cold dark matter ($Lambda$CDM) model. This amplitude can be extrapolated to the present epoch, yielding constraints on the value of the $sigma_8$ parameter. On the other hand the abundance of Sunyaev-Zeldovich (SZ) clusters detected by Planck, with masses inferred by using hydrostatic equilibrium estimates, leads to a significantly lower value of the same parameter. This discrepancy is often dubbed the $sigma_8$ tension in the literature and is sometimes regarded as a possible sign of new physics. Here, we examine a direct determination of $sigma_8$ at the present epoch in $Lambda$CDM, and thereby the cluster mass calibrations using cosmological data at low redshift, namely the measurements of $fsigma_8$ from the analysis of the completed Sloan Digital Sky Survey (SDSS): we combine redshift-space distortions measurements with Planck CMB constraints, X-ray, and SZ cluster counts within the $Lambda$CDM framework, but leaving the present day amplitude of matter fluctuations as an independent parameter (i.e. no extrapolation is made from high-redshift CMB constraints). The calibration of X-ray and SZ masses are therefore left as free parameters throughout the whole analysis. Our study yields tight constraints on the aforementioned calibrations, with values entirely consistent with results obtained from the full combination of CMB and cluster data only. Such agreement suggests an absence of tension in the $Lambda$CDM model between CMB-based estimates of $sigma_8$ and constraints from low-redshift on $fsigma_8$ but indicates a tension with the standard calibration of clusters masses.
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.
Recent measurements of the D(p,$gamma)^3$He, nuclear reaction cross-section and of the neutron lifetime, along with the reevaluation of the cosmological baryon abundance from cosmic microwave background (CMB) analysis, call for an update of abundance predictions for light elements produced during the big-bang nucleosynthesis (BBN). While considered as a pillar of the hot big-bang model in its early days, BBN constraining power mostly rests on deuterium abundance. We point out a new $simeq1.8sigma$-tension on the baryonic density, or equivalently on the D/H abundance, between the value inferred on one hand from the analysis of the primordial abundances of light elements and, on the other hand, from the combination of CMB and baryonic oscillation data. This draws the attention on this sector of the theory and gives us the opportunity to reevaluate the status of BBN in the context of precision cosmology. Finally, this paper presents an upgrade of the BBN code PRIMAT.
Evolution and abundance of the large-scale structures we observe today, such as clusters of galaxies, is sensitive to the statistical properties of dark matter primordial density fluctuations, which is assumed to follow a Gaussian probability distribution function. Within this assumption, a significant disagreement have been found between clusters counts made by Planck and their prediction when calibrated by CMB angular power spectrum. The purpose of this work is to relax the Gaussianty assumption and test if Non-Gaussianity in dark matter primordial density fluctuations, could alleviate the tension.
We present an improved Minimal Variance (MV) method for using a radial peculiar velocity sample to estimate the average of the three-dimensional velocity field over a spherical volume, which leads to an easily interpretable bulk flow measurement. The only assumption required is that the velocity field is irrotational. The resulting bulk flow estimate is particularly insensitive to smaller scale flows. We also introduce a new constraint into the MV method that ensures that bulk flow estimates are independent of the value of the Hubble constant $H_o$; this is important given the tension between the locally measured $H_o$ and that obtained from the cosmic background radiation observations. We apply our method to the textit{CosmicFlows-3} catalogue and find that, while the bulk flows for shallower spheres are consistent with the standard cosmological model, there is some tension between the bulk flow in a spherical volume with radius $150$hmpc and its expectations; we find only a $sim 2%$ chance of obtaining a bulk flow as large or larger in the standard cosmological model with textit{Planck} parameters
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