Monday, November 27, 2017

The Impossible Early Galaxy Problem And Other Cracks In The ΛCDM Model

Another problem with the ΛCDM "standard model of cosmology."
The current hierarchical merging paradigm and ΛCDM predict that the z ∼ 4−8 universe should be a time in which the most massive galaxies are transitioning from their initial halo assembly to the later baryonic evolution seen in star-forming galaxies and quasars. However, no evidence of this transition has been found in many high redshift galaxy surveys including CFHTLS, CANDELS and SPLASH, the first studies to probe the high-mass end at these redshifts. Indeed, if halo mass to stellar mass ratios estimated at lower-redshift continue to z ∼ 6−8, CANDELS and SPLASH report several orders of magnitude more M ∼ 10^12−13 M⊙ halos than are possible to have formed by those redshifts, implying these massive galaxies formed impossibly early. We consider various systematics in the stellar synthesis models used to estimate physical parameters and possible galaxy formation scenarios in an effort to reconcile observation with theory. Although known uncertainties can greatly reduce the disparity between recent observations and cold dark matter merger simulations, even taking the most conservative view of the observations, there remains considerable tension with current theory.
Charles L. Steinhardt, et al., "The Impossibly Early Galaxy Problem" (June 3, 2015).

Along the same lines (and note that wCDM is not WDM):
We continue to build support for the proposal to use HII galaxies (HIIGx) and giant extragalactic HII regions (GEHR) as standard candles to construct the Hubble diagram at redshifts beyond the current reach of Type Ia supernovae. Using a sample of 25 high-redshift HIIGx, 107 local HIIGx, and 24 GEHR, we confirm that the correlation between the emission-line luminosity and ionized-gas velocity dispersion is a viable luminosity indicator, and use it to test and compare the standard model ΛCDM and the Rh=ct  Universe by optimizing the parameters in each cosmology using a maximization of the likelihood function. For the flat ΛCDM model, the best fit is obtained with Ωm=0.40+0.090.09. 
However, statistical tools, such as the Akaike (AIC), Kullback (KIC) and Bayes (BIC) Information Criteria favor Rh=ct over the standard model with a likelihood of 94.8%98.8% versus only 1.2%5.2%     . For wCDM (the version of ΛCDM with a dark-energy equation of state wdepde/ρde rather than wde=wΛ=1), a statistically acceptable fit is realized with Ωm=0.22+0.160.14 and wde=0.51+0.150.25  which, however, are not fully consistent with their concordance values. In this case, wCDM has two more free parameters than Rh=ct, and is penalized more heavily by these criteria. We find that Rh=ct is strongly favored over wCDM with a likelihood of 92.9%99.6% versus only 0.4%7.1%. The current HIIGx sample is already large enough for the BIC to rule out ΛCDM/wCDM in favor of Rh=ct at a confidence level approaching 3σ.
Jun-Jie Wei, et al., "The HII Galaxy Hubble Diagram Strongly Favors R_h=ct over ΛCDM" (August 6, 2016).

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