Is high redshift data a problem for ΛCDM?

[ohwilleke]

A couple of papers in the last couple of years identify problems with the ΛCDM "standard model of cosmology" based upon high redshift astronomy observations. Have there been adequate responses to these concerns?

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+0.09−0.09.

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 wde≡pde/ρde rather than wde=wΛ=−1), a statistically acceptable fit is realized with Ωm=0.22+0.16−0.14 and wde=−0.51+0.15−0.25 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).

 

[kimbyd]

It's hard to say. The issue is that this relies upon the details of how galaxies formed in the early universe. Those details are not well-known and are subject to large uncertainties.

There might be some discrepancy here. Or it might just be that our simulations of how galaxies form are off due to one or more of the many assumptions that are made in order to get the simulations to actually run in a reasonable amount of time. As they note in Steinhardt paper, they do try to account for these uncertainties (the systematics), and the discrepancy seems to remain even after doing this. But it's hard to be certain.

The way forward is to propose a model which solves this discrepancy in the data, and also solves another discrepancy somewhere else in a completely unrelated dataset (e.g. the CMB). This is especially useful if the number of independent tests of this model can be increased further. The reason why the independent checks are important is that no matter how careful scientists are, there's always a chance that something they haven't thought of is messing up their interpretation of the data. This is especially critical when the system in question is as complex as the one involved in galaxy formation.

With regards to the $R_h=ct$ model, I really do think that it is a non-starter as a realistic model. It is a mathematical curiosity that it does so well with the data, but it has no theoretical backing and has no hope of explaining the CMB data. Also bear in mind that it doesn't fit the data better. It uses fewer parameters, and they use a method to apply a penalty to fits with more parameters, which gives the LCDM model a penalty in the comparison. I looked into doing this kind of calculation a number of years back, and it turns out that it is impossible to objectively choose the penalty one should use. You have to make a subjective choice about what the penalty should be, and that choice can have a large impact on how much the lower-parameter model is favored. So while the abstract of this paper claims that $R_h=ct$ is favored to 2-sigma versus LCDM, the fact that this calculation includes a subjective penalty should give one pause.

 

[ohwilleke]

A follow up paper to the second one that I quote in the OP by the same author uses a different statistical methodology to reach the same conclusion, making it somewhat more robust. https://arxiv.org/abs/1711.10793

 

[kimbyd]

A follow up paper to the second one that I quote in the OP by the same author uses a different statistical methodology to reach the same conclusion, making it somewhat more robust. https://arxiv.org/abs/1711.10793

I don't see this as being more helpful. It's curious, but ultimately doesn't really say anything about the universe. It's likely just a consequence of the fact that matter and dark energy have been close to one another in density for the last few billion years, combined with the fact that the model gets a boost in the comparison due to having fewer parameters.

Try combining this with data across a much broader range of redshifts and the $R_h=ct$ model will look much worse. The CMB is particularly problematic for the model.

 

[Garth]

We have discussed here on PF the question of whether there is an Age Problem in the early universe for many years now, since 2005:


Is there an Age Problem in the Mainstream Model? (Oct 2005)
Cosmic age problem ? (Nov 2008)
Is There An Age Problem In The Early LCDM Model? (Jun 2010)
Massive galaxy cluster could upend theory of universe evolution, (Dec 2014)
and An Age Problem (again)? (Jan 2015)

There are many objects at high z that are difficult to explain under the standard model at such an early epoch in the universe's history . The OP is highlighting this concern. On today's physics ArXiv we have another example: J1342+0928 Confirms the Cosmological Timeline in R_h=ct where the author is advocating a linearly expanding (or coasting) model to resolve the tension.

Garth

 

[kimbyd]

We have discussed here on PF the question of whether there is an Age Problem in the early universe for many years now, since 2005:


Is there an Age Problem in the Mainstream Model? (Oct 2005)
Cosmic age problem ? (Nov 2008)
Is There An Age Problem In The Early LCDM Model? (Jun 2010)
Massive galaxy cluster could upend theory of universe evolution, (Dec 2014)
and An Age Problem (again)? (Jan 2015)

There are many objects at high z that are difficult to explain under the standard model at such an early epoch in the universe's history . The OP is highlighting this concern. On today's physics ArXiv we have another example: J1342+0928 Confirms the Cosmological Timeline in R_h=ct where the author is advocating a linearly expanding (or coasting) model to resolve the tension.

Garth

The claim of that author still has all of the problems I discussed above. $R_h=ct$ is an absurd model that requires a rather dramatic deviation from current physics, and doesn't fit early-universe data at all.

 

[Garth]

The claim of that author still has all of the problems I discussed above. $R_h=ct$ is an absurd model that requires a rather dramatic deviation from current physics, and doesn't fit early-universe data at all.

Nevertheless the Age Problem in the early universe appears to remain, as does the tension between the CMB derivation of Hubble's Parameter and that derived from weak lensing KiDS-450: testing extensions to the standard cosmological model. The 'Coasting Model' may indeed have problems but it also has to be recognised that so does the standard $\Lambda$CDM Model.

Garth

 

[kimbyd]

Nevertheless the Age Problem in the early universe appears to remain, as does the tension between the CMB derivation of Hubble's Parameter and that derived from weak lensing KiDS-450: testing extensions to the standard cosmological model. The 'Coasting Model' may indeed have problems but it also has to be recognised that so does the standard $\Lambda$CDM Model.

Garth

There's a large difference between these two, though. A simple comparison:

$\Lambda$CDM: Solid theoretical foundation but may be over-simplification of reality (e.g. dark matter may not be completely "cold", dark energy may not be constant). Fits nearly all data extremely well, though may have some discrepancies in some areas. These discrepancies may indicate that a more accurate model exists.

$R_h=ct$: No theoretical foundation, as it's pure curve-fitting. Implies extreme deviation from known physics. Doesn't fit CMB data remotely well (or anything earlier, such as BBN). On the up side, it has fewer parameters than $\Lambda$CDM.

The most likely resolution to $\Lambda$CDM's discrepancies is some combination of systematic errors and dark matter and/or dark energy having important differences from purely cold and non-interacting dark matter and a cosmological constant (respectively).