# How safe is the LCDM model

1. Feb 8, 2016

### wolram

I noticed this in the arxivs, i thought the LCDM model was irifutable but it seems some are trying to better it.

arXiv:1602.02103 [pdf, ps, other]
First evidence of running cosmic vacuum: challenging the concordance model
Joan Sola, Adria Gomez-Valent, Javier de Cruz Perez
Comments: LaTeX, 6 pages, 2 tables and 3 figures
Subjects: Cosmology and Nongalactic Astrophysics (astro-ph.CO); General Relativity and Quantum Cosmology (gr-qc); High Energy Physics - Phenomenology (hep-ph); High Energy Physics - Theory (hep-th)

Despite the fact that a rigid $\Lambda$-term is a fundamental building block of the concordance $\Lambda$CDM model, we show that a large class of cosmological scenarios with dynamical vacuum energy density $\rho_{\Lambda}$ and/or gravitational coupling $G$, together with a possible non-conservation of matter, are capable of seriously challenging the traditional phenomenological success of the $\Lambda$CDM. In this Letter, we discuss these "running vacuum models" (RVM's), in which $\rho_{\Lambda}=\rho_{\Lambda}(H)$ consists of a nonvanishing constant term and a series of powers of the Hubble rate. Such generic structure is potentially linked to the quantum field theoretical description of the expanding Universe. By performing an overall fit to the cosmological observables $SNIa+BAO+H(z)+LSS+BBN+CMB$ (in which the WMAP9, Planck 2013 and Planck 2015 data are taken into account), we find that the RVM's appear definitely more favored than the $\Lambda$CDM, namely at an unprecedented level of $\sim 4\sigma$, implying that the $\Lambda$CDM is excluded at $\sim 99.99\%$ c.l. Furthermore, the Akaike and Bayesian information criteria confirm that the dynamical RVM's are strongly preferred as compared to the conventional rigid $\Lambda$-picture of the cosmic evolution.

2. Feb 8, 2016

### Orodruin

Staff Emeritus
Of course someone is trying to find a better model, this is what science is all about. If a model was irrefutable it would not be a good scientific model.

3. Feb 8, 2016

### Chalnoth

My bet is that this will turn out to be a result of some subtle systematic error. But it would be very exciting if it turned out to be accurate!

4. Feb 8, 2016

### bcrowell

Staff Emeritus
From a quick scan of the paper, it appears to have no discussion of possible sources of systematic error. Unless there's some such discussion in there that I'm missing, I would not believe this result at all. We should all have learned our lesson from the bogus BICEP2 result.

5. Feb 8, 2016

### Chalnoth

It looks like they're aggregating data released through other experiments. My bet is that there's some subtle differences between the way the different data sets were calibrated (or some similar systematic effect) that leads to a spurious signal. It'll take a fair amount of work to see where the discrepancy lies, however.

6. Feb 8, 2016

### ohwilleke

LCDM has some pretty meaningful error bars itself, and is based upon some very subtle experimental observations some of which like the cosmic background radiation, have nonetheless been measured to pretty much maximal precision.

Indeed, in some sense, we know for a fact that LCDM is not correct as a matter of physics, because it ignores some factors (e.g. radiation) which we know exist and have an impact, because the improvement added by including all known factors is overshadowed by the reduction of statistical power per degree of freedom involved in omitting those factors. Until such time as we have much greater precision measurements (which may be never) we may never be able to distinguish between some of the alternatives that make similar predictions at the cosmological scale observation level.

Overall, it is much more fruitful to, for example, look at galactic and cluster level phenomena to better understand dark matter and dark energy and then to insert that insight back into discriminating between LCDM and its competitors, than to try to distinguish between theories that are experimentally indistinguishable given the amount of noise in the existing data.

7. Feb 9, 2016

### Jorrie

How do you mean "radiation is ignored"? It features strongly in the LCDM equations at times earlier than the CMB release. The Planck results give a radiation/matter equality redshift of z~3400, which is not that long before the CMB origin. It is so that in later time observations its effects are negligible, but it is not ignored.

8. Feb 9, 2016

### Chalnoth

The CMB has only been measured at close to maximal precision for temperature anisotropies. There's still quite a long way to go with regard to polarization.

Radiation is usually ignored for late-time expansion because its magnitude is so small. The actual energy density of the radiation isn't a free parameter at all, but is extremely accurately-measured through CMB observations (It's of the order of 0.001% of the current energy density). Taking the radiation energy density into account is important for modeling the CMB, but doesn't have much impact for anything after that.

The tricky thing there is that galaxy and cluster physics are much, much more complicated, so that it becomes difficult to control for systematic errors for these systems. Not impossible, just tricky.