# Is the LCDM model correct

1. Jul 31, 2015

### wolram

Not according to this paper.

arXiv:1507.08279 [pdf, ps, other]
A Test of Cosmological Models using high-z Measurements of H(z)
Fulvio Melia, Thomas M. McClintock
Comments: 22 pages, 1 figure, 1 table. Accepted for publication in the Astronomical Journal
Subjects: Cosmology and Nongalactic Astrophysics (astro-ph.CO); Astrophysics of Galaxies (astro-ph.GA); High Energy Physics - Phenomenology (hep-ph)

The recently constructed Hubble diagram using a combined sample of SNLS and SDSS-II Type Ia SNe, and an application of the Alcock-Paczynski (AP) test using model-independent Baryon Acoustic Oscillation data, have suggested that the principal constraint underlying the cosmic expansion is the total equation-of-state of the cosmic fluid, rather than that of its dark energy. These studies have focused on the critical redshift range (0 < z < 2) within which the transition from decelerated to accelerated expansion is thought to have occurred, and they suggest that the cosmic fluid has zero active mass, consistent with a constant expansion rate. The evident impact of this conclusion on cosmological theory calls for an independent confirmation. In this paper, we carry out this crucial one-on-one comparison between the R_h=ct Universe (an FRW cosmology with zero active mass) and wCDM/LCDM, using the latest high-z measurements of H(z). Whereas the Type Ia SNe yield the integrated luminosity distance, while the AP diagnostic tests the geometry of the Universe, the Hubble parameter directly samples the expansion rate itself. We find that the model-independent cosmic chronometer data prefer R_h}=ct over wCDM/LCDM with a BIC likelihood of ~95% versus only ~5%, in strong support of the earlier SNeIa and AP results. This contrasts with a recent analysis of H(z) data based solely on BAO measurements which, however, strongly depend on the assumed cosmology. We discuss why the latter approach is inappropriate for model comparisons, and emphasize again the need for truly model-independent observations to be used in cosmological tests.

2. Jul 31, 2015

### Garth

Their Figure 1 from the OP eprint.

They stop at z = 2, of course they could have taken the red shift out to z ~ 2.4 as illustrated by the diagram in my thread 'Utility of observational Hubble parameter data on DE', here: Utility of observational Hubble parameter data on DE. (Ignore my comments on that post for now).

At z < 1 the earlier analysis clearly favours $\Lambda$CDM and the two plots for $\Lambda$CDM and R = ct cross around z = 1.7 so predictions of the two theories are the same there, however beyond z > 2 that data set points clearly favours R = ct. It is therefore interesting here that Melia and McClintock concentrate on the lower red shift range and still find that they favour linear expansion.

Once a model derived from interpreted data has been established to be the standard one it will take "extraordinary evidence" to support the "extraordinary claim" that it might be wrong, I doubt whether the OP eprint provides such evidence, however their analysis is intriguing.

Garth

Last edited: Jul 31, 2015
3. Jul 31, 2015

### wolram

I agree it will take extraordinary evidence to beat the LCDM model, however people are all ways pecking around the edges of it.

4. Jul 31, 2015

### ohwilleke

It is hard to believe that the data set in Figure 1 really distinguishes between the two scenarios at a 19-1 odds. It looks quite inconclusive to me just eyeballing it carefully.

5. Jul 31, 2015

### bcrowell

Staff Emeritus
6. Jul 31, 2015

### Chronos

One thing that struch me was his rather peculiar focus on discrediting work by Daniel Shafer. Melia offers little new to his long standing feud over an R_h=ct universe with other reputable cosmologists. Futhermore the argument that his approach is model independent appears poorly subtantiated and possibly circular.

7. Aug 1, 2015

### Garth

As a theoretical model the linearly expanding model will get nowhere unless there is a 'mechanism' that delivers the "zero effective mass" requirement i.e. an equation of state $\omega = - \frac{1}{3}$.

On the other hand, as far as observations go, the question of how the universe is expanding is now being explored out to higher and higher red shifts.

It will be interesting to see what the next generation of surveys at higher red shift bring in, in the regime where the H(z) v z plots really diverge.

As it said in the paper Utility of observational Hubble parameter data on dark energy evolution
Garth

Last edited: Aug 1, 2015
8. Aug 11, 2015

### yogi

Perhaps you can embellish upon that a bit... it would seem to me that the w = -1/3 is tantamount to a mandated state for a net zero energy universe. i.e., expanding negative pressure creates positive energy (a la McCrea) at the same rate that negative potential grows due to the expanding volume - if the cosmological constant is left out of the equation, the expansion is linear i.e., R double dot is zero, whereas if the CC is included, the expansion is exponential, and the acceleration corresponds to q = -1. What other equation of state corresponds to zero energy during expansion?

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