# Re acceleration of the Universe

1. Jan 11, 2016

### wolram

Just thought i would flag this up.

arXiv:1601.01701 [pdf, ps, other]
A 6% measurement of the Hubble parameter at $z\sim0.45$: direct evidence of the epoch of cosmic re-acceleration
Michele Moresco, Lucia Pozzetti, Andrea Cimatti, Raul Jimenez, Claudia Maraston, Licia Verde, Daniel Thomas, Annalisa Citro, Rita Tojeiro, David Wilkinson
Comments: 30 pages, 9 figures, 5 tables, submitted to JCAP. The H(z) data can be downloaded at this http URL
Subjects: Cosmology and Nongalactic Astrophysics (astro-ph.CO)

Deriving the expansion history of the Universe is a major goal of modern cosmology. To date, the most accurate measurements have been obtained with Type Ia Supernovae and Baryon Acoustic Oscillations, providing evidence for the existence of a transition epoch at which the expansion rate changes from decelerated to accelerated. However, these results have been obtained within the framework of specific cosmological models that must be implicitly or explicitly assumed in the measurement. It is therefore crucial to obtain measurements of the accelerated expansion of the Universe independently of assumptions on cosmological models. Here we exploit the unprecedented statistics provided by the Baryon Oscillation Spectroscopic Survey (BOSS) Data Release 9 to provide new constraints on the Hubble parameter $H(z)$ using the em cosmic chronometers approach. We extract a sample of more than 130000 of the most massive and passively evolving galaxies, obtaining five new cosmology-independent $H(z)$ measurements in the redshift range $0.3<z<0.5$, with an accuracy of $\sim$11-16\% incorporating both statistical and systematic errors. Once combined, these measurements yield a 6\% accuracy constraint of $H(z=0.4293)=91.8\pm5.3$ km/s/Mpc. The new data are crucial to provide the first cosmology-independent determination of the transition redshift at high statistical significance, measuring $z_{t}=0.4\pm0.1$, and to significantly disfavor the null hypothesis of no transition between decelerated and accelerated expansion at 99.9\% confidence level. This analysis highlights the wide potential of the cosmic chronometers approach: it permits to derive constraints on the expansion history of the Universe with results competitive with standard probes, and most importantly, being the estimates independent of the cosmological model, it can constrain cosmologies beyond -and including- the $\Lambda$CDM model

2. Jan 12, 2016

### Garth

Yes, I found that paper interesting. Particularly the emphasis on being able to "derive constraints on the expansion history of the Universe with results competitive with standard probes, and most importantly, being the estimates independent of the cosmological model". (emphasis mine)

Notice their statement:
If you plot this value on my diagram of the 'H(z) v z' expansion history of the universe,
https://www.physicsforums.com/threa...n-from-type-ia-sne.817386/page-3#post-5137199, the error bars straddle the R =ct solid line but just miss the $\Lambda$CDM red dashed line.

Note: those lines were based on a h0 = 0.67, the 'R = ct' plot would be exactly over the 'H(z=0.4293)=91.8$\pm$5.3 km/s/Mpc' data point, if h0 was lower, say ~ 0.65.

Just a thought,
Garth

Last edited: Jan 12, 2016