One of the main points of cosmology is that the Hubble rate of distance expansion is not constant. In the standard cosmic model it evolves according to the Friedman equation. Recently I've been noticing more professional cosmologists referring to it as a rate, like these people (Roy Maartens is a prominent world-class figure, Clarkson is also a leader in the field)(adsbygoogle = window.adsbygoogle || []).push({});

http://arxiv.org/abs/1205.3431

Using H(z) data as a probe of the concordance model

Marina Seikel, Sahba Yahya, Roy Maartens, Chris Clarkson (Cape Town and Western Cape)

(Submitted on 15 May 2012)

Direct observations of the Hubble rate, from cosmic chronometers and the radial baryon acoustic oscillation scale, can out-perform supernovae observations in understanding the expansion history, because supernovae observations need to be differentiated to extract H(z). We use existing H(z) data and smooth the data using a new Gaussian Processes package, GaPP, from which we can also estimate derivatives. The obtained Hubble rate and its derivatives are used to reconstruct the equation of state of dark energy and to perform consistency tests of the LCDM model, some of which are newly devised here. Current data is consistent with the concordance model, but is rather sparse. Future observations will provide a dramatic improvement in our ability to constrain or refute the concordance model of cosmology. We produce simulated data to illustrate how effective H(z) data will be in combination with Gaussian Processes.

Comments: 9 pages, 8 figures.

Or take this earlier paper by Chris Clarkson and friends.

http://arxiv.org/abs/1011.3959

TheHubble ratein averaged cosmology

Obinna Umeh, Julien Larena, Chris Clarkson (ACGC, University of Cape Town)

(Submitted on 17 Nov 2010)

The calculation of the averagedHubble expansion ratein an averaged perturbed Friedmann-Lemaitre-Robertson-Walker cosmology leads to small corrections to the background value of the expansion rate, which could be important for measuring the Hubble constant from local observations. It also predicts an intrinsic variance associated with the finite scale of any measurement of H_{0}, theHubble ratetoday. Both the meanHubble rateand its variance depend on both the definition of theHubble rateand the spatial surface on which the average is performed. We quantitatively study different definitions of the averaged Hubble rate encountered in the literature by consistently calculating the backreaction effect at second order in perturbation theory, and compare the results. We employ for the first time a recently developed gauge-invariant definition of an averaged scalar. We also discuss the variance of theHubble ratefor the different definitions.

Comments: 12 pages, 25 figures.

**Physics Forums | Science Articles, Homework Help, Discussion**

Join Physics Forums Today!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

# It's a rate (not a constant) so let's call it that!

**Physics Forums | Science Articles, Homework Help, Discussion**