Michele Moresco

A 6% measurement of the Hubble parameter at z∼0.45: direct evidence of the epoch of cosmic re-acceleration

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 (SNe) and Baryon Acoustic Oscillations (BAO), 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, [1-3]) Data Release 9 to provide new constraints on the Hubble parameter H(z) using the 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 ~11–16% incorporating both statistical and systematic errors. Once combined, these measurements yield a 6% accuracy constraint of H(z = 0.4293) = 91.8 ± 5.3 km/s/Mpc. The new data are crucial to provide the first cosmology-independent determination of the transition redshift at high statistical significance, measuring zt = 0.4 ± 0.1, 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 ΛCDM model.

Raising the bar: new constraints on the Hubble parameter with cosmic chronometers at z ∼ 2

One of the most compelling tasks of modern cosmology is to constrain the expansion history of the Universe, since this measurement can give insights on the nature of dark energy and help to estimate cosmological parameters. In this letter are presented two new measurements of the Hubble parameter H(z) obtained with the cosmic chronometer method up to zxa0∼xa02. Taking advantage of near-infrared spectroscopy of the few very massive and passive galaxies observed at zxa0>xa01.4 available in literature, the differential evolution of this population is estimated and calibrated with different stellar population synthesis models to constrain H(z), including in the final error budget all possible sources of systematic uncertainties (star formation history, stellar metallicity, model dependences). This analysis is able to extend significantly the redshift range coverage with respect to present-day constraints, crossing for the first time the limit at zxa0∼xa01.75. The new H(z) data are used to estimate the gain in accuracy on cosmological parameters with respect to previous measurements in two cosmological models, finding a small but detectable improvement (∼5u2009peru2009cent) in particular on Ω_M and w_0. Finally, a simulation of a Euclid-like survey has been performed to forecast the expected improvement with future data. The provided constraints have been obtained just with the cosmic chronometers approach, without any additional data, and the results show the high potentiality of this method to constrain the expansion history of the Universe at these redshifts.

An improved measurement of baryon acoustic oscillations from the correlation function of galaxy clusters at z ∼ 0.3

We detect the peak of baryon acoustic oscillations (BAO) in the two-point correlation function of a spectroscopic sample of

Inferring the star formation histories of the most massive and passive early-type galaxies at z<0.3

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Constraining the time evolution of dark energy, curvature and neutrino properties with cosmic chronometers

clusters selected from the Sloan Digital Sky Survey. Galaxy clusters, as tracers of massive dark matter haloes, are highly biased structures. The linear bias

Disentangling interacting dark energy cosmologies with the three-point correlation function

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Passive galaxies as tracers of cluster environments at z 2

of the sample considered in this work, that we estimate from the projected correlation function, is

Old age and supersolar metallicity in a massive z ∼ 1.4 early-type galaxy from VLT/X-Shooter spectroscopy

b sigma_8 = 1.72 pm 0.03

Measuring the distance–redshift relation with the baryon acoustic oscillations of galaxy clusters

. Thanks to the high signal in the cluster correlation function and to the accurate spectroscopic redshift measurements, we can clearly detect the BAO peak and determine its position,

CosmoBolognaLib: C++ libraries for cosmological calculations

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