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# COSMOGRAIL: the COSmological MOnitoring of GRAvItational lenses - VII. Time delays and the Hubble constant from WFI J2033-4723

Astronomy and Astrophysics (Impact Factor: 4.38). 03/2008; 488(2). DOI: 10.1051/0004-6361:200809866

Source: arXiv

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**ABSTRACT:**We present accurate time delays for the quadruply imaged quasar HE 0435-1223, from the COSMOGRAIL collaboration. A new way of turning the delays into H0 is proposed, using Nbody realisations of the lensing galaxy. The delays are measured from 575 independent photometric points obtained between January 2004 and March 2010. With 6 years of data, we clearly show that quasar image A is affected by strong microlensing variations and that the time delays are best expressed relative to quasar image B. We measure delta_t(BA) = 8.4+/-2.1 days, delta_t(BC) = 7.8+/-0.8 days and delta_t(BD) = -6.5+/-0.7 days. HST NICMOS2 images are deconvolved in order to derive accurate astrometry of the quasar images and to infer the light profile of the lensing galaxy. In combination with VLT spectroscopy of the lens, the HST images are used to estimate the baryonic fraction, fb, in the Einstein radius. We measure fb = 0.65+0.13-0.10 if the lensing galaxy has a Kroupa IMF and fb = 0.45+0.04-0.07 if it has a Salpeter IMF. N-body realisations of the lensing galaxy are used to infer its dark matter profile, given the measured rest-frame stellar velocity dispersion, sigma_ap = 222+/-34 km/s and the baryonic fraction. These dynamical models and baryonic fraction are also required to match the lensing observables. We find that only the lensing galaxies with Kroupa IMF match all the data simultaneously. Using the time delays to estimate the Hubble constant under this assumption leads to H0 = 62+6-4 km/s/Mpc. While the relatively small formal error bars reflect the high potential of the method to provide an accurate estimate of H0, the value itself might be revised when new observational constraints are available, in particular a high precision velocity dispersion measurement (or velocity dispersion profile) of the lens and a measurement of the external shear, from integral-field spectroscopy and/or deep X-ray images. - [Show abstract] [Hide abstract]

**ABSTRACT:**The basis for a new model of gravitation is presented, as are its basic cosmological consequences. Gravity is conceived as a process of outward movement of matter and space whose cumulative effect is the exponential expansion of the Universe. In the cosmological extreme the model thus resembles Masreliez's Expanding Spacetime Theory. [1] Unlike the latter theory, the new model predicts novel effects that can be tested in a modest laboratory. The next most noteworthy feature of the model is that it gives new meaning to the well-known "large numbers coincidences." This new ap-proach encompasses a broader range of physical reality than usual, including now the cosmic background radiation and the density of atomic nuclei. -
##### Article: The Hubble Constant

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**ABSTRACT:**I review the current state of determinations of the Hubble constant, which gives the length scale of the Universe by relating the expansion velocity of objects to their distance. In the last 20 years, much progress has been made and estimates now range between 60 and 75 km/s/Mpc, with most now between 70 and 75km/s/Mpc, a huge improvement over the factor-of-2 uncertainty which used to prevail. Further improvements which gave a generally agreed margin of error of a few percent rather than the current 10% would be vital input to much other interesting cosmology. There are several programmes which are likely to lead us to this point in the next 10 years.Living Reviews in Relativity 09/2007; 10. DOI:10.12942/lrr-2007-4 · 19.25 Impact Factor