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4.6. H0 with PG 1115+080

Although many lensed quasars are potentially as powerful as PG 1115+080 for constraining H0, this nice quadruple remains one of the best studied cases so far, along with the double Q 0957+561. Basically all possible observational tools have been used to study PG 1115+080, and we have adopted it as an example to illustrate how observations help the theorist in quasar lensing. Some of the main results obtained for PG 1115+080 are summarized in Table 1, following the chronological order of the observational saga of the object.

As the observations of PG 1115+080 were improving, various values were found for H0 along the years. This is mainly a consequence of the mass sheet degeneracy and to the lack of observational constraints on the actual mass profile of the lens. Among the range of possible values, the highest ones were obtained for lenses with constant mass-to-light ratios, and lower values of H0 were found for models including a dark matter halo. If our knowledge of the physics of galaxies is any good, the latter lens model is closer to the truth than the former. This results in values for H0 that disagree (see Table 1) with local estimates, based on standard candles, i.e., H0 = 72 ± 8 km s-1 Mpc-1 (Freedman et al. 2001).