Gaia and the use of White Dwarfs as Advanced Physics laboratories

White dwarfs are the final remnants of low and intermediate mass stars and their evolution is essentially a long lasting process of cooling. The tool that allows to compare the theoretical models with the observations is their luminosity function, that is, the number of stars per unit volume and luminosity interval. The shape of the bright branch of this function is only sensitive to the average cooling rate and, thus, it is possible to use it to check for the possible existence of additional non standard sources or sinks of energy able to modify the expected 'normal evolution'. Despite the recent improvements introduced by cosmological surveys like the SDSS and the SCSS catalogues, one of the main difficulties to achieve this goal is still the size of the samples. Gaia, launched in 2013, will allow to obtain, in a next future, the fundamental properties of ~400,000 white dwarfs. Such a sample will be statistically significant and will allow the detection of small deviations from the normal cooling process. As an example, we describe here the case of axions, a not yet detected weakly interacting particle introduced to solve the so called CP-problem of the Standard Model of particles. In particular, we show that their inclusion noticeably improves the agreement between the theoretical and observational white dwarf luminosity functions, thus providing a first hint that axions could exist. This improvement is not only valid for the luminosity function obtained with all three catalogues but also for the luminosity function of the galactic thin and thick disks, suggesting that the change of the luminosity function shape is due to an intrinsic property of white dwarfs and not to a fluctuation of the star formation rate. The best fit is obtained for axion masses around 6 meV, and values larger than 16 meV could probably be excluded.