"One of the most intriguing recent results in the physical sciences, has been the growing body of evidence that 70% of the energy content in the universe comes from an unknown energy field (‘dark energy’). One of the most promising methods for quantifying dark energy, is using SNe Ia as standard candles. However, a key issue in SN Ia studies is whether the results are biased by cosmic dust. The interstellar medium (ISM) of galaxies causes light passing through to be partially extinguished. The extinction depends on wavelength, and is directly related to the dust composition of the ISM. While the extinction from the ISM has been mapped extensively in the Milky Way, little is known about the extinction properties of more distant galaxies. Therefore, when correcting for dust, typical Milky Way dust extinction is assumed, even though dust properties in galaxies are expected to depend on redshift. This proposal aims at measuring extinction curves of distant galaxies using gravitationally lensed multiply imaged background sources. The proposal is in part based on proven methods, and in part based on a new technique, extending the method to a larger sample. The importance of this work is not limited to SNe Ia surveys, but impacts any astrophysical probe, as incorrect assumption on the dust properties can cause a non-negligible bias in derived quantities. In addition, the measured evolution of dust with redshift will give insight into galaxy formation. The fellowship provides the fellow with an excellent opportunity to perform independent research at cutting edge institutions (Princeton and NBI), and collaborate with experts in the field. This gives the fellow an opportunity to become an independent researcher with expertise in the ISM. The fellowship will be mutually beneficial to the researcher and hosts well beyond the training period. The forging of strong, productive links between the hosts and researcher will form the basis of continued collaborations."