Fundamental physics with ESPRESSO: Precise limit on variations in the fine-structure constant towards the bright quasar HE0515-4414

Vol. 658
3. Cosmology

Fundamental physics with ESPRESSO: Precise limit on variations in the fine-structure constant towards the bright quasar HE0515-4414

One of the main scientific drivers for constructing the new ESPRESSO spectrograph at the Very Large Telescope was to measure possible variations in the strength of electromagnetism over cosmic time with unprecedented accuracy. Previous attempts, over two decades, have found possible hints of variations in the fine-structure constant with time and/or space, but the slit-based spectrographs they relied on were plagued by calibration problems that undermined their reliability. If the fine-structure constant varies, we expect relative wavelength shifts between different absorption lines arising in gas clouds along the sight lines to distant quasars. This work is the first precise measurement of the fine-structure constant with ESPRESSO, whose high-accuracy calibration with a laser frequency comb has almost completely removed wavelength calibration errors from consideration. The authors find that the fine-structure constant is the same as on Earth, to within just 1.4 parts per million, in an absorption complex at redshift 1.15 – a look-back time of 8.4 billion years – seen against one of the brightest high-redshift quasars known.