In the Deep, Dark Lyman-Alpha Forest: Exploring Dark Energy and Galaxy Formation Using the Intergalactic Medium – DEEPDARKFOREST

During the last 5 billion years (Gyr), the universe has been accelerating in its expansion due to a repulsive force that we have named ‘dark energy’. My goal is to understand the nature of this mysterious phenomenon that dominates our universe. Cosmologists are working toward achieving this goal by studying its emergence with measurements across the 14 Gyr history of the universe. As part of the BOSS survey, I am exploring the universe at high redshift (when it was 11 Gyr old).

Galaxies and the intergalactic gas between them are distributed in a gargantuan ‘cosmic web’. Sound waves from the Big Bang are frozen into this web and act as a ruler on the sky, allowing us to measure its expansion. The light of bright background quasars can be used to reveal this gas through absorption imprinted on the spectra on its long journey to Earth. We have observed nearly 140,000 of the expected 160,000 quasars spectra so far, and so can build up a 3D picture of the cosmic web. With the BOSS survey coming to an end, I have obtained the most precise measurement of the expansion rate of the universe at any epoch since the formation of the Cosmic Microwave Background (CMB). This is rapid growth given that the 1st ever measurement occurred only a year ago. With these high-precision observations, I am revealing intriguing tension with standard models based on Planck measurements of the CMB, perhaps hinting at new physics.

Intergalactic gas reveals more than just this peaceful picture of slowing and accelerating expansion. Forming galaxies produce violent outflows that reach far into remote regions of space bringing with them heavy elements born in stars. They stir gas up and change the way that stars and galaxies form and evolve. I am using quasar observations to study these explosions by looking at the presence and properties of heavy elements like oxygen and carbon. I have performed measurements of gas around galaxies with exquisite precision using BOSS data, with a wealth of information about galaxy formation waiting to be tapped. It is already clear that these measurements are sensitive to structure on tiny scales, more than a million time smaller than the sound waves described above. Hence my proposal aims to understand both the smallest and the largest scales assessed by modern cosmology using the same rich data set.

I am a core member of the small team that conducted the research I outline, as part of the larger BOSS collaboration. BOSS itself forms part of the most impactful astronomical collaboration in astronomy – the US-led Sloan Digital Sky Survey (SDSS). eBOSS will pick up where BOSS leaves off in 2014, improving the precision at high redshift and potentially extending to intermediate redshifts using carbon absorption. I wrote the single author paper proposing the approach and am uniquely equipped to drive this as the lead expert.

I lead both the next generation dark energy surveys of this type set to begin in late 2018. One is part of the Dark Energy Spectroscopic Instrument (or DESI) and builds substantially on the intellectual heritage of SDSS. The other is part of WEAVE (a survey mode for the William Hershel Telescope), which is a European endeavour. The host institution has full access to both these surveys.

I recently accepted an Idex Excellence Chair (A*MIDEX ‘Rising Stars’) to join the host, Laboratoire d'Astrophysique de Marseille (LAM), in September 2014. Accepting this chair results in losing membership of SDSS as it embarks upon eBOSS. In order to fully perform the leadership of both next generation dark energy surveys, and lead the development of novel probes, it is of the utmost importance that I remain a core member of the SDSS team. I request funding sufficient to provide membership for myself and one ANR funded postdoc (to pursue my novel dark energy prove), and 3rd year funding for my A*MIDEX postdoc in order to carry out interpretation and simulation of galactic outflows.