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I work in cosmology, studying the large scale structure of the Universe. I am a Ramón y Cajal Fellow based at the Institut de Ciències del Cosmos at the University of Barcelona.

My research focuses on the study of cosmology using the large scale structure of the Universe. I seek to answer one of the most fundamental questions within the field of fundamental physics and cosmology in the last 20 years: what is driving the late-time accelerated expansion of the Universe. In order to answer this question, astronomers have been surveying the cosmos during the last 30 years locating the most bright and luminous galaxies at different distances and epochs. Using galaxies and neutral hydrogen gas as tracers of the underlying matter structure of the cosmos,
we have been able to understand how the Universe has been expanding since it origin, 14 Gyr ago. In addition, the study of the cosmic microwave light and nearby Supernovae have complemented the picture of this expansion. One of the most striking conclusions is that this expansion has been decelerating during the first half of the Universe's age, and started to accelerate during the second half, 7Gyr ago. Direct measures of the current expansion rate using the dimming light of nearby Supernovae indicate that the distant galaxies are currently expanding at 73 km/s/Mpc.

The Dark Energy Spectroscopic Instrument (DESI) will deliver by 2026 cosmological information with an unprecedented precision which will allow cosmologists to put the current standard cosmological model under a stress test to investigate its validity.

In order to understand what is driving the current acceleration of the expansion of the Universe, physicists and cosmologists have postulated the existence of an exotic form of energy which we refer to as Dark Energy. Such form of energy would be associated with the energy pool that the quantum vacuum possesses and somehow would counter strike the attractive force of gravity at those times when the Universe is sufficiently sparse to start dominating. Beside dark energy, astronomers have been aware since the 1930s that there must be more matter in the Universe than the one associated with visible light in order to explain the observed dynamics of galaxies and their interactions. We refer to this extra amount of matter as Dark Matter, and as Dark Energy its origin and composition is still an unresolved puzzle. Dark Matter and Dark Energy constitute the current standard model of cosmology, which we refer to as ΛCDM.

The ΛCDM model predicts a very precise expansion rate of the Universe along different epochs. In order to test this model astronomers have been measuring this expansion rate through the distribution of galaxies at different epochs. I led the final analysis of some of these projects: BOSS (2009-2014), eBOSS (2014-2019), and DESI which has started to deliver data in 2020. Using a technique called, Baryon Acoustic Oscillations (BAO), we have been able to identify a specific scale in this distribution of galaxies, which is unaffected by physics of galaxy dynamics (galaxy bias), neither by non-linear effects of gravity. Measuring this scale at different epochs have allowed us to trace this expansion during the last 10 Gyr. The forthcoming DESI survey will exploit this technique at maximum allowing for a dramatic leap in the understanding of the laws of our Universe and possibly on the nature of Dark Energy.

Héctor Gil-Marín

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