We presented reliable constraints on the nature of dark matter particles using state-of-the-art hydrodynamical simulations of structure formation (theory) and high-resolution quasar spectra (data).

“Galaxies like the one we are living in are believed to reside along the filaments and at their intersections, while most of the space is empty or at very low density.”

It is the first time that constraints on warm dark matter particles like gravitinos and sterile neutrinos are obtained using hydrodynamical simulations that address the growth of intergalactic structure in the high redshift universe.

We performed an extensive set of state-of-the-art simulations using supercomputer facilities at the COSMOS supercomputer hosted in Cambridge at the Department of Applied Mathematics and Theoretical Physics. Furthermore, we accurately estimated the systematics involved in such a measurement, using the high-resolution data of the European Southern Observatory (ESO) Large Programme taken at the Very Large Telescope (VLT) in Chile. This presents a unique sample of high-resolution quasars.

The two figures show a 6.6 million light years slice of a simulated universe populated by cold dark matter particles and warm dark matter particles. They display the filamentary gaseous (baryonic) structure, when the universe was only about 10% of its present age.

Galaxies like the one we are living in are believed to reside along the filaments and at their intersections, while most of the space is empty or at very low density. The network of filaments consists mainly of neutral hydrogen that is observed in absorption at a particular (Lyman-alpha) wavelength in the spectra of distant sources that emit photons (quasars).

These structures trace the underlying dark matter field. Thus, by studying the absorptions, one can model the dark matter distribution that keeps the structure together gravitationally. One can see that, if the dark matter is in the form of massive and warm particles, then the structures are more diffused and will give rise to different observed absorption features when compared to cold dark matter.

I started to get interested in this topic when my former Ph.D. advisor, Prof. Sabino Matarrese, put me in touch with researchers that were also involved in similar research at the Physics Department of Padua University: Dr. Julien Lesgourgues (visiting from Lausanne) and Dr. Antonio Riotto (now at Cern). At that time, I was in Cambridge working as a postdoc with Dr. Martin Haehnelt on the recovery of cosmological parameters from quasar absorption lines, so we decided to bring all of our expertise together.

We had theoretical expertise, along with the observational data collected at the VLT in Chile and computational facilities (COSMOS supercomputer). There were no real problems along the way; however we were particularly careful in presenting this cutting-edge topic in a clear and accessible manner which could be easily understood both by particle physicists and astronomers.