Vincent Desjacques group picture


What is the Universe made of? How did the structures in the Universe form? These are only some of the concepts that have intrigued Vincent Desjacques since an early age growing up in Switzerland. After completing a master’s degree in physics, researching the field of astrophysics and cosmology, he was posted in Israel as part of his mandatory French army service, and started working on a project with Adi Nusser at the Technion. Once he finished his service, he decided to stay at the Technion and completed a Ph.D. in physics. After a postdoc position at Hebrew University and a second one at Zurich University he moved to Geneva where he was an assistant professor for several years. In 2016 Vincent returned to the Technion as faculty and as a member of the astrophysics and cosmology group. “I enjoyed the atmosphere at the Technion, both in terms of research and on a personal level, and was happy to return here and continue my work in Cosmology”, says Vincent.


Cosmology, a branch in astrophysics, is the study of the formation, structure and evolution of the Universe as a whole. We can describe our Universe with the standard cosmological paradigm. The paradigm combines the standard model of particle physics that classifies the elementary particles and three of the four fundamental forces (weak, strong and electro-magnetic) with general relativity as the theory for gravity (the fourth fundamental force), along with cosmic inflation that translates to the initial condition of the Universe as part of the “big bang” model. Cosmic inflation is an event that took place in the very early Universe when the Universe expanded exponentially for a very short time, beginning 10-36 seconds after the “big bang” and ending at 10-32 seconds. The cosmological model for the Universe also points to the existence of dark energy that currently drives our Universe to expand at an accelerating rate. In addition, observations indicate the existence of dark matter that accounts for most of the matter in the Universe. The picture becomes even more complex when we look at the large-scale structures in the Universe.


Vincent arrived at the Technion after winning an ISF (Israel Science Foundation) grant in 2016. The aim of his research is setting theoretical constraints on cosmic structure formation in our Universe from several aspects. Understanding the large-scale structure distribution of galaxies will enable us the extract information on the early Universe, on the nature of dark matter and on the nature of gravity (is it indeed as Einstein described it in the theory of general relativity?). Vincent’s research aims to explore two extensions of the cosmological model. The first is the observational evidence that neutrinos have a non-zero mass, and the second is related to inflation and the fluctuations in the early Universe known us “primordial non Gaussianities”. These two extensions of the model leave signatures (small effects) on the large-scale structures observed in today’s Universe.


Neutrinos are weakly-interacting, electrically neutral, elementary particles that are supposed to be mass-less according to the standard model of particle physics. Neutrinos are associated with three elementary light particles (leptons): electron, muon and tau, and because of that come in three “flavors” (or types): electron neutrinos, muon neutrinos and tau neutrinos. Our sun, for example, produces only electron neutrinos as a byproduct of nuclear reactions in its core. A problem with this picture has risen in the 1960s when Ray Davis led an experiment that measured the flux of the solar electron neutrinos on earth and found it to be only a third of that expected. It was later understood that neutrinos can actually “change their nature” and quantum mechanically oscillate from one type to another as they propagate through interplanetary space, causing pure electron neutrinos to arrive at Earth as muon neutrinos and tau neutrinos.


Neutrino oscillations can be explained if at least two of the neutrinos have a non-zero, slightly different mass (“massive neutrinos” hereafter). Davis won the 2002 Nobel prize in physics along with Masatoshi Koshiba for the detection of cosmic neutrinos, and a Nobel prize was awarded in 2015 to Art McDonald for the discovery of neutrino oscillations with experiments that detected and measured the flavors of solar neutrinos. These experiments measured the difference in mass between the neutrino flavors, but in order to get the exact mass value for each flavor a second type of experiment is needed that will give the sum of their masses. This is where cosmology comes into play.


Since neutrinos have a very low mass, mc2 less than 1eV (almost 6 orders of magnitudes lighter than the electron), they can easily escape gravitational potential wells like those of galaxies. Once escaping the well, they will reduce its gravity since they are massive. Hence the larger the mass of the neutrino – the stronger the cosmological effect. The overall mass “removed” will add the information needed to extract the mass of each neutrino flavor. Vincent’s group at the Technion looks at the theoretical aspect of massive neutrinos by modeling what will be the effect of massive neutrinos on the structure of the Universe depending on their masses. Since neutrinos have a small mass, the effects are expected to be small and a careful analytical model must be constructed. Erez Zinman, a student at the end of his master’s degree with Vincent, conducted analytical approximations based on the understanding that neutrinos are collision-less particles hence allowing him to make simplifications to the equations that describe the effect. The next step will be to test the analytical predictions with simulations where the initial condition is the Universe right after the epoch of inflation that was then smooth up to small fluctuations in the matter and radiation distributions.


Dimitry Ginzburg is currently working on his Ph.D. under the supervision of Vincent while focusing on aspects related to the distribution of galaxy clustering. How were these structures formed? How can we model the distribution of galaxies? From observational data we extract the position of galaxies in space, both the angular position in the sky and the distance (redshift) of each galaxy. Redshift is an effect where the wavelength (and the frequency) emitted by an object changes due to a combination of the doppler effect (blueshift or redshift) and the expansion of the Universe (a cosmological redshift). Overall, the expansion redshift dominates at large distances and causes distant astrophysical objects to appear redder. Hence, the value of the redshift is an indication of distances.


Since the galaxies do not have a continuous distribution but, rather, a discrete one, galaxy data are “noisy”. This noise is not just the simple Poisson noise of randomly distributed galaxies. It depends strongly on the properties of the surveyed galaxies, which evolve with redshift. In other words, when modeling the galaxy distribution to extract information on the neutrino masses e.g., we need to also model the noise properly. These theoretical models of the galaxy distribution can then be tested against numerical simulations before being applied to real data. The group at the Technion extracts the information needed from open source simulations in order to construct “simulated” or “synthetic” galaxy catalogues which mimic the properties of real surveys. Vincent notes that, since the characteristics of galaxy catalogues depend significantly on redshift, one needs to select redshifts in the simulations according to the redshift window surveyed by the telescope during the observations.


Another puzzle in cosmology that the group is working on is the nature of dark matter particles. There is an ongoing search for these enigmatic particles from several fronts, but they haven’t been detected yet. Vincent studies an alternative dark matter model called “axion dark matter”, where hypothetical elementary, particles called axions might be components of dark matter. In this analytical research, Vincent and collaborators take steps towards understanding the impact of light axions on the cosmic structure, from large gaseous filaments at high redshift down to nearby astrophysical objects such as binary pulsars. “I find the questions surrounding what happened in the early Universe to be the most exciting, and they drive my work and follow me from the point I wake up in the morning and until I go to sleep. I believe that in my scientific lifetime we will resolve the puzzle surrounding the nature of the dark matter and answer other open questions, while in the meantime new questions will appear. This is what makes science exciting”, summarizes Vincent.



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by Efrat Sabach