JWST | "With this mission, we hope to provide answers to two great mysteries of the Universe"

Two of the major questions that will be studied with JWST concern the understanding of the history of the Universe and the study of dark matter, which seems to constitute more than 80% of the matter in the Universe.

Our Universe is expanding, a bit like a raisin cake in an oven. The grapes would be galaxies, which in our cake-universe are moving away from each other. By measuring the speed at which the galaxies are moving away from each other, i.e. the rate of expansion of the Universe or Hubble-Lemaître constant, we can reconstruct its history and deduce when it was created (when the cake was still a ball of dough). If we study in detail the baking of the cake by comparing it to a model, we could deduce the temperature of the oven but also the quantity of yeast and flour. Astrophysicists use this approach to determine the content of the Universe from the movement of galaxies. They find that among the necessary ingredients, there is a mystery ingredient that has been called dark matter because it is not visible. The problem is that we still don't know what constitutes this dark matter and the JWST should help us to better understand its properties.  

The phenomenon of gravitational mirages is fascinating, even if it is perfectly explained by the theory of general relativity. These are multiple images of the same object, usually a distant galaxy, which appear because during their journey to us the light rays have been deflected by a foreground galaxy. We generally observe 2 or 4 images of the most distant galaxy, images that are formed around the nearest galaxy. The latter acts a bit like a distorting lens. The path followed by the light is not exactly the same for each image, so that if the source varies, we see the variations appear in one image, then in another. It is by measuring these temporal delays that we can deduce the difference in path between the rays that lead us to the measurement of the Hubble-Lemaître constant. It is this method that we have been developing for many years in the TDCOSMO collaboration.

I am now not alone in Liège in TDCOSMO since a PhD student, Lyne van de Vyvere, and an American post-doc, Matt Gomer, help me in my work. The fantastic thing about these systems is that the position of the observed images depends on the total mass of the deflecting galaxy (or lens), whether this mass is luminous or not. The luminous mass can be estimated from the light. What remains is the dark matter. Gravitational lenses allow us to "see" dark matter. We can study two big questions at the same time: the history of the Universe and the dark matter. It is not surprising that gravitational mirages have a prominent place in research.   

The nature of dark matter is totally unknown and this is the problem. It is quite challenging to think that most of the matter in the Universe is made of particles of unknown nature. This said, there is no lack of theoretical candidates. We can divide these into two categories: cold dark matter, made up of "heavy" particles that move slowly, and hot dark matter, made up of lighter particles that move more quickly. If these particles are massive and moving slowly, they are more likely to clump together and form lumps of different sizes, whereas if they are light and fast, there will be fewer lumps. To count the lumps, one must use methods that are sensitive to mass and this is where gravitational lenses come in. Observing a large sample of gravitational lenses with the James Webb will allow us to estimate the average amount of lumps in galaxies, for a wide range of masses of these lumps and thus identify the type of particles that would be the best candidate to compose the dark matter.  

The measurement of the Hubble's law is very delicate and this is the reason why there have been many debates and controversies for over a century. Modern instrumentation has allowed us to make a giant step forward and improve the precision of the measurements by more than a factor of ten. But this improvement in precision, and the multiplication of measurement methods has revealed an unexpected result: measurements based on the study of the "young" Universe predict a slower expansion than those based on the "old" Universe. Is this due to measurement errors that are underestimated or is it real? If it is real, it will be the collapse of the cosmological model as we know it. None of the proposed variants seems to be able to explain all the observations at our disposal. This could be the beginning of a great crisis or a major change in the cosmological model.  

What is certain is that the JWST will provide many key observations, impossible to obtain with current instruments. It is designed to study the first galaxies that formed. It allows many other fine observations of distant galaxies that are currently impossible. In the field of observational cosmology, it may shake up the established paradigm. Dark matter and dark energy could be the modern equivalent of the aether that was thought to bathe the Universe until the end of the 19th century. We need strong arguments to completely question our cosmological model and I think that the JWST is able to provide the observations that could make us reconsider our copy ... if it is necessary! 

I am involved in 2 projects that will be part of the first cycle of observations to be obtained with JWST. One project in which I am involved with American colleagues will use the MIRI instrument to observe a large number of gravitational lenses and probe their dark matter content, and in particular estimate the number of dark matter lumps. My role is primarily to assess the extent to which stars in the galaxy can give rise to observational signatures similar to those produced by dark matter lumps. The second project uses the NIRSPEC instrument and is conducted with my colleagues of the TDCOSMO collaboration. It aims at measuring the mass of a gravitational lensing galaxy by studying the motion of stars in this galaxy. This is a very difficult measurement from the ground. By comparing it to the measurement deduced from the mirage itself, we will be able to estimate if the method we use to measure the Hubble constant with gravitational mirages is robust enough. This is an important test that we will do on a mirage that also has a Liège note since it is one of the gravitational mirages discovered by Liège researchers in the last decades.