Nabila Aghanim: what measuring the cosmos reveals
Nabila Aghanim is an astrophysicist and cosmologist at the Institute of Space Astrophysics (IAS - Université Paris-Saclay, CNRS). In an ongoing dialogue between theory, modelling and instrumentation, her mission is to predict the models of the cosmos to be studied, and then to analyse the data proceeding from space telescopes in the major international programmes. What is the end goal? To track down any trace of signals in the cosmic microwave background to learn more about the nature of the Universe and the formation of the first clusters of galaxies.
Since she completed and defended her PhD thesis at the IAS under the direction of Jean-Loup Puget in 1996, Nabila Aghanim has worked on the Planck space telescope project for the European Space Agency (ESA). The telescope measures with great precision tiny temperature variations in the cosmic microwave background - the fossil radiation from the Big Bang 13.7 billion years ago. The mission was finally selected by the ESA in 1996 and the telescope was launched in 2009. "My research was to study the secondary effects of the cosmic microwave background, i.e. everything that has an impact on the fossil radiation signal from the moment it was emitted. I first worked on the scientific predictions of the measurement of the so-called SZ (Sunyaev Zel'dovich) effect, i.e. the trace of hot gas in the clusters that alters the cosmic microwave background." The study of the latter informs scientists about the history of galaxy clusters and the first stars.
The study of the cosmic microwave background
Nabila Aghanim's work consisted of predicting the most interesting physical models to measure from among all possible theoretical scenarios. More specifically, she was responsible for showing, at each stage, that the mission was feasible with the proposed instrument concept. "Once you have the data, you have to translate it into something that can be tested against theories. If you don't have it, you have to make the connection between the instrumentation and the scientific need." From 2009 to 2016, Nabila Aghanim was in charge of one of the Planck scientific programmes, coordinating the international teams while analysing the satellite's first measurements.
Research spread over time
Astronomical time is spread over an infinitely long scale. "Some areas of space enable launching a satellite every two years, to study Mars for example. An experiment like Planck takes place on average every 15 years!" emphasises Nabila Aghanim, who feels deeply privileged to have taken part in each phase of the evolution of the project, "a cornerstone of international cosmology." "There is an indescribable satisfaction in contributing to the overall project and its collective successes," she enthuses.
Finding the hidden matter of the Universe
In 2017, in addition to being awarded the CNRS Silver Medal, Nabila Aghanim was awarded an Advanced Fellowship from the European Research Council (ERC) for her ByoPIC project (the Baryon Picture of the Cosmos). "The cosmological model as we understand it today is made up of about 68% dark energy (which makes the expansion of the Universe accelerate), 25% dark matter (whose nature is not known) and less than 5% ordinary particles, of which half are not detected. The description of the Universe is therefore based on elements that we are not familiar with." The goal of ByoPIC is to combine Planck data with other existing information to try and find the missing ordinary matter.
Since galaxies are distributed in the Universe in the form of a complex network of nodes, connected to each other by filaments that form the cosmic web, Nabila Aghanim's team looked for gas emissions in these filaments. They effectively found the signal associated with these missing particles (baryons) on the cosmic microwave background via the SZ effect. "We detected it both in Planck data and in X-rays of 20-year-old data, and thus proved that it was indeed this."
Discovering the history of the Universe
Under construction since 2016 and presented to the press in March 2023, ESA's Euclid space telescope will be launched in July 2023 to map the geometry of the dark Universe. At the same time as she uses Planck data, Nabila Aghanim has been collaborating on this mission for several years, with the IAS technical and scientific teams. She takes part in the construction and scientific use of the VIS imager (observing in the visible range) on the telescope.
While waiting for the images from Euclid to arrive, Nabila Aghanim remains active. In particular, she is thinking about a mission concept applied to the observation of fossil radiation, whose goal would be to detect the variations of its emission spectrum. "We say that the cosmic microwave background is a black body emission, that is to say, an emission in perfect thermal equilibrium," she explains. “In fact, we detect small variations caused by energy input, such as that of stars whose photon emission distorts the spectrum of the microwave background very slightly. If we can measure it with ultimate precision, we will know the thermal and energetic history of the Universe from the Big Bang to the present day."
BISOU (Balloon Interferometer for Spectral Observations of the primordial Universe) in the stratosphere
Nabila Aghanim also takes part in research projects with more modest means, including the CNES stratospheric balloon project "BISOU", for which she works on the scientific instrumentation in collaboration with her colleague Bruno Maffei. “The instrumental aspect allows us to understand the limits of the observations," says the researcher. "The more detailed the interpretation of the data, the more detailed the access to the way the instrument measures it."
From Algeria to the cosmos
Nabila Aghanim's journey started in Algeria, her native country. As a pupil, she was dazzled by a book explaining the formation of the Earth-Moon system. At the age of nine, her decision was made: she would study the Universe! After taking a degree in science and a first Master's degree in Physics, she went to France to take a second Master's degree in Astrophysics and Space Instrumentation, this time in Paris. After her PhD, she became a temporary teaching fellow (ATER in French) at Université Paris-Sud (now Université Paris-Saclay) for one year, before leaving in 1997 for a short post-doctoral fellowship at the University of California at Berkeley. She continued her post-doctorate at CNES, at the Institute of Astrophysics in Paris and the IAS, where she was hired as a research fellow in 1999. She was appointed CNRS director of research in 2010. She then held the positions of deputy director of the IAS, then director of the Science Observatory at Université Paris-Sud (now the Science Observatory at Université Paris-Saclay) from 2017 to 2021.
Nabila Aghanim now forms part of numerous scientific bodies: she sits on one of the scientific committees of the CNES and on the Council of the European Astronomical Society. In 2022, she received the Huy Duong Bui Grand Prize from the French Academy of Science, "in recognition of the work undertaken with my collaborators."
She advises the new generations not to be afraid to go for it despite all the barriers, as her own journey has not always been easy. She was affected by the recommendations of the previous government to increase the registration fees for international students. "I am extremely grateful for the choice made by Université Paris-Saclay not to follow these decisions, thereby allowing students to keep the hope of continuing their studies and giving everyone a great chance to succeed," she concludes.
