The Planck satellite mission has delivered its final results, confirming the accuracy of the hypotheses formulated by Georges Lemaître. Prof. Christophe Ringeval, who co-authored several articles presenting the results, reviews the mission’s achievements as well as assumptions that still need to be confirmed.
What a coincidence: the European Space Agency published the Planck mission’s final results on 17 July, the birthday of Canon Georges Lemaître. He and Alexander Friedmann were fathers of the standard model of cosmology, the most complete and effective description to date of the observable universe.
Launched by ESA in 2009, the Planck satellite mission was to scan space via nine frequencies in order to map the universe. Christophe Ringeval, a professor at UCLouvain’s Institute for Research in Mathematics and Physics (IRMP) and member of the Cosmology, Universe and Relativity at Louvain research group, can’t contain his delight when talking about the accomplishments made possible by the satellite. ‘We now see the universe like we can see earth on a map with continents, oceans, mountains. But with one important difference: earth maps reflect our planet at a given moment. Plank has mapped the entire universe from today all the way back to a very remote time, about 300,000 years after the Big Bang, which was about 13.4 billion years ago. This is a pivotal moment in the history of the universe since it was previously opaque: no stars or separate galaxies existed, only a very hot soup (plasma) whose density prevented light from escaping. Around 300,000 years ago, the universe cooled and expanded enough – thus becoming less dense – for light to finally travel out of the original soup. It’s this fossil radiation, originally hot but today very cold (2.73 K), often called cosmic microwave background (CMB), that was recorded by the Planck satellite. Thus the maps show light emitted by the universe when it was 300,000 years old, when it became transparent. But that’s not all: they also show the interaction of light with all that has happened since, that is, the making of the universe to present day.’
Prof. Ringeval added, ‘The Friedmann-Lemaître model works incredibly well. It helps explain the universe from 300,000 years ago to today. We try to find faults in it but nothing significant has been found.’
It’s impossible to report here all the lessons delivered by the Planck mission. But while they confirm magnificently the intuition of Georges Lemaître and all those who developed the Big Bang model, including the dark matter and energy whose nature remains mysterious, they also pave the way for future progress. That’s what Prof. Ringeval and his team are working on.
He explains, ‘Part of our research focuses on the primordial universe, that is, the universe long before it became transparent. So a priori we shouldn’t see anything via Planck, which captured the light of the universe when it ceased to be a plasma and became transparent. But any new phenomenon in the primordial universe can change the behaviour of the plasma and thus leave traces in the cosmological radiation; it becomes a tool that could allow us to learn more about this primordial universe. A first example is cosmic strings (not to be confused with string theory).’ At the very beginning, particle physicists expect the fundamental forces of nature (electromagnetism, strong nuclear force, weak interaction, gravitation) to be unified; as the universe cools, these forces ‘specialise’ in some way. Their separation corresponds to a phase change or transition, such as when water passes from solid to liquid.
‘It may be that defects appear in the structure of the universe. Defects, called cosmic strings, that take the form of filiform objects in the space-time structure and must have left signatures, additional traces in the fossil radiation.’ Prof. Ringeval has therefore developed theoretical synthetic maps in order to show what the CMB radiation maps would look like were these filiform objects present. The researchers compared these maps with the Planck mission’s and found no trace of elongated defects. But the signature strongly depends on the energy in which these objects form; if they’re not found, it may be because researchers have to look into another energy range. ‘Thanks to the Planck mission we know we have to look at energies less than 1015 GeV. This is important for both particle physics and future fossil radiation observation missions.’
Cosmologists also think that during its opaque phase, shortly after the Big Bang, the universe experienced a brutal, rapid, quasi-exponential expansion. But how to prove it? By detecting, for example, a Higgs particle. Not the one that was detected by CERN’s LHC in 2012, but another. In theory, there may have been a very rapid, brief expansion phase if the universe was dominated at that moment by a Higgs particle in a certain state. ‘Each unification of fundamental interactions corresponds to an associated Higgs particle. The one that was discovered recently corresponds to the unification of the electromagnetic force with the weak interaction. There must therefore be a particle corresponding to the unification of the three fundamental forces, which corresponds to a much stronger energy.’ It’s useless to hope for spotting such a particle on Earth: our accelerators will never reach such energies. Thus it’s the primordial universe that has to play the role of accelerator and the CMB the role of detector. Although the Planck mission failed to unequivocally detect this new Higgs field, it provided consistent clues of the existence of the resulting exponential expansion.
The first clue: the shape of the universe. The Friedmann-Lemaitre standard model doesn’t allow for determining it because it allows for any of three shapes: spherical, flat or saddle-shaped. But the theory of exponential inflation predicts a flat universe. ‘And that’s what the Planck mission measured’, Prof. Ringeval confirms. ‘We live in a very flat universe, which is a natural consequence of exponential inflation.’
The second clue: Plank’s CMB map reveals small clumps, embryos of future galaxies called fossil radiation anisotropies. Planck has shown that these anisotropies are not identical, and the way they differ points in the direction of an quasi-exponential expansion.
The final clue: Planck tested in spectacular fashion whether there may have been different initial densities in the plasma between particle species. For example, an overdensity of photons in one place, of quarks in another. Plank found no such density differences: all particles are born with the same relative overdensity everywhere in the universe. An observation that fits in with the fact that everything comes from a single particle, the same one that would have generated the inflation.
‘But all these clues are not enough’, Prof. Ringeval cautions. ‘It's like in a police investigation, you can’t settle for clues, you need proof! And the proof could be gravitational wave detection. The brutal inflation experienced by the universe must indeed have generated such waves, small fluctuations of space-time that propagate and should still be visible in fossil radiation.’ But how to detect them? Via light polarisation: diffuse background light is indeed polarised and it’s known that gravitational waves act on the generation of one of the two modes of light polarisation. It should therefore be possible to identify this action on the light. But even though it drew up two CMB polarisation maps, Planck wasn’t equipped for such a discovery and must leave it to future missions, including the Japanese LiteBird mission and US ground-based detector networks.
Europe won’t be left out. Funding for a new polarisation satellite, CORE, is under discussion, and the LISA space mission for direct gravitational wave detection is making headway. ‘That’s part of my research,’ Prof. Ringeval enthuses, ‘creating “templates”, theoretical maps that indicate what measurement instruments should “see” if our theories are correct.’
A glance at Christophe Ringeval's bio
Since 2016: Professor, Centre for Cosmology, Particle Physics and Phenomenology (CP3), UCLouvain
2006-16: Associate Professor, (CP3), UCLouvain
2004-06: Postdoctoral Researcher, Imperial College London (Theoretical Physics Group)
2002-04: Postdoctoral Researcher, University of Geneva (Department of Theoretical Physics)
1999-2002: Assistant, Université de Paris XI.
1999-2002: PhD in Science, Institut d’Astrophysique de Paris and Université de Paris VI
1997-98: DEA, Université de Paris VI.
1997: Bachelor’s Degree in Physics, Université de Toulouse III