The structuration of our Universe

The cosmological paradigm is now well established thanks to many observational probes covering a wide range of the cosmic history, i.e. a wide range of redshifts (from the observation of the Cosmic Microwave Background at z ~ 1100 to the observation of the Large Scale Structures at z ~ 0.1 ), as well as a wide range of cosmological scales (from the cluster scales of ~ 10 Mpc, to the Hubble scale of ~ 10 Gpc). The success of this paradigm in explaining such a series of cosmological observables is usually summarized in the very accurate measurement of the six parameters of the standard, ΛCDM cosmological model. However, detailed inspection and analysis of the various cosmological datasets reveal tensions within this simple model. These tensions may either be hints of departures from ΛCDM, or indicate the failure of this effective model to account for complex astrophysical processes at play during the different eras of cosmic history. In both cases, such tensions may bear the promise of either discovering new physics or understanding better the standard physical processes at play in the Universe.

The aim of this programme is precisely to explore some of these tensions, and to question their origins and their implications. We will therefore gather participants with different perspectives and expertise, from theory to data analysis, in order to address the issue in a comprehensive and cross-disciplinary manner. Tensions within the cosmological model may originate from across the entire cosmic history, from the CMB liberated at the Last Scattering Surface (LSS) at redshift ~ 1100 to the formation and evolution of the Large Scale Structures (LSS) at redshift ~ 1 and below.

We will focus in particular on the following series of four questions:

  • Week 1 : What is the importance of cosmological magnetic fields?
  • Week 2 : How did the Universe leave the Dark Ages?
  • Week 3 :Does the Universe tell us the same story from high to low redshifts?
  • Week 4 :Where are the missing baryons at low redshifts?

How did the Universe leave the Dark Ages?

Recent results of the Planck collaboration confirmed that Cosmic Reionization, the transition of the cosmic baryons from an essentially neutral state to an ionized state, ended rather late, at a redshift of about 8. In addition, the evolution of the global ionisation fraction has been found to be compatible with a rather late onset, and fully consistent with the first galaxies being the major driver of Reionization, as corroborated by observations of the most distant galaxies. What was then the rôle of the very first, so-called Population III stars that came before the first galaxies? How comes that their radiation did not have a more significant impact on the ionisation state of the Universe? What were their properties? In which observable do we have to look for their signature? (Image credit: Aubert et al.)

Where are the missing baryons at low redshifts?

In addition to not knowing exactly what dark energy and dark matter are, we face the disturbing, embarrassing fact that we do not know where, in the present day Universe, up to half of the ordinary matter resides... Are the missing baryons hidden in the cosmic web, as numerical simulations suggest? Have they been blown away, back into the underdense regions of cosmic voids? How can we detect them, if they are neither dense nor hot enough? What are their physical and dynamical properties? How do they fuel the nodes of the cosmic web to sustain star formation in present day clusters of galaxies? (Image credit: Bonjean et al.)

Does the Universe tell us the same story from high to low redshifts?

In the last decade, intriguing tensions between datasets have emerged. A couple of cosmological parameters (notably the present day expansion rate H0 and the variance of the matter density field σ8 ) seem very sensitive to the way you determine them, i.e. either from the measurement of primary CMB anisotropies (the Last Scattering Surface), from the distribution of galaxies and galaxy clusters (the Large Scale Structure) or from low redshift supernovae. What does it mean? Could the high redshift Universe be different from the low redshift Universe? Are those tensions signs of a dynamical, beyond the cosmological constant Dark Energy? Or maybe of unexpected properties of Dark Matter? Could laws of gravitation alternative to General Relativity be responsible for these tensions? But perhaps these tensions indicate that we do not understand properly how to extract cosmological information from low redshift data? Indeed, to which extent are the latter affected by biases of astrophysical origin (baryonic feedback, non-thermal processes, mass biases, etc.)? And how is the propagation of light, and consequently the information it carries, affected by the presence of large matter inhomogeneities on its way to us? (Image credit: Salvati et al.)

What is the importance of cosmological magnetic fields?

An increasing bundle of complementary data suggest that baryonic matter is currently magnetized at the scale of the entire Universe. In addition, cosmological magnetic fields have also been detected up to redshifts of about 3.5, that is when the Universe was only 1.8 billion years old, i.e. 13% of its present age. Where did those fields come from? How were they seeded before being amplified by dynamo processes? Were they generated in the primordial Universe through non standard inflation or phase transitions? Have they been induced by astrophysical batteries soon after Last Scattering? Were they injected into the intergalactic medium from the densest and most active Large Scale Structures? How did they evolve on the larges scales of the Cosmic Web? What rôle did they play, if already present, in the regulation of star formation during Reionization? Since non-linear, often turbulent plasma physics has the effect of erasing the initial properties of magnetic fields, what observations will enable us to disentangle between primordial and astrophysical seeding mechanisms? (Image credit: Durrive et al.)

Application

Application are possible at the following page.
We will review applications asap.