On the nature of dark matter


  • Astroparticle Physics


Identifying the nature of the dark matter component of the Universe remains one of the fundamental and most pressing open problem in Science today. Within the Standard Model for Cosmology, dark matter is simply described in terms of a classical pressure-less fluid, interacting with the other components of the Universe only through gravity, impacting on the Universe dynamics and whose perturbations drive structure formation. While there are a number of predictions within this framework matching observations, some possible shortcomings may be addressed within specific particle physics scenarios, as well as point to non-standard particle interactions.

Status of project and perspectives:

Non-minimally coupled Dark Matter

PIs: Andrea Lapi, Stefano Liberati

Despite its broad success the cold dark matter framework has been facing in recent times growing difficulties in fitting the growing wealth of data concerning dark matter phenomenology. At the same time some modified gravity approaches have obtained some success at small scales while struggling in explaining observations at large scales. In recent years we have explored an alternative approach which modifies the dynamics of cold dark matter at small scales by introducing dynamically a non-minimal coupling between the latter and the Einstein tensor. This proposal aims to further investigate the implications of non- minimally coupled dark matter within the framework of general relativity and explore its potential generalization in modified theories of gravity such as fractional and non-local gravity.

In these recent years the local group has focused on testing the non-minimal gravity framework (in its minimal formulation, with just a free parameter) by comparing its predictions with observational evidence such as galactic density profiles, core surface densities, rotation curves and the radial acceleration relation. Also we recently investigated the effectiveness of the model at the scale of galaxy clusters. In all these cases the proposed framework showed a remarkable consistency in reproducing the observed phenomenology despite its simplicity. Similar investigations were carried out in the context of fractional gravity. In the future we intend to extend this research with the following objectives:

  • Theoretical Framework: Develop a comprehensive theoretical framework for non-minimally coupled dark matter, in particular its possible origin and connection with more general ansatz such as fractional gravity.
  • Cosmological Consequences: Investigate the cosmological consequences of non-minimal coupling, including its impact on cosmography, structure formation, and the growth of dark matter perturbations.
  • Observational Signatures: Identify new observable signatures of non-minimally coupled dark matter and fractional gravity, such as distinctive effects on the cosmic microwave background, large-scale structure, and gravitational lensing.
  • Comparative Analysis: Compare the predictions of our model with the standard cold dark matter one and modified gravity theories, to discern the unique features and advantages of this approach.
  • Implications for Dark Energy: Investigate the possible connections between our framework for dark matter and the nature of dark energy, considering potential unification or interaction between these two cosmic components.

Paradigms and Scenarios for the Nature of the Dark Matter

PI: Paolo Salucci

The distribution of the dark matter in galaxies of different luminosity and Hubble type, has revealed itself to be much more than the evidence that dark particles govern the virialized structures of the Universe. In fact, we have found the existence of a well-ordered and amazing observational scenario in which the structural properties of the dark matter halos are in strong relationship with the corresponding ones of the baryonic components that they host. In detail, several unexpected correlations between the dark and the luminous sector have been discovered. This has clearly indicated that the mystery of the dark matter phenomenon has to be addressed by means of reverse-engineering the structural properties of the dark and luminous matter in galaxies. Furthermore, this has encouraged us to propose a change of the current paradigm, according to which, the correct scenario for the dark particle must originate from the simplest and most useful “First Principles of Physics”, as in the case of the well-known LCDM scenario. Instead, evidence has directed us to a different paradigm that exploits the dark and luminous entanglement, detected in the structural properties of galaxies, to obtain the correct scenario of the dark particle. This strategy is also suggested by the fact that WIMPs, the by-far preferred particles coming from the DM scenario subject to the first principles in Physics, fail in being detected. Noticeably, this new approach opens the way to currently poorly studied scenarios, as that in which the dark particles interact, over cosmological times, with Standard Model Particles.

In the framework of this new paradigm, our IFPU research line is set to obtain/investigate a) the kinematics of galaxies so as b) the weak/strong lensing signals around them and c) any other tracer of their gravitational fields, sometime also in cooperation with proper N-Body hydro-dynamical simulations. The aim of this is to understand the (likely complex) Nature of the dark particle. This investigation will be extended to the whole mass scale of virialized objects, i.e., to clusters of galaxies.