Seminar: Rescuing ΛCDM with cosmological neutrinos

ABSTRACT

The standard Planck ΛCDM model is well known to encounter several tensions when confronted with various cosmological data, including the neutrino mass and the DESI BAO tensions. The former tension refers to the neutrino mass constraint from CMB fitting being substantially below the lower bound from neutrino oscillation experiments. In the latter, the BAO scales measured by DESI disagree significantly with the Planck ΛCDM prediction at redshifts below 1. Both tensions could be alleviated by allowing the dark energy to vary with time, which would be an exciting breakthrough in Cosmology. However, we offer a simpler alternative to resolving these tensions. We examine a simple and natural extension of the standard ΛCDM model, which allows relic neutrinos to have finite chemical potentials. We confront this ΛCDMξ model, ΛCDM with neutrino mass Mν and degeneracy ξ3 as additional parameters, with various cosmological datasets. Fitting the ΛCDMξ model to the CMB data, we find 3σ evidence for a nonzero neutrino mass (Mν = 0.57 + 0.17 − 0.13 eV) and degeneracy (ξ3 = 1.13 + 0.41 − 0.19), and the O(1) neutrino degeneracy parameter is compatible with galaxy pairwise velocity and Big Bang Nucleosynthesis (BBN) data. Furthermore, the recent DESI BAO data strongly prefer the ΛCDMξ model to the Planck ΛCDM model.

BIOGRAPHY   

The speaker obtained his BSc and PhD degrees from the California Institute of Technology (Caltech).  He held research positions at MIT and Caltech before joining the Chinese University of Hong Kong in 1995.  His current research interest includes astrophysics, cosmology, and particle physics.  In particular, he has been the PI of the Hong Kong team of the Daya Bay Reactor Neutrino Experiment, which has discovered a new kind of neutrino oscillation that bears important implications for cosmology and particle physics. In 2014, he and colleagues from The Chinese University of Hong Kong, the University of Hong Kong, and the Hong Kong University of Science and Technology formed a Hong Kong Cluster and joined the ATLAS Collaboration at the Large Hadron Collider, CERN.